Large-Scale Operational Pyriproxyfen Autodissemination Deployment to Suppress the Immature Asian Tiger Mosquito (Diptera: Culicidae) Populations

Yellow Fever in Non-Human Primates: A Veterinary Guide from a One Health Perspective

Yellow fever (YF) causes severe morbidity and mortality in Africa and South America. It is an arthropod-borne viral disease endemic to tropical regions of Africa and South America. Yellow fever virus (YFV) is transmitted by mosquitoes and frequently affects both non-human primates (NHPs) and humans. Neotropical primates (NTPs) are generally more severely afflicted by YFV than African primates. Asian primates appear not to be susceptible to this disease. Susceptibility varies among NTP species: asymptomatic infections are described in some NTP species, whereas severe epizootic mortality events are described in others. The genus Alouatta (howler monkeys) is considered to be the most susceptible among the NTPs. Epizootic events resulting in the death of thousands of NTPs have been recorded in recent history. As a result, YFV poses a threat to the survival of some NTP species. In most cases, NTPs are found dead without showing prior clinical signs. In cases where clinical signs are observed, they are mostly non-specific. Due to their high susceptibility, epizootic events in NTPs are used as epidemiological predictors for human YF outbreaks. YFV infection may be diagnosed by means of virus isolation, reverse transcription polymerase chain reaction, serology, histopathology, or immunohistochemistry. Animals that survive the disease develop neutralizing antibodies to YFV. Currently, no specific treatment is available. Sustained YF control strategies must rely on surveillance and accurate diagnostics to allow for early detection of outbreaks and rapid implementation of control measures. Prophylaxis should be based on a One Health perspective that recognizes the intricate interplay between human health, primate health, and the environment. Vaccines for YF are available, with the human 17DD vaccine effectively preventing disease in primates. However, mitigation strategies continue to rely more and more on vector control, preferably using eco-friendly methods. Climate change and human activities, and their impact on local ecology, are assumed to increase the risk of YF transmission in the next decades.

Little disease but lots of bites: social, urbanistic, and entomological risk factors of human exposure to Aedes aegypti in South Texas, U.S

BACKGROUND

Aedes aegypti presence, human-vector contact rates, and Aedes-borne virus transmission are highly variable through time and space. The Lower Rio Grande Valley (LRGV), Texas, is one of the few regions in the U.S. where local transmission of Aedes-borne viruses occurs, presenting an opportunity to evaluate social, urbanistic, entomological, and mobility-based factors that modulate human exposure to Ae. aegypti.

METHODOLOGY & PRINCIPAL FINDINGS

Mosquitoes were collected using BG-Sentinel 2 traps during November 2021 as part of an intervention trial, with knowledge, attitudes, and practices (KAP) and housing quality surveys to gather environmental and demographic data. Human blood samples were taken from individuals and a Bitemark Assay (ELISA) was conducted to quantify human antibodies to the Ae. aegypti Nterm-34kDa salivary peptide as a measure of human exposure to bites. In total, 64 houses were surveyed with 142 blood samples collected. More than 80% of participants had knowledge of mosquito-borne diseases and believed mosquitoes to be a health risk in their community. Our best fit generalized linear mixed effects model found four fixed effects contributed significantly to explaining the variation in exposure to Ae. aegypti bites: higher annual household income, younger age, larger lot area, and higher female Ae. aegypti abundance per trap night averaged over 5 weeks prior to human blood sampling.

CONCLUSIONS

Most surveyed residents recognized mosquitoes and the threat they pose to individual and public health. Urbanistic (i.e., lot size), social (i.e., income within a low-income community and age), and entomological (i.e., adult female Ae. aegypti abundance) factors modulate the risk of human exposure to Ae. aegypti bites. The use of serological biomarker assays, such as the Bitemark Assay, are valuable tools for surveillance and risk assessment of mosquito-borne disease, especially in areas like the LRGV where the transmission of target pathogens is low or intermittent.

Diflubenzuron larvicide auto-dissemination: A modeling study

Proposing substitutes for Pyriproxyfen (PPF) in the auto-dissemination strategy is essential to ensure the continuity of the strategy in the field, especially in the case of the emergence of populations resistant to this larvicide. One possible substitute among the compounds already in use in Brazil is the larvicide Diflubenzuron (DFB). The equation that defines the proportion of oviposition sites (habitats) contaminated by the auto-dissemination strategy was modified to account for the number of visits required to reach the necessary concentration of DFB for contamination, considering scenarios with varying numbers of oviposition sites and mosquito densities. The dissemination was evaluated in oviposition sites of 2 L, 1.5 L, 1 L, 0.5 L, 0.2 L, and 0.1 L. The minimum concentration of active ingredient (a.i) of DFB required for a commercial product to contaminate at least 50% of oviposition sites was also investigated, along with the impact of other vector control methods, such as the removal/destruction of oviposition sites and the use of insecticides to kill adult 'females, on the auto-dissemination approach. The use of pure DFB compounds enabled contamination efficiency of more than 50% in oviposition sites with a volume of less than 2 L in scenarios with fewer oviposition sites. On the other hand, with the use of the commonly used concentration of the product, similar efficacy was only achieved in oviposition sites of 0.1 L and 0.2 L in medium and high infestation scenarios. Strategies that reduce the number of available oviposition sites work synergistically with the auto-dissemination strategy, making it possible to use less concentrated products and contaminated sites of larger volume. The strategy proved to be resilient in situations of insecticide application according to the concentration of DFB used, abundance of females, and low number of oviposition sites. Increasing the number of dissemination traps on the field also contributes to better results, especially for oviposition sites of 0.5 L and 1 L. The results of the model obtained under the stipulated conditions provide further support for the potential use of DFB as a substitute for PPF in the auto-dissemination strategy.

Sterilized Anopheles funestus can autodisseminate sufficient pyriproxyfen to the breeding habitat under semi-field settings

BACKGROUND

Anopheles funestus, the main malaria vector, prefer to oviposit in permanent and/or semi-permanent breeding habitats located far from human dwellings. Difficulties in identifying and accessing these habitats jeopardize the feasibility of conventional larviciding. In this way, a semi-field study was conducted to assess the potential of autodissemination of pyriproxyfen (PPF) by An. funestus for its control.

METHODS

The study was conducted inside a semi-field system (SFS). Therein, two identical separate chambers, the treatment chamber with a PPF-treated clay pot (0.25 g AI), and the control chamber with an untreated clay pot. In both chambers, one artificial breeding habitat made of a plastic basin with one litre of water was provided. Three hundred blood-fed female An. funestus aged 5-9 days were held inside untreated and treated clay pots for 30 min and 48 h before being released for oviposition. The impact of PPF on adult emergence, fecundity, and fertility through autodissemination and sterilization effects were assessed by comparing the treatment with its appropriate control group.

RESULTS

Mean (95% CI) percentage of adult emergence was 15.5% (14.9-16.1%) and 70.3% (69-71%) in the PPF and control chamber for females exposed for 30 min (p < 0.001); and 19% (12-28%) and 95% (88-98%) in the PPF and control chamber for females exposed for 48 h (p < 0.001) respectively. Eggs laid by exposed mosquitoes and their hatch rate were significantly reduced compared to unexposed mosquitoes (p < 0.001). Approximately, 90% of females exposed for 48 h retained abnormal ovarian follicles and only 42% in females exposed for 30 min.

CONCLUSION

The study demonstrated sterilization and adult emergence inhibition via autodissemination of PPF by An. funestus. Also, it offers proof that sterilized An. funestus can transfer PPF to prevent adult emergence at breeding habitats. These findings warrant further assessment of the autodissemination of PPF in controlling wild population of An. funestus, and highlights its potential for complementing long-lasting insecticidal nets.

Rapid Elimination of Aedes aegypti and Culex quinquefasciatus Mosquitoes from Puerco Island, Palawan, Philippines with Odor-Baited Traps

Globalization and climate change are key drivers for arboviral and parasitic infectious diseases to expand geographically, posing a growing threat to human health and biodiversity. New non-pesticidal approaches are urgently needed because of increasing insecticide resistance and the negative human and environmental health impacts of synthetic pyrethroids used for fogging. Here, we report the complete and rapid removal of two mosquito species (Aedes aegypti L. and Culex quinquefasciatus Say), both arboviral disease vectors, with odor-baited mosquito traps (at a density of 10 traps/hectare) from a 7.2-hectare island in the Philippines in just 5 months. This rapid elimination of mosquitoes from an island is remarkable and provides further proof that high-density mosquito trapping can play a significant role in mosquito- and vector-borne disease elimination in small islands around the world.

Community efforts to monitor and manage Aedes mosquitoes (Diptera: Culicidae) with ovitraps and litter reduction in east Tennessee

BACKGROUND

East Tennessee (USA) is burdened by mosquito-borne La Crosse virus disease, but minimal resources for mosquito surveillance, management, or related community education exist in the region. To address these needs, we developed a program to train middle and high school educators in basic medical entomology. The educators then used their skills in the classroom to teach students about La Crosse virus disease and conduct mosquito collection experiments. As a case study of a potential application of classroom-collected data, we also partnered with a local non-profit organization to assess the potential for a volunteer litter cleanup to reduce mosquito populations in a Tennessee neighborhood.

METHODS

Our first objective was to investigate the ability for educators and their students (schools) to collect high-quality mosquito surveillance data. In 2019 and 2020, we collected Aedes (Diptera: Culicidae) eggs during the same study period as schools and assessed whether data collected by schools reflected the same findings as our own data. Our second objective was to investigate the impact of a volunteer litter cleanup event on Aedes mosquito abundance. In 2021, we collected Aedes eggs before and after a neighborhood trash cleanup while schools conducted their own mosquito egg collections. Using the school collections as non-treatment sites, we used a Before-After-Control-Impact analysis to determine if there was a significant decline in egg abundance after the cleanup.

RESULTS

In 2019, mosquito abundance trends were similar between our data and school data but differed significantly during some weeks. After refining our protocols in 2020, school data was highly similar to our data, indicating that schools consistently collected high-quality surveillance data in the program's second year. In 2021, we found a significant decline in Aedes egg abundance after the litter cleanup event in comparison to the schools, but the number of adults reared from those eggs did not differ between sites after the cleanup.

CONCLUSION

The results of our work demonstrate the potential for community-driven programs to monitor mosquito abundance trends and for volunteer-based cleanup events to reduce the burden of Aedes mosquitoes. In the absence of infrastructure and resources, academic-community partnerships like the ones evaluated here, provide opportunities to help resource limited areas.

Mass Trapping and Larval Source Management for Mosquito Elimination on Small Maldivian Islands

Globally, environmental impacts and insecticide resistance are forcing pest control organizations to adopt eco-friendly and insecticide-free alternatives to reduce the risk of mosquito-borne diseases, which affect millions of people, such as dengue, chikungunya or Zika virus. We used, for the first time, a combination of human odor-baited mosquito traps (at 6.0 traps/ha), oviposition traps (7.2 traps/ha) and larval source management (LSM) to practically eliminate populations of the Asian tiger mosquito Aedes albopictus (peak suppression 93.0% (95% CI 91.7-94.4)) and the Southern house mosquito Culex quinquefasciatus (peak suppression 98.3% (95% CI 97.0-99.5)) from a Maldivian island (size: 41.4 ha) within a year and thereafter observed a similar collapse of populations on a second island (size 49.0 ha; trap densities 4.1/ha and 8.2/ha for both trap types, respectively). On a third island (1.6 ha in size), we increased the human odor-baited trap density to 6.3/ha and then to 18.8/ha (combined with LSM but without oviposition traps), after which the Aedes mosquito population was eliminated within 2 months. Such suppression levels eliminate the risk of arboviral disease transmission for local communities and safeguard tourism, a vital economic resource for small island developing states. Terminating intense insecticide use (through fogging) benefits human and environmental health and restores insect biodiversity, coral reefs and marine life in these small and fragile island ecosystems. Moreover, trapping poses a convincing alternative to chemical control and reaches impact levels comparable to contemporary genetic control strategies. This can benefit numerous communities and provide livelihood options in small tropical islands around the world where mosquitoes pose both a nuisance and disease threat.

New tools for Aedes control: mass trapping

Aedes aegypti, the main vector of dengue, chikungunya, and Zika viruses uses artificial containers around homes to undergo immature development, making household-level detection and control extremely difficult in large urban areas. Mass trapping is an emerging methodology to control container-Aedes species such as Aedes aegypti and Aedes albopictus because effective traps for adult stages of these mosquitoes were developed recently. There are three main approaches to mass-trapping these mosquitoes: 1) Pull (attract/kill), 2) push (repel)-pull (attract/kill), and 3) pull (attract/contaminate/infect)-push (fly away). Effective mass-trapping depends on trap quality (capture efficiency, sturdiness, frequency of servicing), trap density and areal coverage, community involvement, and safety. Recent studies showed that Ae. aegypti populations can be sustainably controlled by mass trapping, although more area-wide studies showing effectiveness at preventing disease are needed for all trapping systems. Cost-effectiveness studies are needed for all emerging Aedes control approaches.

Small-scale field assessment against the dengue vector Aedes aegypti using the auto-dissemination approach in an urban area of Vientiane, Lao PDR

Background

In Lao PDR, dengue fever is the most important vector borne disease and vector control remains the principal method to fight against Aedes aegypti the primary transmitter mosquito species. Vector control management programs need new strategies in addition to conventional larviciding and adulticiding interventions in the country. In this study, we examined the In2Care® Mosquito Trap’s efficacy using insecticide auto-dissemination strategy. The insecticide pyriproxyfen, present in powder form inside the trap station, contaminates the body of gravid female mosquitoes visiting the traps and is later on disseminated via the mosquitoes in breeding sites surrounding the traps. We tested the attractiveness of the Traps, their efficacy to reduce the larval and adult abundance, and the impact on emergence rates. Specifically, we tested if the servicing interval of the In2Care® Mosquito Trap could be extended to 12 weeks.

Methods

Two black plastic ovitrap buckets and two BG® sentinel traps were placed in the premises of the Science campus of Vientiane Capital located in an urban area to measure weekly the larval and adult relative abundance of Aedes mosquitoes from 2017 to 2019. Twenty-five In2Care® Mosquito Traps were evenly distributed in this area and two studies of 12 weeks were implemented during January and April 2018 and, July to October 2018 (dry and rainy season, respectively). Every 2 weeks, water samples from 5 In2Care® Traps were randomly selected and tested at the laboratory with Ae. aegypti larvae to measure the larval and pupal mortality. The relative abundance of Aedes mosquitoes in the BG traps® with the presence of In2Care® Traps in 2018, was compared with the surveillance results obtained in 2017 and 2019 without In2Care® Traps. Every week, water samples from the ovitrap buckets were tested for Emergence Inhibition (EI).

Results

The In2Care® Traps were very attractive to gravid Ae. aegypti mosquitoes specifically during the rainy seasons with 96% of the traps colonized with larvae/pupae within four weeks. The bioassays showed 100% mortality in the water samples from the traps during the twelve weeks studies showing the good efficacy over time of the pyriproxyfen without additional servicing in the 12 week period. In addition, the larvicide was successfully disseminated into the ovitrap buckets placed in the treated area where 100% of EI during all weeks of intervention was measured. There was no significant effect of the treatment on adult abundance reduction in the treated area, probably due to recolonization of adult mosquitoes surrounding the field experiment.

Conclusions

The observed potential of the In2Care® Mosquito Trap using the auto-dissemination strategy could lead to the use of this new tool in combination with conventional control methods against Dengue vectors in urban tropical areas. Large scale field trials should be implemented in Lao PDR to prove its efficacy for Public Health programs.

Effectiveness of autocidal gravid trapping and chemical control in altering abundance and age structure of Aedes albopictus

BACKGROUND

Aedes albopictus is a nuisance pest mosquito of public health importance commonly managed with adulticides and larvicides. We investigated whether adding Gravid Aedes Traps (GATs), Autocidal Gravid Ovitraps (AGOs) or In2Care traps would extend the effectiveness of chemical control methods in Wake County, North Carolina, USA, by combining barrier sprays and larval habitat management (LHM) with each trap type at suburban households. We compared these three treatment groups to untreated controls and to backyards treated only with barrier sprays and LHM. Once a week, for ten weeks, we collected adult mosquitoes at each house using lure-baited surveillance traps and dissected a portion of Ae. albopictus females to determine parity.

RESULTS

Barrier sprays and LHM alone or combined with any supplemental autocidal ovitrap significantly reduced female Ae. albopictus through Week 3 post-treatment. GATs significantly extended chemical control effectiveness for the duration of the study. Compared to the untreated control, the AGO and GAT treatment groups had significant overall female Ae. albopictus reductions of 74% and 80.4%, respectively, with populations aging significantly slower at houses treated with AGOs.

CONCLUSION

This household-level study, though limited in size, observed significant reductions in nuisance Ae. albopictus when combining AGOs and GATs with chemical controls for an eight-week period. Delayed population aging in AGO-treated yards suggests that traps also could mitigate disease transmission risk. Future studies should test these control methods at the neighborhood level to evaluate large-scale effectiveness as well as assess the effect of autocidal ovitraps without chemical intervention. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A Field Efficacy Evaluation of In2Care Mosquito Traps in Comparison with Routine Integrated Vector Management at Reducing Aedes aegypti

Aedes aegypti is the predominant vector of dengue, chikungunya, and Zika viruses. This mosquito is difficult to control with conventional methods due to its container-inhabiting behavior and resistance to insecticides. Autodissemination of pyriproxyfen (PPF), a potent larvicide, has shown promise as an additional tool to control Aedes species in small-scale field trials. However, few large-scale field evaluations have been conducted. We undertook a 6-month-long large-scale field study to compare the effectiveness and operational feasibility of using In2Care Mosquito Traps (In2Care Traps, commercially available Aedes traps with PPF and Beauveria bassiana) compared to an integrated vector management (IVM) strategy consisting of source reduction, larviciding, and adulticiding for controlling Ae. aegypti eggs, larvae, and adults. We found that while the difference between treatments was only statistically significant for eggs and larvae (P < 0.05 for eggs and larvae and P > 0.05 for adults), the use of In2Care Traps alone resulted in 60%, 57%, and 57% fewer eggs, larvae, and adults, respectively, collected from that site compared to the IVM site. However, In2Care Trap deployment and maintenance were more time consuming and labor intensive than the IVM strategy. Thus, using In2Care Traps alone as a control method for large areas (e.g., >20 ha) may be less practical for control programs with the capacity to conduct ground and aerial larviciding and adulticiding. Based on our study results, we conclude that In2Care Traps are effective at suppressing Ae. aegypti and have the most potential for use in areas without sophisticated control programs and within IVM programs to target hotspots with high population levels and/or risk of Aedes-borne pathogen transmission.

Heterodissemination: precision insecticide delivery to mosquito larval habitats by cohabiting vertebrates

Conventional larvicide delivery strategies originally developed for permanent and floodwater mosquitoes have proved suboptimal in the small, scattered, and cryptic larval habitats preferred by container-inhabiting Aedes mosquitoes. New methods such as autodissemination, wherein adult mosquitoes spread insecticides to their own larval habitats, have been under study. Another novel delivery method termed heterodissemination, i.e. larvicide delivery by other species sharing the same habitats, has also been proposed. We conducted a proof-of-concept study with four independent experiments using American bullfrogs (Lithobates catesbeianus) and green frogs Lithobates clamitans as carriers of pyriproxyfen, an insect growth regulator, under semi-field conditions in three different locations, two in New Jersey, and one in Utah. Frogs with attached slow-release pyriproxyfen tablets were introduced into outdoor enclosures with water containers. Water samples from the containers were periodically tested using larval Aedes albopictus and Culex pipiens mosquitoes to assess mortality and percent eclosure inhibition. Overall pupal mortality [95% credible intervals] estimated by Bayesian analysis for the treatment group was 73.4% [71.3–75.2] compared to 4.1% [2.9–5.5] for the control group. Mortality within treatment groups in four different experiments ranged from 41 to 100%, whereas control mortalities ranged from 0.5% to 11%. We conclude that heterodissemination is a promising and effective approach deserving of further study.

Susceptibility of South Texas Aedes aegypti to Pyriproxyfen

Simple Summary

We evaluated the susceptibility of an Ae. aegypti strain from the Lower Rio Grande Valley (LRGV) of South Texas to the insect growth regulator pyriproxyfen. We observed a difference in the inhibition of emergence to the lowest doses of pyriproxyfen tested between our field strain and a susceptible strain. However, the doses used are 10 times lower from the recommended application of <50 ppb for vector control programs. Our results suggest that pyriproxyfen should be an effective active ingredient in the LRGV to help reduce Ae. aegypti populations in the LRGV.

Abstract

An integral part to integrated mosquito management is to ensure chemical products used for area-wide control are effective against a susceptible population of mosquitoes. Prior to conducting an intervention trial using an insect growth regulator, pyriproxyfen, in South Texas to control Aedes aegypti, we conducted a larval bioassay to evaluate baseline levels of susceptibility. We used seven serially-diluted doses ranging from 2.5 ppb to 6.3 × 10⁻⁴ ppb. We observed 100% inhibition emergence (IE) at even the lowest dose of 6.3 × 10⁻⁴ ppb in our susceptible reference colony of Ae. aegypti Liverpool. In our field strain of Ae. aegypti (F5 colonized from South Texas) we observed 79.8% IE at 6.3 × 10⁻⁴ ppb, 17.7% IE at 1.25 × 10⁻³ ppb, 98.7% IE at 1.25 × 10⁻² ppb, and 100% emergence inhibition for the remainder of the doses. Given that commercial pyriproxyfen products are labeled for doses ranging to 50 ppb, we conclude that the field population sampled by this study are susceptible to this insect growth regulator.

Comparing sterile male releases and other methods for integrated control of the tiger mosquito in temperate and tropical climates

The expansion of mosquito species worldwide is creating a powerful network for the spread of arboviruses. In addition to the destruction of breeding sites (prevention) and mass trapping, methods based on the sterile insect technique (SIT), the autodissemination of pyriproxyfen (ADT), and a fusion of elements from both of these known as boosted SIT (BSIT), are being developed to meet the urgent need for effective vector control. However, the comparative potential of these methods has yet to be explored in different environments. This is needed to propose and integrate informed guidelines into sustainable mosquito management plans. We extended a weather-dependent model of Aedes albopictus population dynamics to assess the effectiveness of these different vector control methods, alone or in combination, in a tropical (Reunion island, southwest Indian Ocean) and a temperate (Montpellier area, southern France) climate. Our results confirm the potential efficiency of SIT in temperate climates when performed early in the year (mid-March for northern hemisphere). In such a climate, the timing of the vector control action was the key factor in its success. In tropical climates, the potential of the combination of methods becomes more relevant. BSIT and the combination of ADT with SIT were twice as effective compared to the use of SIT alone.

Stage-specific action of juvenile hormone analogs

The discovery of juvenile hormones (JH) and their synthetic analogs (JHA) generated excitement and hope that these compounds will replace first- and second-generation insecticides that have not so desirable environmental and human safety profiles. However, JHAs used commercially during the past four decades did not meet these expectations. The recent availability of advanced molecular and histological methods and the discovery of key players involved in JH action provided some insights into the functioning of JHA in a stage and species-specific manner. In this review, we will summarize recent findings and stage-specific action of JHA, focusing on three commercially used JHA, methoprene, hydroprene and pyriproxyfen and economically important pests, the red flour beetle, Tribolium castaneum, and the tobacco budworm, Heliothis virescens, and disease vector, the yellow fever mosquito, Aedes aegypti.

Deployment and Fact Analysis of the In2Care® Mosquito Trap, A Novel Tool for Controlling Invasive Aedes Species

During April-October 2019, the West Valley Mosquito and Vector Control District (Ontario, CA) deployed large numbers of In2Care® mosquito traps in a preliminary study to evaluate the trap's potential effectiveness at controlling invasive Aedes aegypti (L.) and Ae. albopictus (Skuse) in 6 cities of San Bernardino County, CA. The trap was used to attract ovipositing females, expose them to the juvenile hormone mimic pyriproxyfen and the entomopathogenic fungus Beauveria bassiana, and autodisseminate pyriproxyfen to other water sources prior to their death from fungal infection. The trap attracted Ae. aegypti and Culex quinquefasciatus, with the latter species predominating at much higher larval densities in the trap reservoirs. Field-collected larvae and pupae from the trap reservoirs showed complete adult emergence inhibition. Furthermore, the trap reservoirs retained high levels of residual larvicidal, pupicidal, and emergence inhibition activity after they were retrieved from the field, as indicated by laboratory bioassays against laboratory colony of Cx. quinquefasciatus. Results of this study support more detailed quantitative local evaluations on trap efficacy to measure the impact of the In2Care mosquito trap on wild invasive Aedes and Culex populations in future mosquito control efforts.

Measuring mosquito control: adult-mosquito catches vs egg-trap data as endpoints of a cluster-randomized controlled trial of mosquito-disseminated pyriproxyfen

BACKGROUND

Aedes aegypti and Culex quinquefasciatus are the main urban vectors of arthropod-borne viruses causing human disease, including dengue, Zika, or West Nile. Although key to disease prevention, urban-mosquito control has met only limited success. Alternative vector-control tactics are therefore being developed and tested, often using entomological endpoints to measure impact. Here, we test one promising alternative and assess how three such endpoints perform at measuring its effects.

METHODS

We conducted a 16-month, two-arm, cluster-randomized controlled trial (CRCT) of mosquito-disseminated pyriproxyfen (MD-PPF) in central-western Brazil. We used three entomological endpoints: adult-mosquito density as directly measured by active aspiration of adult mosquitoes, and egg-trap-based indices of female Aedes presence (proportion of positive egg-traps) and possibly abundance (number of eggs per egg-trap). Using generalized linear mixed models, we estimated MD-PPF effects on these endpoints while accounting for the non-independence of repeated observations and for intervention-unrelated sources of spatial-temporal variation.

RESULTS

On average, MD-PPF reduced adult-mosquito density by 66.3% (95% confidence interval, 95% CI: 47.3-78.4%); Cx. quinquefasciatus density fell by 55.5% (95% CI: 21.1-74.8%), and Ae. aegypti density by 60.0% (95% CI: 28.7-77.5%). In contrast, MD-PPF had no measurable effect on either Aedes egg counts or egg-trap positivity, both of which decreased somewhat in the intervention cluster but also in the control cluster. Egg-trap data, therefore, failed to reflect the 60.0% mean reduction of adult Aedes density associated with MD-PPF deployment.

CONCLUSIONS

Our results suggest that the widely used egg-trap-based monitoring may poorly measure the impact of Aedes control; even if more costly, direct monitoring of the adult mosquito population is likely to provide a much more realistic and informative picture of intervention effects. In our CRCT, MD-PPF reduced adult-mosquito density by 66.3% in a medium-sized, spatially non-isolated, tropical urban neighborhood. Broader-scale trials will be necessary to measure MD-PPF impact on arboviral-disease transmission.