The impact of livestock on the abundance, resting behaviour and sporozoite rate of malaria vectors in southern Tanzania

Treatment of cattle with ivermectin and its effect on dung degradation and larval abundance in a tropical savanna setting

When ingested as part of a blood meal, the antiparasitic drug ivermectin kills mosquitoes, making it a candidate for mass drug administration (MDA) in humans and livestock to reduce malaria transmission. When administered to livestock, most ivermectin is excreted unmetabolized in the dung within 5 days post administration. Presence of ivermectin, has been shown to adversely affect dung colonizers and dung degradation in temperate settings; however, those findings may not apply to, tropical environment, where ivermectin MDA against malaria would occur. Here we report results of a randomized field experiment conducted with dung from ivermectin-treated and control cattle to determine the effect of ivermectin on dung degradation in tropical Tanzania. For intact pats, we measured termite colonization, larval numbers and pat wet and dry weights. Pat organic matter was interpolated from a subsample of the pat (10 g wet weight). Additionally, we counted larvae growing in the treated and untreated pats in a semi-field setting. We found that termites colonized ivermectin pats more readily than controls. Despite this, wet weight decreased significantly slower in the ivermectin-treated pats in the first two weeks. As water was lost, sub-sample dry weight increased, and organic matter decreased similarly over time for the treatment and control. Interpolated for whole pats, total organic matter was higher, and larval counts were lower in the ivermectin-treated pats after the first month. Our results demonstrate an effect of ivermectin and its metabolites on dung degradation and fauna in a tropical savanna setting. Because slow dung degradation and low insect abundance negatively impact pastureland, these non-target, environmental effects must be further investigated within the context of real-world implementation of ivermectin MDA in cattle and weighed against the potential benefits for malaria control.

Understudied anophelines may sustain residual transmission during the dry season in a pre-elimination setting in southern Zambia

Malaria control is a public health priority but common control methods like indoor residual spraying and the use of bednets do not target outdoor-biting vectors. In settings with seasonal residual malaria transmission, we lack critical knowledge regarding anopheline species composition and their role in transmission. This study aimed to determine relative seasonal vector species abundance and associated household level factors in a low transmission setting in Choma District, Zambia. Indoor and outdoor adult vector collections were embedded in a community-based longitudinal cohort study in 60 households that were visited monthly for 2 years between 2018 and 2020. Surveys conducted at the time of trap placement collected information on animal ownership, housing structure, and the receipt of malaria interventions. Anopheline species identities were molecularly confirmed by polymerase chain reaction, and enzyme-linked immunosorbent assay was used to detect the circumsporozoite protein of Plasmodium falciparum. Generalized linear mixed effects negative binomial regression with zero-inflation models were used to describe the relationship between risk factors and the outcome of monthly anopheline counts at each household, stratified by season. The study collected 1,532 female anophelines, 76% of which were caught outdoors. The relative abundance differed by season: in the dry season, 90% of female anophelines were caught outdoors. Anopheles arabiensis was overall the most common vector, but made up only 28% of outdoor collections; the remainder were understudied anophelines including An. coustani, An. leesoni, An. rufipes, and An. squamosus. The only Plasmodium falciparum-infected mosquito was an An. squamosus that was caught outdoors. Owning more goats was associated with a 3.5 (IRR 4.47, 95% confidence interval [CI]: 2.00, 10.01) and 7.7 (IRR 8.73, 95% CI: 4.40, 17.32) times increase in indoor and outdoor anopheline collections in the dry season and a 1.2 (IRR 2.18, 95% CI: 1.12, 4.23) times higher risk of outdoor anophelines in the rainy season. Improved housing structure was associated with fewer indoor anophelines in the rainy season, but not during dry season or outdoor anopheline abundance any time of year. Vector control in this low transmission setting, therefore, needs to target anopheline mosquitoes year-round, must be expanded to target traditionally zoophillic mosquitoes, and leverage known risk factors when selecting methods of control.

Feeding habits and malaria parasite infection of Anopheles mosquitoes in selected agroecological areas of Northwestern Ethiopia

BACKGROUND

Surveillance of the host-anopheline mosquitoes' interaction is important for assessing malaria transmission risk and guiding vector control. We assume that changes in malaria vector species' feeding habits, as well as the surrounding environment, have a substantial impact on varied malaria transmission. In this study, we determined the vertebrate host feeding patterns of anopheline mosquitoes to characterize entomologic risk factors for malaria in Jabi Tehnan, Northwestern Ethiopia.

METHODS

Blood-fed anophelines surveyed during malaria surveillance in Jabi Tehnan district of northwestern Ethiopia were utilized in this study. They were collected using Centers for Disease Control and Prevention (CDC) light traps deployed in selected households per village, placed indoors and outdoors, spanning three agroecological settings (dry mountain, plateau, and semiarid highlands) between June 2020 and May 2021. The engorged mosquitoes were analyzed for host blood meal sources and Plasmodium infection via polymerase chain reaction (PCR) and/or sequencing. Infection rates and bovine and human blood indices were calculated and compared for abundant species; between indoors and outdoors and between agroecology using a chi-squared test for equality of proportion in R package at a significant level of p ≤ 0.05.

RESULTS

A total of 246 mosquitoes were successfully typed (indoor, 121; outdoor, 125), with greater relative abundance indoors in mountain and plateau highlands, and outdoors in semiarid areas. Despite ecological differences in blood-fed capture rates, cattle served as the most utilized blood meal source by 11 anopheline species with an overall bovine blood index (BBI) of 74.4%. This trend was dictated by Anopheles gambiae s.l. (198/246; BBI = 73.7%), which exhibited the most plastic feeding habits that included humans (human blood index = 15.7%) and other livestock and rodents. A total of five anopheline species (An. gambiae s.l., An. funestus s.l., An. coustani s.l., An. pretoriensis, and An. pharoensis) fed on humans, of which the first three were found infected with Plasmodium parasites. Most of the infected specimens were An. arabiensis (5.6%, 11/198) and had recently fed mainly on cattle (72.7%, 8/11); one each of infected An. funestus s.l. and An. coustani s.l. had fed on humans and cattle, respectively.

CONCLUSIONS

The results demonstrate communal feeding on cattle by anophelines including primary and secondary malaria vectors. This study also indicates the importance of cattle-targeted interventions for sustainable control of malaria vectors in the study areas.

Livestock keeping, mosquitoes and community viewpoints: a mixed methods assessment of relationships between livestock management, malaria vector biting risk and community perspectives in rural Tanzania

BACKGROUND

Livestock keeping is one of the potential factors related to malaria transmission. To date, the impact of livestock keeping on malaria transmission remains inconclusive, as some studies suggest a zooprophylactic effect while others indicate a zoopotentiation effect. This study assessed the impact of livestock management on malaria transmission risks in rural Tanzania. Additionally, the study explored the knowledge and perceptions of residents about the relationships between livestock keeping and malaria transmission risks in a selected village.

METHODS

In a longitudinal entomological study in Minepa village, South Eastern Tanzania, 40 households were randomly selected (20 with livestock, 20 without). Weekly mosquito collection was performed from January to April 2023. Indoor and outdoor collections used CDC-Light traps, Prokopack aspirators, human-baited double-net traps, and resting buckets. A subsample of mosquitoes was analysed using PCR and ELISA for mosquito species identification and blood meal detection. Livestock's impact on mosquito density was assessed using negative binomial GLMMs. Additionally, in-depth interviews explored community knowledge and perceptions of the relationship between livestock keeping and malaria transmission risks.

RESULTS

A total of 48,677 female Anopheles mosquitoes were collected. Out of these, 89% were Anopheles gambiae sensu lato (s.l.) while other species were Anopheles funestus s.l., Anopheles pharoensis, Anopheles coustani, and Anopheles squamosus. The findings revealed a statistically significant increase in the overall number of An. gambiae s.l. outdoors (RR = 1.181, 95%CI 1.050-1.862, p = 0.043). Also, there was an increase of the mean number of An. funestus s.l. mosquitoes collected in households with livestock indoors (RR = 2.866, 95%CI: 1.471-5.582, p = 0.002) and outdoors (RR = 1.579,95%CI 1.080-2.865, p = 0.023). The human blood index of Anopheles arabiensis mosquitoes from houses with livestock was less than those without livestock (OR = 0.149, 95%CI 0.110-0.178, p < 0.001). The majority of participants in the in-depth interviews reported a perceived high density of mosquitoes in houses with livestock compared to houses without livestock.

CONCLUSION

Despite the potential for zooprophylaxis, this study indicates a higher malaria transmission risk in livestock-keeping communities. It is crucial to prioritize and implement targeted interventions to control vector populations within these communities. Furthermore, it is important to enhance community education and awareness regarding covariates such as livestock that influence malaria transmission.

New directions for malaria vector control using geography and geospatial analysis

As we strive towards the ambitious goal of malaria elimination, we must embrace integrated strategies and interventions. Like many diseases, malaria is heterogeneously distributed. This inherent spatial component means that geography and geospatial data is likely to have an important role in malaria control strategies. For instance, focussing interventions in areas where malaria risk is highest is likely to provide more cost-effective malaria control programmes. Equally, many malaria vector control strategies, particularly interventions like larval source management, would benefit from accurate maps of malaria vector habitats - sources of water that are used for malarial mosquito oviposition and larval development. In many landscapes, particularly in rural areas, the formation and persistence of these habitats is controlled by geographical factors, notably those related to hydrology. This is especially true for malaria vector species like Anopheles funestsus that show a preference for more permanent, often naturally occurring water sources like small rivers and spring-fed ponds. Previous work has embraced geographical concepts, techniques, and geospatial data for studying malaria risk and vector habitats. But there is much to be learnt if we are to fully exploit what the broader geographical discipline can offer in terms of operational malaria control, particularly in the face of a changing climate. This chapter outlines potential new directions related to several geographical concepts, data sources and analytical approaches, including terrain analysis, satellite imagery, drone technology and field-based observations. These directions are discussed within the context of designing new protocols and procedures that could be readily deployed within malaria control programmes, particularly those within sub-Saharan Africa, with a particular focus on experiences in the Kilombero Valley and the Zanzibar Archipelago, United Republic of Tanzania.

A systematic review of the factors associated with malaria infection among forest rangers

INTRODUCTION

Malaria is a vector-borne disease that initially manifests as fever, headache, and chills. The illness could progress to more severe conditions, including lethargy, impaired consciousness, convulsions, shortness of breath, blood in urine, jaundice, and haemorrhage if left untreated. The risk of contracting malaria is considerably heightened in specific occupational settings, particularly among forest rangers, following frequent exposure to natural habitats. Consequently, advancing the understanding of malaria and emphasising how specific occupational environments (including those of forest rangers) contribute to disease risk and management is imperative.

OBJECTIVE

The present study aims to determine the factors associated with malaria infection among forest rangers by systematically reviewing electronic articles from three databases (EBSCOhost, ScienceDirect, and ResearchGate).

METHODS

The current review was prepared based on the updated preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. First, three independent reviewers screened the titles and abstracts of the data collected. The information was then stored in Endnote20 based on the inclusion and exclusion criteria. The articles were critically appraised with the mixed methods appraisal tool (MMAT) to assess their quality.

RESULT

A total of 103, 31, and 51 articles from EBSCOhost, ScienceDirect, and ResearchGate, respectively, were selected, resulting in 185 unique hits. Nevertheless, only 63 full-text publications were assessed following a rigorous selection screening, from which only five were included in the final review. The studies revealed that several factors contribute to malaria infection among forest rangers. The parameters were classified into sociodemographic, individual, and living condition-related.

CONCLUSION

A better understanding of malaria progresses and identifying its potential risk factors is essential to impact worker well-being. The findings might be utilised to improve malaria infection prevention programme implementations, hence maximising their success. Pre-employment and regular health screenings could also aid in evaluating and identifying potential risks for malaria infection among forest rangers.

Vegetation index and livestock practices as predictors of malaria transmission in Nigeria

Nigeria is the most malaria-endemic country in the world. Vegetation and livestock practices have been linked to malaria transmission but little is known about these in Nigeria. The study aimed to evaluate the influence of vegetation and livestock as predictors of malaria transmission in Nigeria. Secondary data obtained from the Nigerian Demographic and Health Survey's Geospatial Covariate Datasets Manual were used for the analysis. The survey was carried out successfully in 1389 clusters of thirty (30) households each using a two-stage stratified random sampling design. Hierarchical beta regression models were used to model the associations between malaria incidence, enhanced vegetation index (EVI), and livestock practices. The correlation coefficients for vegetation index and livestock-related variables ranged from - 0.063 to 0.074 and varied significantly with the incidence of malaria in Nigeria (P < 0.001). The model showed vegetation index, livestock goats, and sheep as positive predictors of malaria transmission. Conversely, livestock chicken and pigs were observed to reduce the risk of malaria. The study recommends the need to take into account local differences in transmission when developing malaria early warning systems that utilize environmental and livestock predictors.

In-vitro susceptibility and ex-vivo evaluation of macrocyclic lactone endectocides sub-lethal concentrations against Plasmodium vivax oocyst development in Anopheles arabiensis

BACKGROUND

Asymptomatic malaria transmission has become a public health concern across malaria-endemic Africa including Ethiopia. Specifically, Plasmodium vivax is more efficient at transmitting earlier in the infection and at lower densities than Plasmodium falciparum. Consequently, a greater proportion of individuals infected with P. vivax can transmit without detectable gametocytaemia. Mass treatment of livestock with macrocyclic lactones (MLs), e.g., ivermectin and doramectin, was suggested as a complementary malaria vector tool because of their insecticidal effects. However, the effects of MLs on P. vivax in Anopheles arabiensis has not yet been fully explored. Hence, comparative in-vitro susceptibility and ex-vivo studies were conducted to evaluate the effects of ivermectin, doramectin and moxidectin sub-lethal concentrations on P. vivax oocyst development in An. arabiensis.

METHODS

The 7-day sub-lethal concentrations of 25% (LC25) and 5% (LC5) were determined from in-vitro susceptibility tests on female An. arabiensis in Hemotek® membrane feeding assay. Next, an ex-vivo study was conducted using P. vivax gametocytes infected patient's blood spiked with the LC25 and LC5 of the MLs. At 7-days post-feeding, each mosquito was dissected under a dissection stereo microscope, stained with 0.5% (w/v) mercurochrome solution, and examined for the presence of P. vivax oocysts. Statistical analysis was based on a generalized mixed model with binomially distributed error terms.

RESULTS

A 7-day lethal concentration of 25% (LC25, in ng/mL) of 7.1 (95% CI: [6.3;8.0]), 20.0 (95%CI:[17.8;22.5]) and 794.3 (95%CI:[716.4;1516.3]) were obtained for ivermectin, doramectin and moxidectin, respectively. Similarly, a lethal concentration of 5% (LC5, in ng/mL) of 0.6 (95% CI: [0.5;0.7]), 1.8 (95% CI:[1.6;2.0]) and 53.7 (95% CI:[ 48.4;102.5]) were obtained respectively for ivermectin, doramectin and moxidectin. The oocyst prevalence in treatment and control groups did not differ significantly (p > 0.05) from each other. Therefore, no direct effect of ML endectocides on P. vivax infection in An. arabiensis mosquitoes was observed at the sub-lethal concentration (LC25 and LC5).

CONCLUSIONS

The effects of ivermectin and doramectin on malaria parasite is more likely via indirect effects, particularly by reducing the vectors lifespan and causing mortality before completing the parasite's sporogony cycle or reducing their vector capacity as it affects the locomotor activity of the mosquito.

Effects of cattle on vector-borne disease risk to humans: A systematic review

Vector-borne pathogens (VBPs) causing vector-borne diseases (VBDs) can circulate among humans, domestic animals, and wildlife, with cattle in particular serving as an important source of exposure risk to humans. The close associations between humans and cattle can facilitate the transmission of numerous VBPs, impacting public health and economic security. Published studies demonstrate that cattle can influence human exposure risk positively, negatively, or have no effect. There is a critical need to synthesize the information in the scientific literature on this subject, in order to illuminate the various ecological mechanisms that can affect VBP exposure risk in humans. Therefore, the aim of this systematic review was to review the scientific literature, provide a synthesis of the possible effects of cattle on VBP risk to humans, and propose future directions for research. This study was performed according to the PRISMA 2020 extension guidelines for systematic review. After screening 470 peer-reviewed articles published between 1999-2019 using the databases Web of Science Core Collection, PubMed Central, CABI Global Health, and Google Scholar, and utilizing forward and backward search techniques, we identified 127 papers that met inclusion criteria. Results of the systematic review indicate that cattle can be beneficial or harmful to human health with respect to VBDs depending on vector and pathogen ecology and livestock management practices. Cattle can increase risk of exposure to infections spread by tsetse flies and ticks, followed by sandflies and mosquitoes, through a variety of mechanisms. However, cattle can have a protective effect when the vector prefers to feed on cattle instead of humans and when chemical control measures (e.g., acaricides/insecticides), semio-chemicals, and other integrated vector control measures are utilized in the community. We highlight that further research is needed to determine ways in which these mechanisms may be exploited to reduce VBD risk in humans.

Effect of non-human hosts on the human biting rate of primary and secondary malaria vectors in Tanzania

BACKGROUND

Malaria vectors vary in feeding preference depending on their innate behaviour, host availability and abundance. Host preference and human biting rate in malaria vectors are key factors in establishing zooprophylaxis and zoopotentiation. This study aimed at assessing the impact of non-human hosts in close proximity to humans on the human biting rate of primary and secondary malaria vectors, with varying host preferences.

METHODS

The effect of the presence of non-human hosts in close proximity to the human host on the mean catches per person per night, as a proxy for mosquito biting rate, was measured using mosquito-electrocuting traps (METs), in Sagamaganga, Kilombero Valley, Tanzania. Two experiments were designed: (1) a human versus a calf, each enclosed in a MET, and (2) a human surrounded by three calves versus a human alone, with each human volunteer enclosed individually in a MET spaced 10 m apart. Each experiment was conducted on alternate days and lasted for 36 nights per experiment. During each experiment, the positions of hosts were exchanged daily (except the human in experiment 2). All anopheline mosquitoes caught were assayed for Plasmodium sporozoites using enzyme-linked immunosorbent assay.

RESULTS

A total of 20,574 mosquitoes were captured and identified during the study, of which 3608 were anophelines (84.4% primary and 15.6% secondary malaria vectors) and 17,146 were culicines. In experiment 1, the primary malaria vector, Anopheles arabiensis, along with Culex spp. demonstrated a preference for cattle, while the primary vectors, Anopheles funestus, preferred humans. In experiment 2, both primary vectors, An. arabiensis and An. funestus, as well as the secondary vector Anopheles rivolurum, demonstrated behaviours amenable to zooprophylaxis, whereas Culex spp. increased their attraction to humans in the presence of nearby cattle. All anopheline mosquitoes tested negative for sporozoites.

CONCLUSIONS

The findings of this study provide support for the zooprophylaxis model for malaria vectors present in the Kilombero Valley, and for the zoopotentiation model, as it pertains to the Culex spp. in the region. However, the factors regulating zooprophylaxis and zoopotentiation are complex, with different species-dependent mechanisms regulating these behaviours, that need to be considered when designing integrated vector management programmes.

Changes in contributions of different Anopheles vector species to malaria transmission in east and southern Africa from 2000 to 2022

BACKGROUND

Malaria transmission in Africa is facilitated by multiple species of Anopheles mosquitoes. These vectors have different behaviors and vectorial capacities and are affected differently by vector control interventions, such as insecticide-treated nets and indoor residual spraying. This review aimed to assess changes in the contribution of different vector species to malaria transmission in east and southern Africa over 20 years of widespread insecticide-based vector control.

METHODS

We searched PubMed, Global Health, and Web of Science online databases for articles published between January 2000 and April 2023 that provided species-specific sporozoite rates for different malaria vectors in east and southern Africa. We extracted data on study characteristics, biting rates, sporozoite infection proportions, and entomological inoculation rates (EIR). Using EIR data, the proportional contribution of each species to malaria transmission was estimated.

RESULTS

Studies conducted between 2000 and 2010 identified the Anopheles gambiae complex as the primary malaria vector, while studies conducted from 2011 to 2021 indicated the dominance of Anopheles funestus. From 2000 to 2010, in 57% of sites, An. gambiae demonstrated higher parasite infection prevalence than other Anopheles species. Anopheles gambiae also accounted for over 50% of EIR in 76% of the study sites. Conversely, from 2011 to 2021, An. funestus dominated with higher infection rates than other Anopheles in 58% of sites and a majority EIR contribution in 63% of sites. This trend coincided with a decline in overall EIR and the proportion of sporozoite-infected An. gambiae. The main vectors in the An. gambiae complex in the region were Anopheles arabiensis and An. gambiae sensu stricto (s.s.), while the important member of the An. funestus group was An. funestus s.s.

CONCLUSION

The contribution of different vector species in malaria transmission has changed over the past 20 years. As the role of An. gambiae has declined, An. funestus now appears to be dominant in most settings in east and southern Africa. Other secondary vector species may play minor roles in specific localities. To improve malaria control in the region, vector control should be optimized to match these entomological trends, considering the different ecologies and behaviors of the dominant vector species.

Community perception of the autodissemination of pyriproxyfen for controlling malaria vectors in south-eastern Tanzania

BACKGROUND

The efficacy of the autodissemination of pyriproxyfen to control malaria vectors has been demonstrated under semi field environment in Tanzania. However, the information on how best communities should be engaged for its routine and large-scale adoption are lacking. This study assessed the community's level of knowledge, perceptions, acceptability of the autodissemination of pyriproxyfen, and the perceived risks on the safety of pyriproxyfen on the environment.

METHODS

This was a concurrent mixed methods study, comprised of a community-based survey of 400 household representatives and eight focus group discussions (FGDs). The study was conducted in two villages in Mlimba district in south-eastern Tanzania between June and August 2022. For the quantitative data analysis, descriptive statistics were applied using R software, while inductive approach was used for qualitative data analysis, using NVivo software.

RESULTS

Knowledge on autodissemination of pyriproxyfen approach was found to be relatively low among both the FGD respondents and surveyed community members (36%, n = 144). Nevertheless, when it was explained to them, the envisioned community support for the autodissemination approach was relatively high (97%, n = 388). One of the major perceived benefits of the autodissemination of pyriproxyfen was the reduction of malaria-transmitting mosquitoes and associated malaria transmission. Environmental impact of pyriproxyfen on non-target organisms and health risk to children were among the major concerns. When provided with information on the safety and its utilization particularly through autodissemination approach, 93.5% (n = 374) of the survey respondents said that they would allow the PPF-contaminated pots to be placed around their homes. Similarly, FGD respondents were receptive towards the autodissemination of pyriproxyfen, but emphasized on the need for raising awareness among community members before related field trials.

CONCLUSION

This study indicates a low knowledge but high support for scaling up of the autodissemination of pyriproxyfen as a complementary tool for malaria control in rural Tanzania. The Findings of this study suggest that community sensitization activities are required to improve the community's acceptability and trust of the approach before respective field trials.

Heritability of biting time behaviours in the major African malaria vector Anopheles arabiensis

BACKGROUND

The use of insecticide-treated nets for malaria control has been associated with shifts in mosquito vector feeding behaviour including earlier and outdoor biting on humans. The relative contribution of phenotypic plasticity and heritability to these behavioural shifts is unknown. Elucidation of the mechanisms behind these shifts is crucial for anticipating impacts on vector control.

METHODS

A novel portable semi-field system (PSFS) was used to experimentally measure heritability of biting time in the malaria vector Anopheles arabiensis in Tanzania. Wild An. arabiensis from hourly collections using the human landing catch (HLC) method were grouped into one of 3 categories based on their time of capture: early (18:00-21:00), mid (22:00-04:00), and late (05:00-07:00) biting, and placed in separate holding cages. Mosquitoes were then provided with a blood meal for egg production and formation of first filial generation (F1). The F1 generation of each biting time phenotype category was reared separately, and blood fed at the same time as their mothers were captured host-seeking. The resultant eggs were used to generate the F2 generation for use in heritability assays. Heritability was assessed by releasing F2 An. arabiensis into the PSFS, recording their biting time during a human landing catch and comparing it to that of their F0 grandmothers.

RESULTS

In PSFS assays, the biting time of F2 offspring (early: 18:00-21:00, mid: 22:00-04:00 or late: 05:00-07:00) was significantly positively associated with that of their wild-caught F0 grandmothers, corresponding to an estimated heritability of 0.110 (95% CI 0.003, 0.208). F2 from early-biting F0 were more likely to bite early than F2 from mid or late-biting F0. Similarly, the probability of biting late was higher in F2 derived from mid and late-biting F0 than from early-biting F0.

CONCLUSIONS

Despite modest heritability, our results suggest that some of the variation in biting time is attributable to additive genetic variation. Selection can, therefore, act efficiently on mosquito biting times, highlighting the need for control methods that target early and outdoor biting mosquitoes.

Baseline characterization of entomological drivers of malaria transmission in Namibia: a targeted operational entomological surveillance strategy

BACKGROUND

Namibia's focus on the elimination of malaria requires an evidence-based strategy directed at understanding and targeting the entomological drivers of malaria transmission. In 2018 and 2019, the Namibia National Vector-borne Diseases Control Program (NVDCP) implemented baseline entomological surveillance based on a question-based approach outlined in the Entomological Surveillance Planning Tool (ESPT). In the present study, we report on the findings of the ESPT-based NVDCP on baseline vector species composition and bionomic traits in malaria endemic regions in northern Namibia, which has the aim of generating an evidence base for programmatic decision-making.

METHODS

Nine representative sentinel sites were included in the 2018 entomological surveillance program (Kunene, Omusati, Oshana, Ohangwena, Oshikoto, Otjozondjupa, Kavango West, Kavango East and Zambezi); the number was reduced to four sites in 2019 due to limited funding (Ohangwena, Kavango West, Kavango East, and Zambezi). In the 2018 baseline collections, multiple sampling methods (human landing catches, pyrethroid spray catches, U.S. Centers for Disease Control and Prevention light traps [CDC-LTs], resting boxes [RBs] and larval sampling) were utilized to evaluate indoor/outdoor human biting rates, resting behaviors and insecticide resistance (IR). CDC-LTs and RBs were not used in 2019 due to low and non-representative sampling efficacies.

RESULTS

Overall, molecular evidence demonstrated the presence of three primary mosquito vectors, namely Anopheles arabiensis, rediscovered Anopheles gambiae sensu stricto and Anopheles funestus sensu stricto, alongside Anopheles squamosus and members of the Anopheles coustani complex. Vectors were found to bite throughout the night (1800 hours 0600 hours) both indoors and outdoors, with An. arabiensis having the highest biting rates outdoors. Low numbers of indoor resting Anopheles point to possible low indoor residual spraying (IRS) efficacy-with An. arabiensis found to be the major vector species resting indoors. The IR tests demonstrated varying country-wide resistance levels to the insecticide deltamethrin, with the resistance levels confirmed to have increased in 2019, evidence that impacts national programmatic decision-making. Vectors demonstrated susceptibility to the insecticides dichlorodiphenyltrichloroethane, bendiocarb and Actellic 300CS in 2018, with mosquitoes from only one site (Kavango West) demonstrating possible resistance to DDT. Targeted and question-based entomological surveillance enabled a rapid and focused evidence base to be built, showing where and when humans were being bitten and providing entomological data on long-lasting insecticidal nets, IRS efficacy and insecticide resistance, which the Ministry of Health and Social Services-Namibia can use to further build a monitoring and evaluation framework for understanding the drivers of transmission.

CONCLUSION

Identification and characterization of species-specific bionomic traits allows for an understanding of where and when vector human contact may occur as well as the potential impact of interventions. Low indoor resting rates as well as the presence of insecticide resistance (and the increase in its frequency) point to the need for mosquito-behavior-directed and appropriate interventions as well as the requirement for a resistance mitigation strategy. The ESPT-based question- and minimal essential indicator-based operational research strategy provides programs with directed and focused data for facilitating decision-making while requiring limited funding and capacity.

Association between domesticated animal ownership and Plasmodium falciparum parasite prevalence in the Democratic Republic of the Congo: a national cross-sectional study

BACKGROUND

Domesticated animal ownership is an understudied aspect of the human environment that influences mosquito biting behaviour and malaria transmission, and is a key part of national economies and livelihoods in malaria-endemic regions. In this study, we aimed to understand differences in Plasmodium falciparum prevalence by ownership status of common domesticated animals in DR Congo, where 12% of the world's malaria cases occur and anthropophilic Anopheles gambiae vectors predominate.

METHODS

In this cross-sectional study, we used survey data from individuals aged 15-59 years in the most recent (2013-14) DR Congo Demographic and Health Survey and previously performed Plasmodium quantitative real-time PCR (qPCR) to estimate P falciparum prevalence differences by household ownership of cattle; chickens; donkeys, horses, or mules; ducks; goats; sheep; and pigs. We used directed acyclic graphs to consider confounding by age, gender, wealth, modern housing, treated bednet use, agricultural land ownership, province, and rural location.

FINDINGS

Of 17 701 participants who had qPCR results and covariate data, 8917 (50·4%) of whom owned a domesticated animal, we observed large differences in malaria prevalence across types of animals owned in both crude and adjusted models. Household chicken ownership was associated with 3·9 (95% CI 0·6 to 7·1) more P falciparum infections per 100 people, whereas cattle ownership was associated with 9·6 (-15·8 to -3·5) fewer P falciparum infections per 100 people, even after accounting for bednet use, wealth, and housing structure.

INTERPRETATION

Our finding of a protective association conferred by cattle ownership suggests that zooprophylaxis interventions might have a role in DR Congo, possibly by drawing An gambiae feeding away from humans. Studies of animal husbandry practices and associated mosquito behaviours could reveal opportunities for new malaria interventions.

FUNDING

The National Institutes of Health and the Bill & Melinda Gates Foundation.

TRANSLATIONS

For the French and Lingala translations of the abstract see Supplementary Materials section.

Association between Livestock Ownership and Malaria Incidence in South-Central Ethiopia: A Cohort Study

Zooprophylaxis is one of the possible environmental vector control strategies for malaria prevention. However, its effect on reducing malaria transmission has been questionable, requiring a detailed understanding of contextual factors. This study aims to evaluate the effect of keeping livestock on malaria incidence in south-central Ethiopia. A cohort of 34,548 people in a total of 6,071 households was followed for 121 weeks from October 2014 to January 2017. Baseline data were collected, including livestock ownership. Weekly home visits were done to actively search for malaria cases, and passive case detection was also carried out. Malaria was diagnosed with rapid diagnostic tests. Log binomial and parametric regression survival-time models were used to estimate effect measures. A total of 27,471 residents had complete follow-ups, and the majority (87.5%) lived in households owning livestock, including cattle, sheep, goats, and chickens. The overall incidence risk of malaria was 3.7%, and there was a 24% reduction in the risk of malaria among livestock owners. The total cohort contributed to 71,861.62 person-years of observation. The incidence rate of malaria was 14.7 cases per 1,000 person-years. There was a 17% reduction in the rate of malaria among livestock owners. Meanwhile, the protective effect of livestock ownership increased as the number of livestock or the livestock-to-human ratio increased. In conclusion, livestock owners had less malaria. In a setup where domestication of livestock is a common practice and the predominant malaria vector tends to feed more on livestock than humans, zooprophylaxis remains a promising strategy for malaria prevention.

Host Feeding Patterns of Mansonia (Diptera, Culicidae) in Rural Settlements near Porto Velho, State of Rondonia, Brazil

Mosquito females of the genus Mansonia (Blanchard) can be a nuisance to humans and animals since they are voraciously hematophagous and feed on the blood of a variety of vertebrates. Despite their relevance, there is a lack of investigation into the blood-feeding patterns of the Mansonia species. Knowledge of the host preference is crucial in establishing the public health importance of a mosquito species and its potential to be involved in the transmission dynamics of pathogens. Species that are primarily anthropophilic can be more effective in spreading vector-borne pathogens to humans. In this study, we used an Illumina Nextera sequencing protocol and the QIIME2 workflow to assess the diversity of DNA sequences extracted in the ingested blood of mosquito species to evaluate the overall and local host choices for three species: Ma. titillans, Ma. Amazonensis, and Ma. humeralis, in rural areas alongside the Madeira River in the vicinities of the Santo Antonio Energia (SAE) reservoir in the municipality of Porto Velho, Rondônia, Western Brazil. By performing our analysis pipeline, we have found that host diversity per collection site showed a significant heterogeneity across the sample sites. In addition, in rural areas, Ma. amazonensis present a high affinity for B. taurus, Ma. humeralis shows an overall preference for C. familiaris and B. taurus, but also H. sapiens and E. caballus in urban areas, and Ma. titillans showed more opportunistic behavior in rural areas, feeding on wild animals and G. gallus, though with an overall preference for H. sapiens.

The interplay between malaria vectors and human activity accounts for high residual malaria transmission in a Burkina Faso village with universal ITN coverage

BACKGROUND

Mosquito and human behaviour interaction is a key determinant of the maximum level of protection against malaria that can be provided by insecticide-treated nets (ITNs). Nevertheless, scant literature focuses on this interaction, overlooking a fundamental factor for efficient malaria control. This study aims to estimate malaria transmission risk in a Burkina Faso village by integrating vector biting rhythms with some key information about human habits.

METHODS

Indoor/outdoor human landing catches were conducted for 16 h (16:00-08:00) during 8 nights (September 2020) in Goden village. A survey about net usage and sleeping patterns was submitted to half the households (October-December 2020). A subsample of collected specimens of Anopheles gambiae sensu lato was molecularly processed for species identification, Plasmodium detection from heads-thoraxes and L1014F pyrethroid-resistance allele genotyping. Hourly mosquito abundance was statistically assessed by GLM/GAM, and the entomological inoculation rate (EIR) was corrected for the actual ITN usage retrieved from the questionnaire.

RESULTS

Malaria transmission was mainly driven by Anopheles coluzzii (68.7%) followed by A. arabiensis (26.2%). The overall sporozoite rate was 2% with L1014F estimated frequency of 0.68 (N = 1070 out of 15,201 A. gambiae s.l. collected). No major shift in mosquito biting rhythms in response to ITN or differences between indoor and outdoor catches were detected. Impressive high biting pressure (mean 30.3 mosquitoes/person/hour) was exerted from 20:00 to 06:00 with a peak at 4:00. Human survey revealed that nearly all inhabitants were awake before 20:00 and after 7:00 and at least 8.7% had no access to bednets. Adjusting for anthropological data, the EIR dropped from 6.7 to 1.2 infective bites/person/16 h. In a scenario of full net coverage and accounting only for the human sleeping patterns, the daily malaria transmission risk not targetable by ITNs was 0.69 infective bites.

CONCLUSIONS

The high mosquito densities and interplay between human/vector activities means that an estimated 10% of residual malaria transmission cannot be prevented by ITNs in the village. Locally tailored studies, like the current one, are essential to explore the heterogeneity of human exposure to infective bites and, consequently, to instruct the adoption of new vector control tools strengthening individual and community protection.

Residual Malaria Transmission: magnitude and drivers of persistent Plasmodium infections despite high coverage of control interventions in Burkina Faso, West Africa

This study collected baseline data on malaria vectors to characterize the drivers and the factors of persistent malaria transmission in two villages in the western part of Burkina Faso. Mosquitoes were collected in each village using the Human landing catch and pyrethrum spray catch and identified using the morphological keys. Molecular analyses were performed for the identification of An. gambiae complex species, the detection of Plasmodium infection and kdr-995F mutation. Anopheles mosquito larvae were also collected in the same villages, reared to adult's stage for the WHO tube and cone tests performing. The physical integrity of the LLINs already used by people in each village was assessed using the proportional hole index (pHI). An. gambiae s.l. was the main malaria vector accounting for 79.82% (5560/6965) of all collected mosquitoes. The biting pattern of An. gambiae s.l. was almost constant during the survey with an early aggressiveness before 8 p.m. and later biting activity after 6 a.m. The EIR varied from 0.13 to 2.55 infected bites per human per night (average: 1.03 infected bites per human per night). An. gambiae s.l. populations were full susceptible to Chlorpyrifos-methyl (0.4%) and Malathion (5%) with high kdr-995F mutation frequencies (>0.8). The physical integrity assessment showed high proportion of good nets in Santidougou compared to those collected in Kimidougou. This study highlighted a persistence of malaria transmission despite the intense use of vector control tools as LLINs and IRS by correlating mosquito biting time and human behavior. It provided a baseline guide for the monitoring of the residual malaria transmission in sub-Saharan Africa and encouraging the development of new alternative strategies to support the current malaria control tools.

Seasonal variation in abundance and blood meal sources of primary and secondary malaria vectors within Kilombero Valley, Southern Tanzania

BACKGROUND

Existing control tools have significantly reduced malaria over the past two decades. However, progress has been stalled due to increased resistance in primary vectors and the increasing role of secondary vectors. This study aimed to investigate the impact of seasonal change on primary and secondary vector abundance and host preference. Understanding the impact of seasonal dynamics of primary and secondary vectors on disease transmission will inform effective strategies for vector management and control.

METHODS

Vector abundance was measured through longitudinal collection of mosquitoes, conducted monthly during the wet and dry seasons, in Sagamaganga, a village in the Kilombero Valley, Tanzania. Mosquitoes were collected indoors using CDC light traps and backpack aspirators, and outdoors using resting buckets baited with cattle urine. In addition, a direct measure of host preference was taken monthly using human- and cattle-baited mosquito electrocuting traps. A host census was conducted to provide an indirect measure of host preference together with monthly blood meal source analysis. All collected mosquitoes were assayed for Plasmodium sporozoites.

RESULTS

A total of 2828 anophelines were collected, of which 78.5% and 21.4%, were primary and secondary vectors, respectively. The abundance of the primary vectors, Anopheles arabiensis and Anopheles funestus, and of the secondary vectors varied seasonally. Indirect measures of host preference indicated that all vectors varied blood meal choice seasonally, with the direct measure confirming this for An. arabiensis. All anopheline mosquitoes tested negative for sporozoites.

CONCLUSIONS

At the study location, the abundance of both primary and secondary vectors changed seasonally. Indirect and direct measures of host preference demonstrated that An. arabiensis varied from being zoophilic to being more opportunistic during the wet and dry seasons. A similar trend was observed for the other vectors.

Natural sugar feeding rates of Anopheles mosquitoes collected by different methods in western Kenya

Attractive targeted sugar baits (ATSBs) are a potential vector control tool that exploits the sugar-feeding behaviour of mosquitoes. We evaluated the sugar-feeding behaviour of Anopheles mosquitoes as part of baseline studies for cluster randomised controlled trials of ATSBs. Mosquitoes were collected indoors and outdoors from two villages in western Kenya using prokopack aspirations, malaise tent traps and ultraviolet (UV) light traps. Individual mosquitoes were subjected to the cold anthrone test to assess the presence of sugar. Overall, 15.7% of collected mosquitoes had fed on natural sugar sources. By species and sex, the proportion sugar-fed was 41.3% and 27.7% in male and female Anopheles funestus, 27.2% and 12.8% in male and female An. arabiensis, and 9.7% and 8.3% in male and female An. coustani, respectively. Sugar-feeding was higher in unfed than blood-fed mosquitoes and higher in male than gravid mosquitoes. Anopheles mosquitoes obtained sugar meals from natural sources during all physiological stages, whether they rest indoors or outdoors. These findings offer a potential avenue to exploit for the control of mosquitoes, particularly with the advent of ATSBs, which have been shown to reduce mosquito densities in other regions.

Molecular screening reveals non-uniform malaria transmission in western Kenya and absence of Rickettsia africae and selected arboviruses in hospital patients

Background

In sub-Saharan Africa, malaria is the common diagnosis for febrile illness and related clinical features, resulting in the under-diagnosis of other aetiologies, such as arboviruses and Rickettsia. While these may not be significant causes of mortality in malaria-endemic areas, they affect the daily life and performance of affected individuals. It is, therefore, important to have a clear picture of these other aetiologies to institute correct diagnoses at hospitals and improve patient outcomes.

Methods

Blood samples were collected from patients with fever and other clinical features associated with febrile illness at selected hospitals in the malaria-endemic counties of Busia, Bungoma, and Kakamega, and screened for Crimean-Congo haemorrhagic fever, Sindbis, dengue and chikungunya viruses, Rickettsia africae, and Plasmodium spp. using high-throughput real-time PCR techniques. A logistic regression was performed on the results to explore the effect of demographic and socio-economic independent variables on malaria infection.

Results

A total of 336 blood samples collected from hospital patients between January 2018 and February 2019 were screened, of which 17.6% (59/336) were positive for Plasmodium falciparum and 1.5% (5/336) for Plasmodium malariae. Two patients had dual P. falciparum/P. malariae infections. The most common clinical features reported by the patients who tested positive for malaria were fever and headache. None of the patients were positive for the arboviruses of interest or R. africae. Patients living in Busia (OR 5.2; 95% CI 2.46–11.79; p < 0.001) and Bungoma counties (OR 2.7; 95% CI 1.27–6.16; p = 0.013) had higher odds of being infected with malaria, compared to those living in Kakamega County.

Conclusions

The reported malaria prevalence is in line with previous studies. The absence of arboviral and R. africae cases in this study may have been due to the limited number of samples screened, low-level circulation of arboviruses during inter-epidemic periods, and/or the use of PCR alone as a detection method. Other sero-surveys confirming their circulation in the area indicate that further investigations are warranted.

Hotspots and super-spreaders: Modelling fine-scale malaria parasite transmission using mosquito flight behaviour

Malaria hotspots have been the focus of public health managers for several years due to the potential elimination gains that can be obtained from targeting them. The identification of hotspots must be accompanied by the description of the overall network of stable and unstable hotspots of malaria, especially in medium and low transmission settings where malaria elimination is targeted. Targeting hotspots with malaria control interventions has, so far, not produced expected benefits. In this work we have employed a mechanistic-stochastic algorithm to identify clusters of super-spreader houses and their related stable hotspots by accounting for mosquito flight capabilities and the spatial configuration of malaria infections at the house level. Our results show that the number of super-spreading houses and hotspots is dependent on the spatial configuration of the villages. In addition, super-spreaders are also associated to house characteristics such as livestock and family composition. We found that most of the transmission is associated with winds between 6pm and 10pm although later hours are also important. Mixed mosquito flight (downwind and upwind both with random components) were the most likely movements causing the spread of malaria in two out of the three study areas. Finally, our algorithm (named MALSWOTS) provided an estimate of the speed of malaria infection progression from house to house which was around 200-400 meters per day, a figure coherent with mark-release-recapture studies of Anopheles dispersion. Cross validation using an out-of-sample procedure showed accurate identification of hotspots. Our findings provide a significant contribution towards the identification and development of optimal tools for efficient and effective spatio-temporal targeted malaria interventions over potential hotspot areas.

Using ecological observations to improve malaria control in areas where Anopheles funestus is the dominant vector

The most important malaria vectors in sub-Saharan Africa are Anopheles gambiae, Anopheles arabiensis, Anopheles funestus, and Anopheles coluzzii. Of these, An. funestus presently dominates in many settings in east and southern Africa. While research on this vector species has been impeded by difficulties in creating laboratory colonies, available evidence suggests it has certain ecological vulnerabilities that could be strategically exploited to greatly reduce malaria transmission in areas where it dominates. This paper examines the major life-history traits of An. funestus, its aquatic and adult ecologies, and its responsiveness to key interventions. It then outlines a plausible strategy for reducing malaria transmission by the vector and sustaining the gains over the medium to long term. To illustrate the propositions, the article uses data from south-eastern Tanzania where An. funestus mediates over 85% of malaria transmission events and is highly resistant to key public health insecticides, notably pyrethroids. Both male and female An. funestus rest indoors and the females frequently feed on humans indoors, although moderate to high degrees of zoophagy can occur in areas with large livestock populations. There are also a few reports of outdoor-biting by the species, highlighting a broader range of behavioural phenotypes that can be considered when designing new interventions to improve vector control. In comparison to other African malaria vectors, An. funestus distinctively prefers permanent and semi-permanent aquatic habitats, including river streams, ponds, swamps, and spring-fed pools. The species is therefore well-adapted to sustain its populations even during dry months and can support year-round malaria transmission. These ecological features suggest that highly effective control of An. funestus could be achieved primarily through strategic combinations of species-targeted larval source management and high quality insecticide-based methods targeting adult mosquitoes in shelters. If done consistently, such an integrated strategy has the potential to drastically reduce local populations of An. funestus and significantly reduce malaria transmission in areas where this vector species dominates. To sustain the gains, the programmes should be complemented with gradual environmental improvements such as house modification to maintain biting exposure at a bare minimum, as well as continuous engagements of the resident communities and other stakeholders.

Modelling spatiotemporal trends in the frequency of genetic mutations conferring insecticide target-site resistance in African mosquito malaria vector species

BACKGROUND

Resistance in malaria vectors to pyrethroids, the most widely used class of insecticides for malaria vector control, threatens the continued efficacy of vector control tools. Target-site resistance is an important genetic resistance mechanism caused by mutations in the voltage-gated sodium channel (Vgsc) gene that encodes the pyrethroid target-site. Understanding the geographic distribution of target-site resistance, and temporal trends across different vector species, can inform strategic deployment of vector control tools.

RESULTS

We develop a Bayesian statistical spatiotemporal model to interpret species-specific trends in the frequency of the most common resistance mutations, Vgsc-995S and Vgsc-995F, in three major malaria vector species Anopheles gambiae, An. coluzzii, and An. arabiensis over the period 2005-2017. The models are informed by 2418 observations of the frequency of each mutation in field sampled mosquitoes collected from 27 countries spanning western and eastern regions of Africa. For nine selected countries, we develop annual predictive maps which reveal geographically structured patterns of spread of each mutation at regional and continental scales. The results show associations, as well as stark differences, in spread dynamics of the two mutations across the three vector species. The coverage of ITNs was an influential predictor of Vgsc allele frequencies, with modelled relationships between ITN coverage and allele frequencies varying across species and geographic regions. We found that our mapped Vgsc allele frequencies are a significant partial predictor of phenotypic resistance to the pyrethroid deltamethrin in An. gambiae complex populations.

CONCLUSIONS

Our predictive maps show how spatiotemporal trends in insecticide target-site resistance mechanisms in African An. gambiae vary across individual vector species and geographic regions. Molecular surveillance of resistance mechanisms will help to predict resistance phenotypes and track their spread.

Genetic markers associated with insecticide resistance and resting behaviour in Anopheles gambiae mosquitoes in selected sites in Kenya

Background

Molecular diagnostic tools have been incorporated in insecticide resistance monitoring programmes to identify underlying genetic basis of resistance and develop early warning systems of vector control failure. Identifying genetic markers of insecticide resistance is crucial in enhancing the ability to mitigate potential effects of resistance. The knockdown resistance (kdr) mutation associated with resistance to DDT and pyrethroids, the acetylcholinesterase-1 (ace-1^R) mutation associated with resistance to organophosphates and carbamates and 2La chromosomal inversion associated with indoor resting behaviour, were investigated in the present study.

Methods

Anopheles mosquitoes sampled from different sites in Kenya and collected within the context of malaria vector surveillance were analysed. Mosquitoes were collected indoors using light traps, pyrethrum spray and hand catches between August 2016 and November 2017. Mosquitoes were identified using morphological keys and Anopheles gambiae sensu lato (s.l.) mosquitoes further identified into sibling species by the polymerase chain reaction method following DNA extraction by alcohol precipitation. Anopheles gambiae and Anopheles arabiensis were analysed for the presence of the kdr and ace-1^R mutations, while 2La inversion was only screened for in An. gambiae where it is polymorphic. Chi-square statistics were used to determine correlation between the 2La inversion karyotype and kdr-east mutation.

Results

The kdr-east mutation occurred at frequencies ranging from 0.5 to 65.6% between sites. The kdr-west mutation was only found in Migori at a total frequency of 5.3% (n = 124). No kdr mutants were detected in Tana River. The ace-1^R mutation was absent in all populations. The 2La chromosomal inversion screened in An. gambiae occurred at frequencies of 87% (n = 30), 80% (n = 10) and 52% (n = 50) in Baringo, Tana River and Migori, respectively. A significant association between the 2La chromosomal inversion and the kdr-east mutation was found.

Conclusion

The significant association between the 2La inversion karyotype and kdr-east mutation suggests that pyrethroid resistant An. gambiae continue to rest indoors regardless of the presence of treated bed nets and residual sprays, a persistence further substantiated by studies documenting continued mosquito abundance indoors. Behavioural resistance by which Anopheles vectors prefer not to rest indoors may, therefore, not be a factor of concern in this study’s malaria vector populations.

The Distribution and Composition of Vector Abundance in Hanoi City, Vietnam: Association with Livestock Keeping and Flavivirus Detection

BACKGROUND

Dengue virus and Japanese encephalitis virus are two common flaviviruses that are spread widely by Aedes and Culex mosquitoes. Livestock keeping is vital for cities; however, it can pose the risk of increasing the mosquito population. Our study explored how livestock keeping in and around a large city is associated with the presence of mosquitoes and the risk of them spreading flaviviruses.

METHODS

An entomological study was conducted in 6 districts with 233 households with livestock, and 280 households without livestock, in Hanoi city. BG-Sentinel traps and CDC light traps were used to collect mosquitoes close to animal farms and human habitats. Adult mosquitoes were counted, identified to species level, and grouped into 385 pools, which were screened for flaviviruses using a pan-flavivirus qPCR protocol and sequencing.

RESULTS

A total of 12,861 adult mosquitoes were collected at the 513 households, with 5 different genera collected, of which the Culex genus was the most abundant. Our study found that there was a positive association between livestock keeping and the size of the mosquito population-most predominantly between pig rearing and Culex species (p < 0.001). One pool of Cx. tritaeniorhynchus, collected in a peri-urban district, was found to be positive for Japanese encephalitis virus.

CONCLUSIONS

The risk of flavivirus transmission in urban areas of Hanoi city due to the spread of Culex and Aedes mosquitoes could be facilitated by livestock keeping.

Malaria at Forest Areas in South Kalimantan, Indonesia: Risk Factors and Strategies for Elimination

BACKGROUND: South Kalimantan is one of province in Indonesia which has endemic area, mainly in the villages at forest area. Understanding the risk factors which can increase the risk of malaria in individuals at forest area will enable more effective use for controlling the disease. The identification of risk factors will provide information about local malaria epidemiology and usefull for making appropriate and effective malaria eradication program policies in this area. AIM: To know the risk factors of malaria prevalence in endemic forest areas in South Kalimantan, Indonesia. METHODS: This cross-sectional study was conducted on 107 adult people who lived in Batu Bulan Village and Batu Paha Village, South Kalimantan. Blood samples for malaria microscopy and rapid diagnostic test is taken from cubital vein. Household factors and demographic data were obtained. Chi-square and logistic regression were performed to analyze the factors associated with malaria prevalence in South Kalimantan. This research didn’t do vector survey, only on the prevalence of malaria and risk factor in human and environment. RESULTS: The prevalence of malaria based RDT examination was 35.5% with 23.68% Plasmodium falciparum, 21.05% Plasmodium vivax, and 55.27% mixed infection. The prevalence malaria based on microscopic examination was 17.75% with 47.36% P. falciparum, 26.32% P. vivax, and 26.32% mix infection. Demographic factors influencing the prevalence of malaria were aged below 25-years-old (p = 0.01, 95% CI, OR = 2.289), villages in Batu Paha (p = 0.048, 95% CI, OR = 3.55), and occupation as a forest worker (p = 0.022, 95% CI, OR = 6.38). House factors that influence the prevalence of malaria were the condition of the walls that are open or not tight (p = 0.048 95% CI, OR = 5.205), the roof is made of plastic (p = 0.015 95% CI, OR = 2.831), and the presence of animal cage around the house (p = 0.015 95% CI, OR = 6.292). CONCLUSIONS: Malaria incidence remains occurs with high prevalence in the pupolation in remote forest areas.

Impact of cattle on the abundance of indoor and outdoor resting malaria vectors in southern Malawi

BACKGROUND

Understanding the blood feeding preferences and resting habits of malaria vectors is important for assessing and designing effective malaria vector control tools. The presence of livestock, such as cattle, which are used as blood meal hosts by some malaria vectors, may impact malaria parasite transmission dynamics. The presence of livestock may provide sufficient blood meals for the vectors, thereby reducing the frequency of vectors biting humans. Alternatively, the presence of cattle may enhance the availability of blood meals such that infectious mosquitoes may survive longer, thereby increasing the risk of malaria transmission. This study assessed the effect of household-level cattle presence and distribution on the abundance of indoor and outdoor resting malaria vectors.

METHODS

Houses with and without cattle were selected in Chikwawa district, southern Malawi for sampling resting malaria vectors. Prokopack aspirators and clay pots were used for indoor and outdoor sampling, respectively. Each house was sampled over two consecutive days. For houses with cattle nearby, the number of cattle and the distances from the house to where the cattle were corralled the previous night were recorded. All data were analysed using generalized linear models fitted with Poisson distribution.

RESULTS

The malaria vectors caught resting indoors were Anopheles gambiae sensu stricto (s.s.), Anopheles arabiensis and Anopheles funestus s.s. Outdoor collections consisted primarily of An. arabiensis. The catch sizes of indoor resting An. gambiae sensu lato (s.l.) were not different in houses with and without cattle (P = 0.34). The presence of cattle near a house was associated with a reduction in the abundance of indoor resting An. funestus s.l. (P = 0.04). This effect was strongest when cattle were kept overnight ≤ 15 m away from the houses (P = 0.03). The blood meal hosts varied across the species.

CONCLUSION

These results highlight differences between malaria vector species and their interactions with potential blood meal hosts, which may have implications for malaria risk. Whereas An. arabiensis remained unaffected, the reduction of An. funestus s.s. in houses near cattle suggests a potential protective effect of cattle. However, the low abundance of mosquitoes reduced the power of some analyses and limited the generalizability of the results to other settings. Therefore, further studies incorporating the vectors' host-seeking behaviour/human biting rates are recommended to fully support the primary finding.

Key Characteristics of Residual Malaria Transmission in Two Districts in South-Eastern Tanzania-Implications for Improved Control

After 2 decades of using insecticide-treated nets (ITNs) and improved case management, malaria burden in the historically-holoendemic Kilombero valley in Tanzania has significantly declined. We review key characteristics of the residual transmission and recommend options for improvement. Transmission has declined by >10-fold since 2000 but remains heterogeneous over small distances. Following the crash of Anopheles gambiae, which coincided with ITN scale-up around 2005-2012, Anopheles funestus now dominates malaria transmission. While most infections still occur indoors, substantial biting happens outdoors and before bed-time. There is widespread resistance to pyrethroids and carbamates; An. funestus being particularly strongly-resistant. In short and medium-term, these challenges could be addressed using high-quality indoor residual spraying with nonpyrethroids, or ITNs incorporating synergists. Supplementary tools, eg, spatial-repellents may expand protection outdoors. However, sustainable control requires resilience-building approaches, particularly improved housing and larval-source management to suppress mosquitoes, stronger health systems guaranteeing case-detection and treatment, greater community-engagement and expanded health education.

Residual Malaria: Limitations of Current Vector Control Strategies to Eliminate Transmission in Residual Foci

The transmission of Plasmodium parasites in residual foci is currently a major roadblock for malaria elimination. Human activities and behavior, along with outdoor biting mosquitoes with opportunistic feeding preferences are the main causes of the inefficacy of the main vector control interventions, long lasting insecticide-impregnated nets and insecticide residual spraying. Several strategies to abate or repel outdoor biting mosquito vectors are currently being researched, but the impact of insecticide resistance on the efficacy of these and current indoor-applied insecticides requires further assessment. Understanding the human, ecological and vector factors, determining transmission in residual foci is necessary for the design and implementation of novel control strategies. Vector control alone is insufficient without adequate epidemiological surveillance and prompt treatment of malaria cases, the participation of endemic communities in prevention and control is required. In addition, malaria control programs should optimize their structure and organization, and their coordination with other government sectors.

Bloodmeal sources and feeding behavior of anopheline mosquitoes in Bure district, northwestern Ethiopia

BACKGROUND

Mosquito bloodmeal sources determine the feeding rates, adult survival, fecundity, hatching rates, and developmental times. Only the female Anopheles mosquito takes bloodmeals from humans, birds, mammals, and other vertebrates for egg development. Studies of the host preference patterns in blood-feeding anopheline mosquitoes are crucial to determine malaria vectors. However, the human blood index, foraging ratio, and host preference index of anopheline mosquitoes are not known so far in Bure district, Ethiopia.

METHODS

The origins of bloodmeals from all freshly fed and a few half-gravid exophagic and endophagic females collected using Centers for Disease Control and Prevention light traps were identified as human and bovine using enzyme-linked immunosorbent assay. The human blood index, forage ratio, and host feeding index were calculated.

RESULTS

A total of 617 specimens belonging to An. arabiensis (n = 209), An. funestus (n = 217), An. coustani (n = 123), An. squamosus (n = 54), and An. cinereus (n = 14) were only analyzed using blood ELISA. Five hundred seventy-five of the specimens were positive for blood antigens of the host bloods. All anopheline mosquitoes assayed for a bloodmeal source had mixed- rather than single-source bloodmeals. The FR for humans was slightly > 1.0 compared to bovines for all Anopheles species. HFI for each pair of vertebrate hosts revealed that humans were the slightly preferred bloodmeal source compared to bovines for all species (except An. squamosus), but there was no marked host selection.

CONCLUSIONS

All anopheline mosquitoes assayed for bloodmeal ELISA had mixed feeds, which tends to diminish the density of gametocytes in the mosquito stomach, thereby reducing the chance of fertilization of the female gamete and reducing the chances of a malaria vector becoming infected. Moreover, An. coustani was the only species that had only human bloodmeals, meaning that this species has the potential to transmit the disease. Therefore, combination zooprophylaxis should be reinforced as a means of vector control because the study sites are mixed dwellings.

Anopheles arabiensis hotspots along intermittent rivers drive malaria dynamics in semi-arid areas of Central Ethiopia

Background

Understanding malaria vector’s population dynamics and their spatial distribution is important to define when and where the largest infection risks occur and implement appropriate control strategies. In this study, the seasonal spatio-temporal dynamics of the malaria vector population and transmission intensity along intermittent rivers in a semi-arid area of central Ethiopia were investigated.

Methods

Mosquitoes were collected monthly from five clusters, 2 close to a river and 3 away from a river, using pyrethrum spray catches from November 2014 to July 2016. Mosquito abundance was analysed by the mixed Poisson regression model. The human blood index and sporozoite rate was compared between seasons by a logistic regression model.

Results

A total of 2784 adult female Anopheles gambiae sensu lato (s.l.) were collected during the data collection period. All tested mosquitoes (n = 696) were identified as Anopheles arabiensis by polymerase chain reaction. The average daily household count was significantly higher (P = 0.037) in the clusters close to the river at 5.35 (95% CI 2.41–11.85) compared to the clusters away from the river at 0.033 (95% CI 0.02–0.05). Comparing the effect of vicinity of the river by season, a significant effect of closeness to the river was found during the dry season (P = 0.027) and transition from dry to wet season (P = 0.032). Overall, An. arabiensis had higher bovine blood index (62.8%) as compared to human blood index (23.8%), ovine blood index (9.2%) and canine blood index (0.1%). The overall sporozoite rate was 3.9% and 0% for clusters close to and away from the river, respectively. The overall Plasmodium falciparum and Plasmodium vivax entomologic inoculation rates for An. arabiensis in clusters close to the river were 0.8 and 2.2 infective bites per person/year, respectively.

Conclusion

Mosquito abundance and malaria transmission intensity in clusters close to the river were higher which could be attributed to the riverine breeding sites. Thus, vector control interventions including targeted larval source management should be implemented to reduce the risk of malaria infection in the area.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03697-z.

Insecticide resistance status of indoor and outdoor resting malaria vectors in a highland and lowland site in Western Kenya

BACKGROUND

Long Lasting Insecticidal Nets (LLINs) and indoor residual spraying (IRS) represent powerful tools for controlling malaria vectors in sub-Saharan Africa. The success of these interventions relies on their capability to inhibit indoor feeding and resting of malaria mosquitoes. This study sought to understand the interaction of insecticide resistance with indoor and outdoor resting behavioral responses of malaria vectors from Western Kenya.

METHODS

The status of insecticide resistance among indoor and outdoor resting anopheline mosquitoes was compared in Anopheles mosquitoes collected from Kisumu and Bungoma counties in Western Kenya. The level and intensity of resistance were measured using WHO-tube and CDC-bottle bioassays, respectively. The synergist piperonyl butoxide (PBO) was used to determine if metabolic activity (monooxygenase enzymes) explained the resistance observed. The mutations at the voltage-gated sodium channel (Vgsc) gene and Ace 1 gene were characterized using PCR methods. Microplate assays were used to measure levels of detoxification enzymes if present.

RESULTS

A total of 1094 samples were discriminated within Anopheles gambiae s.l. and 289 within An. funestus s.l. In Kisian (Kisumu county), the dominant species was Anopheles arabiensis 75.2% (391/520) while in Kimaeti (Bungoma county) collections the dominant sibling species was Anopheles gambiae s.s 96.5% (554/574). The An. funestus s.l samples analysed were all An. funestus s.s from both sites. Pyrethroid resistance of An.gambiae s.l F1 progeny was observed in all sites. Lower mortality was observed against deltamethrin for the progeny of indoor resting mosquitoes compared to outdoor resting mosquitoes (Mortality rate: 37% vs 51%, P = 0.044). The intensity assays showed moderate-intensity resistance to deltamethrin in the progeny of mosquitoes collected from indoors and outdoors in both study sites. In Kisian, the frequency of vgsc-L1014S and vgsc-L1014F mutation was 0.14 and 0.19 respectively in indoor resting malaria mosquitoes while those of the outdoor resting mosquitoes were 0.12 and 0.12 respectively. The ace 1 mutation was present in higher frequency in the F1 of mosquitoes resting indoors (0.23) compared to those of mosquitoes resting outdoors (0.12). In Kimaeti, the frequencies of vgsc-L1014S and vgsc-L1014F were 0.75 and 0.05 respectively for the F1 of mosquitoes collected indoors whereas those of outdoor resting ones were 0.67 and 0.03 respectively. The ace 1 G119S mutation was present in progeny of mosquitoes from Kimaeti resting indoors (0.05) whereas it was absent in those resting outdoors. Monooxygenase activity was elevated by 1.83 folds in Kisian and by 1.33 folds in Kimaeti for mosquitoes resting indoors than those resting outdoors respectively.

CONCLUSION

The study recorded high phenotypic, metabolic and genotypic insecticide resistance in indoor resting populations of malaria vectors compared to their outdoor resting counterparts. The indication of moderate resistance intensity for the indoor resting mosquitoes is alarming as it could have an operational impact on the efficacy of the existing pyrethroid based vector control tools. The use of synergist (PBO) in LLINs may be a better alternative for widespread use in these regions recording high insecticide resistance.

Predictors of malaria rapid diagnostic test positivity in a high burden area of Paletwa Township, Chin State in Western Myanmar

Background

Despite major reductions in malaria burden across Myanmar, clusters of the disease continue to persist in specific subregions. This study aimed to assess the predictors of test positivity among people living in Paletwa Township of Chin State, an area of persistently high malaria burden.

Methods

Four villages with the highest malaria incidence from Paletwa Township were purposively selected. The characteristics of 1045 subjects seeking malaria diagnosis from the four assigned village health volunteers from January to December, 2018 were retrospectively analyzed. Their household conditions and surroundings were also recorded using a checklist. Descriptive statistics and logistic regression models were applied to investigate potential associations between individual and household characteristics and malaria diagnosis.

Results

In 2017, the Paletwa township presented 20.9% positivity and an annual parasite index of 46.9 cases per 1000 people. Plasmodium falciparum was the predominant species and accounted for more than 80.0% of all infections. Among 1045 people presenting at a clinic with malaria symptoms, 31.1% were diagnosed with malaria. Predictors for test positivity included living in a hut [adjusted odds ratios (a OR): 2.3, 95% confidence intervals (CI): 1.2–4.6], owning farm animals (aOR: 1.7, 95% CI: 1.1–3.6), using non-septic type of toilets (aOR: 1.9, 95% CI: 1.1–8.4), presenting with fever (aOR: 1.9, 95% CI: 1.1–3.0), having a malaria episode within the last year (aOR: 2.9, 95% CI: 1.4–5.8), traveling outside the village in the previous 14 days (aOR: 4.5, 95% CI: 1.5–13.4), and not using bed nets (a OR: 3.4, 95% CI: 2.3–5.1). There were no statistically significant differences by age or gender in this present analysis.

Conclusions

The results from this study, including a high proportion of P. falciparum infections, little difference in age, sex, or occupation, suggest that malaria is a major burden for these study villages. Targeted health education campaigns should be introduced to strengthen synchronous diagnosis-seeking behaviors, tighten treatment adherence, receiving a diagnosis after traveling to endemic regions, and using bed nets properly. We suggest increased surveillance, early diagnosis, and treatment efforts to control the disease and then to consider the local elimination.

Socio-economic determinants of malaria in tribal dominated Mandla district enrolled in Malaria Elimination Demonstration Project in Madhya Pradesh

Background

Malaria is known as a disease of poverty because of its dominance in poverty-stricken areas. Madhya Pradesh state in central India is one of the most vulnerable states for malaria morbidity and mortality. Socio-economic, environmental and demographic factors present challenges in malaria control and elimination. As part of the Malaria Elimination Demonstration Project in the tribal district of Mandla in Madhya Pradesh, this study was undertaken to assess the role of different social-economic factors contributing to malaria incidence.

Methods

The study was conducted in the 1233 villages of district Mandla, where 87% population resides in rural areas. The data was collected using the android based mobile application—SOCH for a period of 2 years (September 2017 to August 2019). A wealth index was computed along with analysis of the socio-economic characteristics of houses with malaria cases. Variables with significant variation in malaria cases were used in logistic regression.

Results

More than 70% of houses in Mandla are Kuccha (made of thatched roof or mud), 20% do not have any toilet facilities, and only 11% had an annual income of more than 50,000 INR, which converts to about $700 per year. Households with younger heads, male heads, more number of family members were more likely to have malaria cases. Kuccha construction, improper water supply, low household income houses were also more likely to have a malaria case and the odds doubled in houses with no toilet facilities.

Conclusion

Based on the results of the study, it has been found that there is an association between the odds of having malaria cases and different household variables such as age, gender, number of members, number of rooms, caste, type of house, toilet facilities, water supply, cattle sheds, agricultural land, income, and vector control interventions. Therefore, a better understanding of the association of various risk factors that influence the incidence of malaria is required to design and/or deploy effective policies and strategies for malaria elimination. The results of this study suggest that appropriate economic and environmental interventions even in low-income and poverty-stricken tribal areas could have huge impact on the success of the national malaria elimination goals.

Variability in human attractiveness to mosquitoes

Blood-feeding mosquitoes locate humans spatially by detecting a combination of human-derived chemical signals, including carbon dioxide, lactic acid, and other volatile organic compounds. Mosquitoes use these signals to differentiate humans from other animals. Spatial abiotic factors (e.g. humidity, heat) are also used by mosquitoes to find a host. Mosquitoes cause discomfort and harm to humans, being vectors of many pathogens. However, not all humans suffer from mosquito bites with the same frequency or intensity. Some individuals are more attractive to mosquitoes than others, and this has an important impact on the risk of infection by pathogens transmitted by these vectors, such as arboviruses and malaria parasites. Variability in human attractiveness to mosquitoes is partially due to individual characteristics in the composition and intensity in the release of mosquito attractants. The factors that determine these particularities are diverse, modestly understood and still quite controversial. Thus, this review discusses the role of pregnancy, infection with malaria parasites (Plasmodium spp.), skin microbiota, diet, and genetics in human attractiveness to mosquitoes. In brief, pregnancy and Plasmodium infection increase the host attractiveness to mosquitoes. Skin microbiota and human genetics (especially HLA alleles) modulate the production of mosquito attractants and therefore influence individual susceptibility to these insects. There is evidence pointing to a role of diet on human susceptibility to mosquitoes, with some dietary components having a bigger influence than others. In the last part of the review, other factors affecting human-mosquito interactions are debated, with a special focus on the role of mosquito genetics, pathogens and environmental factors (e.g. wind, environmental disturbances). This work highlights that individual susceptibility to mosquitoes is composed of interactions of different human-associated components, environmental factors, and mosquito characteristics. Understanding the importance of these factors, and how they interact with each other, is essential for the development of better mosquito control strategies and studies focused on infectious disease dynamics.

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Individual human attractiveness to mosquitoes is highly variable.

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Mosquito attractants released into the air vary from person to person.

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Variation in attractiveness to mosquitoes alters the risk of mosquito-borne infections.

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Pregnancy, malaria infection, skin microbiota and genetic factors alter the release of mosquito attractants.

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Environment and mosquito-related factors affect human–mosquito interactions.

Emergence of behavioural avoidance strategies of malaria vectors in areas of high LLIN coverage in Tanzania

Despite significant reductions in malaria transmission across Africa since 2000, progress is stalling. This has been attributed to the development of insecticide resistance and behavioural adaptations in malaria vectors. Whilst insecticide resistance has been widely investigated, there is poorer understanding of the emergence, dynamics and impact of mosquito behavioural adaptations. We conducted a longitudinal investigation of malaria vector host choice over 3 years and resting behaviour over 4 years following a mass long-lasting insecticidal nets (LLINs) distribution in Tanzania. By pairing observations of mosquito ecology with environmental monitoring, we quantified longitudinal shifts in host-choice and resting behaviour that are consistent with adaptation to evade LLINs. The density of An. funestuss.l., declined significantly through time. In tandem, An. arabiensis and An. funestuss.l. exhibited an increased rate of outdoor relative to indoor resting; with An. arabiensis reducing the proportion of blood meals taken from humans in favour of cattle. By accounting for environmental variation, this study detected clear evidence of intra-specific shifts in mosquito behaviour that could be obscured in shorter-term or temporally-coarse surveys. This highlights the importance of mosquito behavioural adaptations to vector control, and the value of longer-term behavioural studies.

Insecticide resistance in indoor and outdoor-resting Anopheles gambiae in Northern Ghana

BACKGROUND

Selection pressure from continued exposure to insecticides drives development of insecticide resistance and changes in resting behaviour of malaria vectors. There is need to understand how resistance drives changes in resting behaviour within vector species. The association between insecticide resistance and resting behaviour of Anopheles gambiae sensu lato (s.l.) in Northern Ghana was examined.

METHODS

F1 progenies from adult mosquitoes collected indoors and outdoors were exposed to DDT, deltamethrin, malathion and bendiocarb using WHO insecticide susceptibility tests. Insecticide resistance markers including voltage-gated sodium channel (Vgsc)-1014F, Vgsc-1014S, Vgsc-1575Y, glutathione-S-transferase epsilon 2 (GSTe2)-114T and acetylcholinesterase (Ace1)-119S, as well as blood meal sources were investigated using PCR methods. Activities of metabolic enzymes, acetylcholine esterase (AChE), non-specific β-esterases, glutathione-S-transferase (GST) and monooxygenases were measured from unexposed F1 progenies using microplate assays.

RESULTS

Susceptibility of Anopheles coluzzii to deltamethrin 24 h post-exposure was significantly higher in indoor (mortality = 5%) than outdoor (mortality = 2.5%) populations (P = 0.02). Mosquitoes were fully susceptible to malathion (mortality: indoor = 98%, outdoor = 100%). Susceptibility to DDT was significantly higher in outdoor (mortality = 9%) than indoor (mortality = 0%) mosquitoes (P = 0.006). Mosquitoes were also found with suspected resistance to bendiocarb but mortality was not statistically different (mortality: indoor = 90%, outdoor = 95%. P = 0.30). Frequencies of all resistance alleles were higher in F1 outdoor (0.11-0.85) than indoor (0.04-0.65) mosquito populations, while Vgsc-1014F in F0 An. gambiae sensu stricto (s.s) was significantly associated with outdoor-resting behaviour (P = 0.01). Activities of non-specific β-esterase enzymes were significantly higher in outdoor than indoor mosquitoes (Mean enzyme activity: Outdoor = : 1.70/mg protein; Indoor = 1.35/mg protein. P < 0.0001). AChE activity was also more elevated in outdoor (0.62/mg protein) than indoor (0.57/mg protein) mosquitoes but this was not significant (P = 0.08). Human blood index (HBI) was predominantly detected in indoor (18%) than outdoor mosquito populations (3%).

CONCLUSIONS

The overall results did not establish that there was a significant preference of resistant malaria vectors to solely rest indoors or outdoors, but varied depending on the resistant alleles present. Phenotypic resistance was higher in indoor than outdoor-resting mosquitoes, but genotypic and metabolic resistance levels were higher in outdoor than the indoor populations. Continued monitoring of changes in resting behaviour within An. gambiae s.l. populations is recommended.

Prevalence and determinants of malaria infection among children of local farmers in Central Malawi

Background

Malaria is a leading cause of morbidity and mortality among children under 5 years in Malawi, and especially among those from rural areas of central Malawi. The goal of this study was to examine the prevalence and determinants of malaria infection among children in rural areas of Dowa district in central Malawi.

Methods

A multistage, cross-sectional study design was used to systematically sample 523 child-mother dyads from postnatal clinics. A survey was administered to mothers and a rapid malaria infection diagnostic test was administered to children. The main outcome was positive malaria diagnostic tests in children. Logistic regressions were used to determine risk factors associated with malaria among children aged 2 to 59 months.

Results

The prevalence of malaria among children under 5 years was 35.4%. Results suggest that children of mothers who experienced recent intimate partner violence (IPV) were more likely to be diagnosed with malaria (AOR: 1.88, 95% CI 1.19–2.97; P = 0.007) than children of mothers who did not. Children of mothers who had no formal education were more likely to be diagnosed with malaria (AOR: 2.77, 95% CI 1.24–6.19; P = 0.013) than children of mothers who had received secondary education. Children aged 2 to 5 months and 6 to 11 months were less likely to be diagnosed with malaria (AOR: 0.21, 95% CI 0.10–0.46; P = 0.000 and AOR: 0.43; 95% CI 0.22–0.85; P = 0.016, respectively) than children aged 24 to 59 months.

Conclusion

The prevalence of malaria infection among children in the study area was comparable to the national level. In addition to available malaria control programmes, further attention should be paid to children whose mothers have no formal education, children aged 24 to 59 months, and children of mothers that are exposed to IPV in the area.

Risk factors for malaria infection prevalence and household vector density between mass distribution campaigns of long-lasting insecticidal nets in North-western Tanzania

BACKGROUND

Long-lasting insecticidal nets (LLINs) are the most widely deployed vector control intervention in sub-Saharan Africa to prevent malaria. Recent reports indicate selection of pyrethroid insecticide resistance is widespread in mosquito vectors. This paper explores risk factors associated with malaria infection prevalence and vector density between mass distribution campaigns, changes in net coverage, and loss of protection in an area of high pyrethroid resistance in Northwest Tanzania.

METHODS

A cross sectional malaria survey of 3456 children was undertaken in 2014 in Muleba district, Kagera region west of Lake Victoria. Vector density was assessed using indoor light traps and outdoor tent traps. Anophelines were identified to species using PCR and tested for Plasmodium falciparum circumsporozoite protein. Logistic regression was used to identify household and environmental factors associated with malaria infection and regression binomial negative for vector density.

RESULTS

LLIN use was 27.7%. Only 16.9% of households had sufficient nets to cover all sleeping places. Malaria infection was independently associated with access to LLINs (OR: 0.57; 95% CI 0.34-0.98). LLINs less than 2 years old were slightly more protective than older LLINs (53 vs 65% prevalence of infection); however, there was no evidence that LLINs in good condition (hole index < 65) were more protective than LLINs, which were more holed. Other risk factors for malaria infection were age, group, altitude and house construction quality. Independent risk factors for vector density were consistent with malaria outcomes and included altitude, wind, livestock, house quality, open eaves and LLIN usage. Indoor collections comprised 4.6% Anopheles funestus and 95.4% Anopheles gambiae of which 4.5% were Anopheles arabiensis and 93.5% were Anopheles gambiae sensu stricto.

CONCLUSION

Three years after the mass distribution campaign and despite top-ups, LLIN usage had declined considerably. While children living in households with access to LLINs were at lower risk of malaria, infection prevalence remained high even among users of LLINs in good condition. While effort should be made to maintain high coverage between campaigns, distribution of standard pyrethroid-only LLINs appears insufficient to prevent malaria transmission in this area of intense pyrethroid resistance.

Passive Outdoor Host Seeking Device (POHD): Designing and Evaluation against Outdoor Biting Malaria Vectors

Odor-baited devices are increasingly needed to compliment long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) for control of residual malaria transmission. However, the odor-baited devices developed so far are bulky, dependent on the source of electricity and carbon dioxide (CO2), and they are logistically unsuitable for scaling up in surveillance and control of malaria vectors. We designed a passive and portable outdoor host seeking device (POHD) and preliminarily evaluated suitable components against Anopheles arabiensis that maintains residual malaria transmission. Experiments were conducted using semifield reared An. arabiensis within the semifield system at Ifakara Health Institute (IHI) in southeastern Tanzania. These mosquitoes were exposed to Suna traps® baited with BG lures or source of light and augmented with carbon dioxide (CO2) in view of identifying best attractants necessary to improve attractiveness of designed POHD. Two Suna traps® were hanged at the corner but outside the experimental hut in a diagonal line and rotated between four corners to control for the effect of position and wind direction on mosquito catches. Furthermore, mosquitoes were also exposed to either a bendiocarb-treated or bendiocarb-untreated POHD baited with Mbita blend, Ifakara blend, and worn socks and augmented with warmth (i.e., 1.5 liter bottle of warm water) inside an experimental hut or a screened rectangular box. This study demonstrated that mosquitoes were more strongly attracted to Suna trap® baited with BG lures and CO2 relative to those traps baited with a source of light and CO2. The POHD baited with synthetic blends attracted and killed greater proportion of An. arabiensis compared with POHD baited with worn socks. Efficacy of the POHD was unaffected by source of warmth, and it was reduced by about 50% when the device was tested inside a screened rectangular box relative to closed experimental hut. Overall, this study demonstrates that the POHD baited with synthetic blends (Mbita and Ifakara blends) and bendiocarb can effectively attract and kill outdoor biting malaria vector species. Such POHD baited with synthetic blends may require the source of CO2 to enhance attractiveness to mosquitoes. Further trials are, therefore, ongoing to evaluate attractiveness of improved design of POHD baited with slow-release formulation of synthetic blends and sustainable source of CO2 to malaria vectors under semifield and natural environments.

Improving the efficiency of reactive case detection for malaria elimination in southern Zambia: a cross-sectional study

Background

Reactive case detection (RCD) seeks to enhance malaria surveillance and control by identifying and treating parasitaemic individuals residing near index cases. In Zambia, this strategy starts with passive detection of symptomatic incident malaria cases at local health facilities or by community health workers, with subsequent home visits to screen-and-treat residents in the index case and neighbouring (secondary) households within a 140-m radius using rapid diagnostic tests (RDTs). However, a small circular radius may not be the most efficient strategy to identify parasitaemic individuals in low-endemic areas with hotspots of malaria transmission. To evaluate if RCD efficiency could be improved by increasing the probability of identifying parasitaemic residents, environmental risk factors and a larger screening radius (250 m) were assessed in a region of low malaria endemicity.

Methods

Between January 12, 2015 and July 26, 2017, 4170 individuals residing in 158 index and 531 secondary households were enrolled and completed a baseline questionnaire in the catchment area of Macha Hospital in Choma District, Southern Province, Zambia. Plasmodium falciparum prevalence was measured using PfHRP2 RDTs and quantitative PCR (qPCR). A Quickbird™ high-resolution satellite image of the catchment area was used to create environmental risk factors in ArcGIS, and generalized estimating equations were used to evaluate associations between risk factors and secondary households with parasitaemic individuals.

Results

The parasite prevalence in secondary (non-index case) households was 0.7% by RDT and 1.8% by qPCR. Overall, 8.5% (n = 45) of secondary households had at least one resident with parasitaemia by qPCR or RDT. The risk of a secondary household having a parasitaemic resident was significantly increased in proximity to higher order streams and marginally with increasing distance from index households. The adjusted OR for proximity to third- and fifth-order streams were 2.97 (95% CI 1.04–8.42) and 2.30 (95% CI 1.04–5.09), respectively, and that for distance to index households for each 50 m was 1.24 (95% CI 0.98–1.58).

Conclusion

Applying proximity to streams as a screening tool, 16% (n = 3) more malaria-positive secondary households were identified compared to using a 140-m circular screening radius. This analysis highlights the potential use of environmental risk factors as a screening strategy to increase RCD efficiency.

Using mid-infrared spectroscopy and supervised machine-learning to identify vertebrate blood meals in the malaria vector, Anopheles arabiensis

BACKGROUND

The propensity of different Anopheles mosquitoes to bite humans instead of other vertebrates influences their capacity to transmit pathogens to humans. Unfortunately, determining proportions of mosquitoes that have fed on humans, i.e. Human Blood Index (HBI), currently requires expensive and time-consuming laboratory procedures involving enzyme-linked immunosorbent assays (ELISA) or polymerase chain reactions (PCR). Here, mid-infrared (MIR) spectroscopy and supervised machine learning are used to accurately distinguish between vertebrate blood meals in guts of malaria mosquitoes, without any molecular techniques.

METHODS

Laboratory-reared Anopheles arabiensis females were fed on humans, chickens, goats or bovines, then held for 6 to 8 h, after which they were killed and preserved in silica. The sample size was 2000 mosquitoes (500 per host species). Five individuals of each host species were enrolled to ensure genotype variability, and 100 mosquitoes fed on each. Dried mosquito abdomens were individually scanned using attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectrometer to obtain high-resolution MIR spectra (4000 cm-1 to 400 cm-1). The spectral data were cleaned to compensate atmospheric water and CO2 interference bands using Bruker-OPUS software, then transferred to Python™ for supervised machine-learning to predict host species. Seven classification algorithms were trained using 90% of the spectra through several combinations of 75-25% data splits. The best performing model was used to predict identities of the remaining 10% validation spectra, which had not been used for model training or testing.

RESULTS

The logistic regression (LR) model achieved the highest accuracy, correctly predicting true vertebrate blood meal sources with overall accuracy of 98.4%. The model correctly identified 96% goat blood meals, 97% of bovine blood meals, 100% of chicken blood meals and 100% of human blood meals. Three percent of bovine blood meals were misclassified as goat, and 2% of goat blood meals misclassified as human.

CONCLUSION

Mid-infrared spectroscopy coupled with supervised machine learning can accurately identify multiple vertebrate blood meals in malaria vectors, thus potentially enabling rapid assessment of mosquito blood-feeding histories and vectorial capacities. The technique is cost-effective, fast, simple, and requires no reagents other than desiccants. However, scaling it up will require field validation of the findings and boosting relevant technical capacity in affected countries.

Species composition, blood meal hosts and Plasmodium infection rates of Anopheles mosquitoes in Ghibe River Basin, southwestern Ethiopia

Background

Vector control interventions using long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are commonly practiced tools for the control of malaria in Ethiopia. In order to evaluate the effectiveness of these control interventions, and understand the prevailing malaria vectors, their incrimination in disease transmission, and their resting and feeding behavior, we set out to identify the Anopheles species, their blood meal sources, and entomological inoculation rate (EIR) in Ghibe and Darge within the Ghibe River basin, southwestern Ethiopia.

Methods

Adult Anopheles mosquitoes were sampled both indoors and outdoors from January 2015 to October 2016 using Centers for Disease Control and Prevention (CDC) light traps, pyrethrum spray catch (PSC), artificial pit shelters and mouth aspirators. Mosquito species were morphologically identified, and their blood meal sources and malaria sporozoite rates were assessed using enzyme-linked immunosorbent assays.

Results

In total, 13 species of Anopheles mosquitoes were identified, among which Anopheles gambiae (s.l.) was the predominant species: 87.9 and 67.7% in Ghibe and Darge, respectively. The mean density of An. gambiae (s.l.) collected per night using CDC light traps was 1.8 and 0.7 outdoors and indoors, respectively, in Ghibe, and 0.125 and 0.07 indoors and outdoors, respectively, in Darge. Anopheles mosquito abundance was higher in houses near the river than in houses far from the river in both study sites. Among Anopheles mosquitoes sampled using CDC light trap catches, 67.6% were unfed and the indoor and outdoor human blood indices of An. gambiae (s.l.) were 58.4 and 15.8%, respectively in Ghibe, while in Darge, they were 57.1 and 50%, respectively. Sporozoite rates were 0.07% for P. vivax and 0.07% for P. falciparum in Ghibe and zero in Darge. In Ghibe, the overall EIRs for P. falciparum and P. vivax were zero and 8.4 infective bites/person/year, respectively, in 2015, while zero and 5.4 infective bites/person/year for P. vivax and P. falciparum, respectively, in 2016. No Plasmodium-positive Anopheles mosquitoes were identified from Darge.

Conclusions

Anopheles gambiae (s.l.), the principal vector of malaria in Ethiopia was the most abundant species both indoors and outdoors, fed both on human and cattle blood and occurred at higher frequencies near rivers. Anopheles gambiae (s.l.) that were circumsporozoite-positive for Plasmodium species were collected from Ghibe, but not Darge.

Mosquito electrocuting traps for directly measuring biting rates and host-preferences of Anopheles arabiensis and Anopheles funestus outdoors

Background

Mosquito biting rates and host preferences are crucial determinants of human exposure to vector-borne diseases and the impact of vector control measures. The human landing catch (HLC) is a gold standard method for measuring human exposure to bites, but presents risks to participants by requiring some exposure to mosquito vectors. Mosquito electrocuting traps (METs) represent an exposure-free alternative to HLCs for measuring human exposure to malaria vectors. However, original MET prototypes were too small for measuring whole-body biting rates on humans or large animals like cattle. Here a much larger MET capable of encompassing humans or cattle was designed, and its performance was evaluated relative to both the original small MET and HLC and for quantifying malaria vector host preferences.

Methods

Human landing catch, small human-baited METs (MET-SH), and large METs baited with either a human (MET-LH) or calves (MET-LC) were simultaneously used to capture wild malaria vectors outdoors in rural southern Tanzania. The four capture methods were compared in a Latin-square design over 20 nights. Malaria vector host preferences were estimated through comparison of the number of mosquitoes caught by large METs baited with either humans or cattle.

Results

The MET-LH caught more than twice as many Anopheles arabiensis than either the MET-SH or HLC. It also caught higher number of Anopheles funestus sensu lato (s.l.) compared to the MET-SH or HLC. Similar numbers of An. funestus sensu stricto (s.s.) were caught in MET-LH and MET-SH collections. Catches of An. arabiensis with human or cattle-baited large METs were similar, indicating no clear preference for either host. In contrast, An. funestus s.s. exhibited a strong, but incomplete preference for humans.

Conclusions

METs are a sensitive, practical tool for assessing mosquito biting rates and host preferences, and represent a safer alternative to the HLC. Additionally these findings suggest the HLC underestimate whole-body human exposure. MET collections indicated the An. funestus s.s. population in this setting had a higher than expected attack rate on cattle, potentially making eliminating of this species more difficult with human-targetted control measures. Supplementary vector control tools targetted at livestock may be required to effectively tackle this species.

Pyriproxyfen-Treated Polypropylene Sheets and Resting Boxes for Controlling Mosquitoes in Livestock Operations

Many insect vector species of medical and veterinary importance are found abundantly in areas where animals are held. In these areas, they often rest for a period of time on objects around the animals both before and after blood feeding. However, the use of neurotoxic insecticides for vector control is not advised for use in such shelters as these chemicals can pose hazards to animals. The present study evaluated the efficacy of pyriproxyfen (PPF), an insect growth regulator, applied to polypropylene sheets and resting boxes on the reproductivity of mosquitoes found in animal shelters in Chiang Mai, Thailand. The sheets sprayed with 666 mg PPF/m² were set on the inner wall of a cowshed and kept in place for 3 h (6.00 to 9.00 pm). During this time, fully blood-fed female mosquitoes that landed and remained continuously on the sheets for 5, 10, and 20 min were collected. The results, involving Anopheles subpictus, An. vagus, Culex gelidus, Cx. tritaeniorhynchus, and Cx. vishnui, revealed significant reductions in oviposition rates, egg hatchability, pupation, and adult emergence in the PPF-treated groups compared to the control groups. Adult emergence rates were reduced to 85.6⁻94.9% and 95.5⁻100% in those exposed for 10 and 20 min, respectively. The sheets retained their effectiveness for three months. The PPF-treated (666 mg/m²) resting boxes (35 × 35 × 55 cm) were placed overnight at a chicken farm where Cx. quinquefasciatus predominated. Blood-fed mosquitoes were collected in the morning and reared in the laboratory. Oviposition rates were reduced by 71.7% and adult emergence was reduced by 97.8% compared to the controls. PPF residual spray on surface materials in animal sheds is a potential method for controlling mosquitoes. Further studies are needed to evaluate the impact of PPF-treated materials on wild populations.

Does livestock protect from malaria or facilitate malaria prevalence? A cross-sectional study in endemic rural areas of Indonesia

Background

Ever since it was discovered that zoophilic vectors can transmit malaria, zooprophylaxis has been used to prevent the disease. However, zoopotentiation has also been observed. Thus, the presence of livestock has been widely accepted as an important variable for the prevalence and risk of malaria, but the effectiveness of zooprophylaxis remained subject to debate. This study aims to critically analyse the effects of the presence of livestock on malaria prevalence using a large dataset from Indonesia.

Methods

This study is based on data from the Indonesia Basic Health Research (“Riskesdas”) cross-sectional survey of 2007 organized by the National Institute of Health Research and Development of Indonesia’s Ministry of Health. The subset of data used in the present study included 259,885 research participants who reside in the rural areas of 176 regencies throughout the 15 provinces of Indonesia where the prevalence of malaria is higher than the national average. The variable “existence of livestock” and other independent demographic, social and behavioural variables were tested as potential determinants for malaria prevalence by multivariate logistic regressions.

Results

Raising medium-sized animals in the house was a significant predictor of malaria prevalence (OR = 2.980; 95% CI 2.348–3.782, P < 0.001) when compared to keeping such animals outside of the house (OR = 1.713; 95% CI 1.515–1.937, P < 0.001). After adjusting for gender, age, access to community health facility, sewage canal condition, use of mosquito nets and insecticide-treated bed nets, the participants who raised medium-sized animals inside their homes were 2.8 times more likely to contract malaria than respondents who did not (adjusted odds ratio = 2.809; 95% CI 2.207–3.575; P < 0.001).

Conclusions

The results of this study highlight the importance of livestock for malaria transmission, suggesting that keeping livestock in the house contributes to malaria risk rather than prophylaxis in Indonesia. Livestock-based interventions should therefore play a significant role in the implementation of malaria control programmes, and focus on households with a high proportion of medium-sized animals in rural areas. The implementation of a “One Health” strategy to eliminate malaria in Indonesia by 2030 is strongly recommended.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2447-6) contains supplementary material, which is available to authorized users.

Malaria vector abundance is associated with house structures in Baringo County, Kenya

Malaria, a major cause of morbidity and mortality, is the most prevalent vector borne disease in Baringo County; a region which has varied house designs in arid and semi-arid areas. This study investigated the association between house structures and indoor-malaria vector abundance in Baringo County. The density of malaria vectors in houses with open eaves was higher than that for houses with closed eaves. Grass thatched roof houses had higher density of malaria vectors than corrugated iron sheet roofs. Similarly, mud walled houses had higher vector density than other wall types. Houses in the riverine zone were significantly associated with malaria vector abundance (p<0.000) possibly due to more varied house structures. In Kamnarok village within riverine zone, a house made of grass thatched roof and mud wall but raised on stilts with domestic animals (sheep/goats) kept at the lower level had lower mosquito density (5.8 per collection) than ordinary houses made of same materials but at ground level (30.5 mosquitoes per collection), suggestive of a change in behavior of mosquito feeding and resting. House modifications such as screening of eaves, improvement of construction material and building stilted houses can be incorporated in the integrated vector management (IVM) strategy to complement insecticide treated bed nets and indoor residual spray to reduce indoor malaria vector density.

Zooprophylaxis as a control strategy for malaria caused by the vector Anopheles arabiensis (Diptera: Culicidae): a systematic review

Background

Zooprophylaxis is the use of wild or domestic animals, which are not the reservoir host of a given disease, to divert the blood-seeking malaria vectors from human hosts. In this paper, we systematically reviewed zooprophylaxis to assess its efficacy as a malaria control strategy and to evaluate the possible methods of its application.

Methods

The electronic databases, PubMed Central®, Web of Science, Science direct, and African Journals Online were searched using the key terms: “zooprophylaxis” or “cattle and malaria”, and reports published between January 1995 and March 2016 were considered. Thirty-four reports on zooprophylaxis were retained for the systematic review.

Results

It was determined that Anopheles arabiensis is an opportunistic feeder. It has a strong preference for cattle odour when compared to human odour, but feeds on both hosts. Its feeding behaviour depends on the available hosts, varying from endophilic and endophagic to exophilic and exophagic. There are three essential factors for zooprophylaxis to be effective in practice: a zoophilic and exophilic vector, habitat separation between human and host animal quarters, and augmenting zooprophylaxis with insecticide treatment of animals or co-intervention of long-lasting insecticide-treated nets and/or indoor residual spraying. Passive zooprophylaxis can be applied only in malaria vector control if cattle and human dwellings are separated in order to avoid the problem of zoopotentiation.

Conclusions

The outcomes of using zooprophylaxis as a malaria control strategy varied across locations. It is therefore advised to conduct a site-specific evaluation of its effectiveness in vector control before implementing zooprophylaxis as the behaviour of Anopheles arabiensis mosquitoes varies across localities and circumstances.

Electronic supplementary material

The online version of this article (10.1186/s40249-017-0366-3) contains supplementary material, which is available to authorized users.