Seasonal and Spatial Dynamics of the Primary Vector of Plasmodium knowlesi within a Major Transmission Focus in Sabah, Malaysia

Real-time fluorescence loop-mediated isothermal amplification assays for detection of zoonotic malaria Plasmodium parasites

BACKGROUND

Natural human infections by Plasmodium cynomolgi and P. inui have been reported recently and gain the substantial attention from Southeast Asian countries. Zoonotic transmission of non-human malaria parasites to humans from macaque monkeys occurred through the bites of the infected mosquitoes. The objective of this study is to establish real-time fluorescence loop-mediated isothermal amplification (LAMP) assays for the detection of zoonotic malaria parasites by combining real-time fluorescent technology with the isothermal amplification technique.

METHODS

By using 18S rRNA as the target gene, the primers for P. cynomolgi, P. coatneyi and P. inui were newly designed in the present study. Four novel real-time fluorescence LAMP assays were developed for the detection of P. cynomolgi, P. coatneyi, P. inui and P. knowlesi. The entire amplification process was completed in 60 min, with the assays performed at 65 °C. By using SYTO-9 as the nucleic acid intercalating dye, the reaction was monitored via real-time fluorescence signal.

RESULTS

There was no observed cross-reactivity among the primers from different species. All 70 field-collected monkey samples were successfully amplified by real-time fluorescence LAMP assays. The detection limit for P. cynomolgi, P. coatneyi and P. knowlesi was 5 × 109 copies/µL. Meanwhile, the detection limit of P. inui was 5 × 1010 copies/µL.

CONCLUSION

This is the first report of the detection of four zoonotic malaria parasites by real-time fluorescence LAMP approaches. It is an effective, rapid and simple-to-use technique. This presented platform exhibits considerable potential as an alternative detection for zoonotic malaria parasites.

Landscape drives zoonotic malaria prevalence in non-human primates

Zoonotic disease dynamics in wildlife hosts are rarely quantified at macroecological scales due to the lack of systematic surveys. Non-human primates (NHPs) host Plasmodium knowlesi, a zoonotic malaria of public health concern and the main barrier to malaria elimination in Southeast Asia. Understanding of regional P. knowlesi infection dynamics in wildlife is limited. Here, we systematically assemble reports of NHP P. knowlesi and investigate geographic determinants of prevalence in reservoir species. Meta-analysis of 6322 NHPs from 148 sites reveals that prevalence is heterogeneous across Southeast Asia, with low overall prevalence and high estimates for Malaysian Borneo. We find that regions exhibiting higher prevalence in NHPs overlap with human infection hotspots. In wildlife and humans, parasite transmission is linked to land conversion and fragmentation. By assembling remote sensing data and fitting statistical models to prevalence at multiple spatial scales, we identify novel relationships between P. knowlesi in NHPs and forest fragmentation. This suggests that higher prevalence may be contingent on habitat complexity, which would begin to explain observed geographic variation in parasite burden. These findings address critical gaps in understanding regional P. knowlesi epidemiology and indicate that prevalence in simian reservoirs may be a key spatial driver of human spillover risk.

Forest Restoration and the Zoonotic Vector Anopheles balabacensis in Sabah, Malaysia

Anthropogenic changes to forest cover have been linked to an increase in zoonotic diseases. In many areas, natural forests are being replaced with monoculture plantations, such as oil palm, which reduce biodiversity and create a mosaic of landscapes with increased forest edge habitat and an altered micro-climate. These altered conditions may be facilitating the spread of the zoonotic malaria parasite Plasmodium knowlesi in Sabah, on the island of Borneo, through changes to mosquito vector habitat. We conducted a study on mosquito abundance and diversity in four different land uses comprising restored native forest, degraded native forest, an oil palm estate and a eucalyptus plantation, these land uses varying in their vegetation types and structure. The main mosquito vector, Anopheles balabacensis, has adapted its habitat preference from closed canopy rainforest to more open logged forest and plantations. The eucalyptus plantations (Eucalyptus pellita) assessed in this study contained significantly higher abundance of many mosquito species compared with the other land uses, whereas the restored dipterocarp forest had a low abundance of all mosquitos, in particular, An. balabacensis. No P. knowlesi was detected by PCR assay in any of the vectors collected during the study; however, P. inui, P. fieldi and P. vivax were detected in An. balabacensis. These findings indicate that restoring degraded natural forests with native species to closed canopy conditions reduces abundance of this zoonotic malarial mosquito vector and therefore should be incorporated into future restoration research and potentially contribute to the control strategies against simian malaria.

Simian malaria: a narrative review on emergence, epidemiology and threat to global malaria elimination

Simian malaria from wild non-human primate populations is increasingly recognised as a public health threat and is now the main cause of human malaria in Malaysia and some regions of Brazil. In 2022, Malaysia became the first country not to achieve malaria elimination due to zoonotic simian malaria. We review the global distribution and drivers of simian malaria and identify priorities for diagnosis, treatment, surveillance, and control. Environmental change is driving closer interactions between humans and wildlife, with malaria parasites from non-human primates spilling over into human populations and human malaria parasites spilling back into wild non-human primate populations. These complex transmission cycles require new molecular and epidemiological approaches to track parasite spread. Current methods of malaria control are ineffective, with wildlife reservoirs and primarily outdoor-biting mosquito vectors urgently requiring the development of novel control strategies. Without these, simian malaria has the potential to undermine malaria elimination globally.

Spatial analyses of Plasmodium knowlesi vectors with reference to control interventions in Malaysia

BACKGROUND

Malaria parasites such as Plasmodium knowlesi, P. inui, and P. cynomolgi are spread from macaques to humans through the Leucosphyrus Group of Anopheles mosquitoes. It is crucial to know the distribution of these vectors to implement effective control measures for malaria elimination. Plasmodium knowlesi is the most predominant zoonotic malaria parasite infecting humans in Malaysia.

METHODS

Vector data from various sources were used to create distribution maps from 1957 to 2021. A predictive statistical model utilizing logistic regression was developed using significant environmental factors. Interpolation maps were created using the inverse distance weighted (IDW) method and overlaid with the corresponding environmental variables.

RESULTS

Based on the IDW analysis, high vector abundances were found in the southwestern part of Sarawak, the northern region of Pahang and the northwestern part of Sabah. However, most parts of Johor, Sabah, Perlis, Penang, Kelantan and Terengganu had low vector abundance. The accuracy test indicated that the model predicted sampling and non-sampling areas with 75.3% overall accuracy. The selected environmental variables were entered into the regression model based on their significant values. In addition to the presence of water bodies, elevation, temperature, forest loss and forest cover were included in the final model since these were significantly correlated. Anopheles mosquitoes were mainly distributed in Peninsular Malaysia (Titiwangsa range, central and northern parts), Sabah (Kudat, West Coast, Interior and Tawau division) and Sarawak (Kapit, Miri, and Limbang). The predicted Anopheles mosquito density was lower in the southern part of Peninsular Malaysia, the Sandakan Division of Sabah and the western region of Sarawak.

CONCLUSION

The study offers insight into the distribution of the Leucosphyrus Group of Anopheles mosquitoes in Malaysia. Additionally, the accompanying predictive vector map correlates well with cases of P. knowlesi malaria. This research is crucial in informing and supporting future efforts by healthcare professionals to develop effective malaria control interventions.

High transmission efficiency of the simian malaria vectors and population expansion of their parasites Plasmodium cynomolgi and Plasmodium inui

BACKGROUND

The elimination of malaria in Southeast Asia has become more challenging as a result of rising knowlesi malaria cases. In addition, naturally occurring human infections with other zoonotic simian malaria caused by Plasmodium cynomolgi and Plasmodium inui adds another level of complexity in malaria elimination in this region. Unfortunately, data on vectors which are responsible for transmitting this zoonotic disease is very limited.

METHODOLOGY/PRINCIPAL FINDINGS

We conducted longitudinal studies to investigate the entomological parameters of the simian malaria vectors and to examine the genetic diversity and evolutionary pattern of their simian Plasmodium. All the captured Anopheles mosquitoes were dissected to examine for the presence of oocysts, sporozoites and to determine the parous rate. Our study revealed that the Anopheles Leucosphyrus Group mosquitoes are highly potential competent vectors, as evidenced by their high rate of parity, survival and sporozoite infections in these mosquitoes. Thus, these mosquitoes represent a risk of human infection with zoonotic simian malaria in this region. Haplotype analysis on P. cynomolgi and P. inui, found in high prevalence in the Anopheles mosquitoes from this study, had shown close relationship between simian Plasmodium from the Anopheles mosquitoes with its vertebrate hosts. This directly signifies the ongoing transmission between the vector, macaques, and humans. Furthermore, population genetic analysis showed significant negative values which suggest that both Plasmodium species are undergoing population expansion.

CONCLUSIONS/SIGNIFICANCE

With constant microevolutionary processes, there are potential for both P. inui and P. cynomolgi to emerge and spread as a major public health problem, following the similar trend of P. knowlesi. Therefore, concerted vector studies in other parts of Southeast Asia are warranted to better comprehend the transmission dynamics of this zoonotic simian malaria which eventually would aid in the implementation of effective control measures in a rapidly changing environment.

Cryptic Diversity and Demographic Expansion of Plasmodium knowlesi Malaria Vectors in Malaysia

Although Malaysia is considered free of human malaria, there has been a growing number of Plasmodium knowlesi cases. This alarming trend highlighted the need for our understanding of this parasite and its associated vectors, especially considering the role of genetic diversity in the adaptation and evolution among vectors in endemic areas, which is currently a significant knowledge gap in their fundamental biology. Thus, this study aimed to investigate the genetic diversity of Anopheles balabacensis, Anopheles cracens, Anopheles introlatus, and Anopheles latens-the vectors for P. knowlesi malaria in Malaysia. Based on cytochrome c oxidase 1 (CO1) and internal transcribed spacer 2 (ITS2) markers, the genealogic networks of An. latens showed a separation of the haplotypes between Peninsular Malaysia and Malaysia Borneo, forming two distinct clusters. Additionally, the genetic distances between these clusters were high (2.3-5.2% for CO1) and (2.3-4.7% for ITS2), indicating the likely presence of two distinct species or cryptic species within An. latens. In contrast, no distinct clusters were observed in An. cracens, An. balabacensis, or An. introlatus, implying a lack of pronounced genetic differentiation among their populations. It is worth noting that there were varying levels of polymorphism observed across the different subpopulations, highlighting some levels of genetic variation within these mosquito species. Nevertheless, further analyses revealed that all four species have undergone demographic expansion, suggesting population growth and potential range expansion for these vectors in this region.

Natural vectors of Plasmodium knowlesi and other primate, avian and ungulate malaria parasites in Narathiwat Province, Southern Thailand

To date, four species of simian malaria parasites including Plasmodium knowlesi, P. cynomolgi, P. inui and P. fieldi have been incriminated in human infections in Thailand. Although the prevalence of malaria in macaque natural hosts has been investigated, their vectors remain unknown in this country. Herein, we performed a survey of Anopheles mosquitoes during rainy and dry seasons in Narathiwat Province, Southern Thailand. Altogether 367 Anopheles mosquitoes were captured for 40 nights during 18:00 to 06:00 h by using human-landing catches. Based on morphological and molecular identification, species composition comprised An. maculatus (37.06%), An. barbirostris s.l. (31.34%), An. latens (17.71%), An. introlatus (10.08%) and others (3.81%) including An. umbrosus s.l., An. minimus, An. hyrcanus s.l., An. aconitus, An. macarthuri and An. kochi. Analyses of individual mosquitoes by PCR, sequencing and phylogenetic inference of the mitochondrial cytochrome genes of both malaria parasites and mosquitoes have revealed that the salivary gland samples of An. latens harbored P. knowlesi (n = 1), P. inui (n = 2), P. fieldi (n = 1), P. coatneyi (n = 1), P. hylobati (n = 1) and an unnamed Plasmodium species known to infect both long-tailed and pig-tailed macaques (n = 2). The salivary glands of An. introlatus possessed P. cynomolgi (n = 1), P. inui (n = 1), P. hylobati (n = 1) and coexistence of P. knowlesi and P. inui (n = 1). An avian malaria parasite P. juxtanucleare has been identified in the salivary gland sample of An. latens. Three other distinct lineages of Plasmodium with phylogenetic affinity to avian malaria species were detected in An. latens, An. introlatus and An. macarthuri. Interestingly, the salivary gland sample of An. maculatus contained P. caprae, an ungulate malaria parasite known to infect domestic goats. Most infected mosquitoes harbored multiclonal Plasmodium infections. All Plasmodium-infected mosquitoes were captured during the first quarter of the night and predominantly occurred during rainy season. Since simian malaria in humans has a wide geographic distribution in Thailand, further studies in other endemic areas of the country are mandatory for understanding transmission and prevention of zoonotic malaria.

Predicting Plasmodium knowlesi transmission risk across Peninsular Malaysia using machine learning-based ecological niche modeling approaches

The emergence of potentially life-threatening zoonotic malaria caused by Plasmodium knowlesi nearly two decades ago has continued to challenge Malaysia healthcare. With a total of 376 P. knowlesi infections notified in 2008, the number increased to 2,609 cases in 2020 nationwide. Numerous studies have been conducted in Malaysian Borneo to determine the association between environmental factors and knowlesi malaria transmission. However, there is still a lack of understanding of the environmental influence on knowlesi malaria transmission in Peninsular Malaysia. Therefore, our study aimed to investigate the ecological distribution of human P. knowlesi malaria in relation to environmental factors in Peninsular Malaysia. A total of 2,873 records of human P. knowlesi infections in Peninsular Malaysia from 1st January 2011 to 31st December 2019 were collated from the Ministry of Health Malaysia and geolocated. Three machine learning-based models, maximum entropy (MaxEnt), extreme gradient boosting (XGBoost), and ensemble modeling approach, were applied to predict the spatial variation of P. knowlesi disease risk. Multiple environmental parameters including climate factors, landscape characteristics, and anthropogenic factors were included as predictors in both predictive models. Subsequently, an ensemble model was developed based on the output of both MaxEnt and XGBoost. Comparison between models indicated that the XGBoost has higher performance as compared to MaxEnt and ensemble model, with AUC_ROC values of 0.933 ± 0.002 and 0.854 ± 0.007 for train and test datasets, respectively. Key environmental covariates affecting human P. knowlesi occurrence were distance to the coastline, elevation, tree cover, annual precipitation, tree loss, and distance to the forest. Our models indicated that the disease risk areas were mainly distributed in low elevation (75-345 m above mean sea level) areas along the Titiwangsa mountain range and inland central-northern region of Peninsular Malaysia. The high-resolution risk map of human knowlesi malaria constructed in this study can be further utilized for multi-pronged interventions targeting community at-risk, macaque populations, and mosquito vectors.

Perspectives of vector management in the control and elimination of vector-borne zoonoses

The complex transmission profiles of vector-borne zoonoses (VZB) and vector-borne infections with animal reservoirs (VBIAR) complicate efforts to break the transmission circuit of these infections. To control and eliminate VZB and VBIAR, insecticide application may not be conducted easily in all circumstances, particularly for infections with sylvatic transmission cycle. As a result, alternative approaches have been considered in the vector management against these infections. In this review, we highlighted differences among the environmental, chemical, and biological control approaches in vector management, from the perspectives of VZB and VBIAR. Concerns and knowledge gaps pertaining to the available control approaches were discussed to better understand the prospects of integrating these vector control approaches to synergistically break the transmission of VZB and VBIAR in humans, in line with the integrated vector management (IVM) developed by the World Health Organization (WHO) since 2004.

Life table analysis of Anopheles balabacensis, the primary vector of Plasmodium knowlesi in Sabah, Malaysia

BACKGROUND

Plasmodium knowlesi has become a major public health concern in Sabah, Malaysian Borneo, where it is now the only cause of indigenous malaria. The importance of P. knowlesi has spurred on a series of studies on this parasite, as well as on the biology and ecology of its principal vector, Anopheles balabacensis. However, there remain critical knowledge gaps on the biology of An. balabacensis, such as life history data and life table parameters. To fill these gaps, we conducted a life table study of An. balabacensis in the laboratory. Characterising vector life cycles and survival rates can inform more accurate estimations of the serial interval, the time between two linked cases, which is crucial to understanding and monitoring potentially changing transmission patterns.

METHODS

Individuals of An. balabacensis were collected in the field in Ranau district, Sabah to establish a laboratory colony. Induced mating was used, and the life history parameters of the progeny were recorded. The age-stage, two-sex life table approach was used in the analysis. The culture conditions in the laboratory were 9 h light:15 h dark, mean temperature 25.7 °C ± 0.05 and relative humidity 75.8% ± 0.31.

RESULTS

The eggs hatched within 2 days, and the larval stage lasted for 10.5 days in total, with duration of instar stages I, II, III and IV of 2.3, 3.7, 2.3, 2.2 days, respectively. The maximum total fecundity was 729 for one particular female, while the maximum female age-specific mean fecundity (m_x) was 142 at age 59 days. The gross reproductive rate or number of offspring per individual was about 102. On average, each female laid 1.81 ± 0.19 (range 1-7) batches of eggs, with 63% of the females producing only one batch; only one female laid six batches, while one other laid seven. Each batch comprised 159 ± 17.1 eggs (range 5-224) and the female ratio of offspring was 0.28 ± 0.06. The intrinsic rate of increase, finite rate of increase, net reproductive rate, mean generation time and doubling time were, respectively, 0.12 ± 0.01 day^-1, 1.12 ± 0.01 day^-1, 46.2 ± 14.97, 33.02 ± 1.85 and 5.97 days.

CONCLUSIONS

Both the net reproductive rate and intrinsic rate of increase of An. balabacensis are lower than those of other species in published studies. Our results can be used to improve models of P. knowlesi transmission and to set a baseline for assessing the impacts of environmental change on malaria dynamics. Furthermore, incorporating these population parameters of An. balabacensis into spatial and temporal models on the transmission of P. knowlesi would provide better insight and increase the accuracy of epidemiological forecasting.

Sero-prevalence of malaria and the knowledge, attitudes and practices relating to the prevention of malaria among indigenous people living in the central forest spine in Peninsular Malaysia: a mixed-methods study

Background

Malaria is still a major public health threat in some parts of the world. Many countries are targeting to achieve malaria free status country. This study aimed to determine the sero-prevalence of malaria and the knowledge, attitudes and practices relating to the prevention of malaria among the indigenous adults living in the central forest spine in Peninsular Malaysia.

Methods

A mixed method study was conducted in indigenous settlements in 2020. Blood film for malaria parasite (BFMP) was used to diagnose malaria in this study. A structured questionnaire was used to collect data from the participants. For the qualitative data, in-depth interviews were conducted and data was collected until data saturation was reached. Multiple linear regression was used to determine the predictors after adjusting for confounders. A p-value of < 0.05 is considered as statistically significant. Meaningful statements from the in-depth interviews were assigned to the relevant codes using NVivo version 12 software.

Results

A total of 284 indigenous people participated in the study. The prevalence of malaria in this study was 0%. Those in the middle age group between 25 and 41 years and tested positive for malaria previously were significantly more likely to have better knowledge and attitude scores. Significant correlations were also observed between knowledge-attitude and knowledge-practice. For the qualitative results, most of the respondents were unsure of monkey malaria, but all were aware of human malaria.

Conclusion

The present study highlighted the absence of malaria in the study population and relatively good knowledge, attitudes and practices relating to the prevention of malaria.

Narrative Review of the Control and Prevention of Knowlesi Malaria

Despite the reduction in the number of cases of human malaria throughout the world, the incidence rate of knowlesi malaria is continuing to rise, especially in Southeast Asia. The conventional strategies for the prevention and control of human malaria can provide some protection against knowlesi malaria. Despite the numerous studies on the risk factors and the innovative methods that may be used to prevent and control the vectors of Plasmodium knowlesi, the incidence rate remains high. An integrated approach that includes environmental intervention should be adopted in order to ensure the successful control of zoonotic malaria. A combination of personal-level protection, vector control and environmental control may mitigate the risk of Plasmodium knowlesi transmission from macaques to humans and, ultimately, reduce the incidence rate of knowlesi malaria.

Non-Human Primate Malaria Infections: A Review on the Epidemiology in Malaysia

Malaria remains a public health problem in many parts of the world, including Malaysia. Although Malaysia has been recognized as one of the countries free from indigenous human malaria since 2018, the rising trend of zoonotic malaria, particularly Plasmodium knowlesi cases, poses a threat to public health and is of great concern to the country’s healthcare system. We reviewed previously scattered information on zoonotic malaria infections in both Peninsular Malaysia and Malaysian Borneo to determine the epidemiology and distribution of emerging zoonotic malaria infections. Given the high prevalence of zoonotic malaria in Malaysia, efforts should be made to detect zoonotic malaria in humans, mosquito vectors, and natural hosts to ensure the success of the National Malaria Elimination Strategic Plan.

Plasmodium spp. in macaques, Macaca fascicularis, in Malaysia, and their potential role in zoonotic malaria transmission

Macaques, Macaca fascicularis, are a known reservoir of Plasmodium knowlesi, the agent of simian malaria which is the predominant zoonotic species affecting humans in Malaysia and other Southeast Asian countries. Recently, a naturally acquired human infection of another simian malaria parasite, P. cynomolgi has been reported. Thus, it is crucial to study the distribution of simian Plasmodium infections with particular attention to the macaques. Four hundred and nineteen (419) long-tailed macaques (Macaca fascicularis) were trapped in selected areas where human cases of P. knowlesi and P. cynomolgi have been reported. Nested polymerase chain reaction (PCR) was conducted to identify the Plasmodium spp., and circumsporozoite protein (CSP) genes of P. knowlesi samples were sequenced. Plasmodium cynomolgi infection was shown to be the most prevalent among the macaque population (68.4%). Although 50.6% of analyzed samples contained single infections either with P. knowlesi, P. cynomolgi, P. inui, P. coatneyi, or P. fieldi, mixed infections with double, triple, quadruple, and all 5 species were also detected. Infection with P. cynomolgi and P. knowlesi were the highest among Malaysian macaques in areas where humans and macaques are in close contact. The risk of zoonotic infection in these areas needs to be addressed since the number of zoonotic malaria cases is on the rise. With the elimination of human malaria, the risk of humans being infected with simian malaria is very high and steps should be taken to mitigate this issue.

Exposure of Primate Reservoir Hosts to Mosquito Vectors in Malaysian Borneo

Several vector-borne pathogens of primates have potential for human spillover. An example is the simian malaria Plasmodium knowlesi which is now a major public health problem in Malaysia. Characterization of exposure to mosquito vectors is essential for assessment of the force of infection within wild simian populations, however few methods exist to do so. Here we demonstrate the use of thermal imaging and mosquito magnet independence traps (MMIT) to assess the abundance, diversity and infection rates in mosquitoes host seeking near long-tailed macaque (Macaca fasicularis) sleeping sites in the Lower Kinabatangan Wildlife Sanctuary, Malaysian Borneo. The primary Plasmodium knowlesi vector, Anopheles balabacensis, was trapped at higher abundance near sleeping sites than control trees. Although none of the An. balabacensis collected (n = 15) were positive for P. knowlesi by PCR screening, two were infected with another simian malaria Plasmodium inui. Analysis of macaque stools from sleeping sites confirmed a high prevalence of Plasmodium infection, suspected to be P. inui. Recently, natural transmission of P. inui has been detected in humans and An. cracens in Peninsular Malaysia. The presence of P. inui in An. balabacensis here and previously in human-biting collections highlight its potential for spillover from macaques to humans in Sabah. We advocate the use of MMITs for non-invasive sampling of mosquito vectors that host seek on wild simian populations.

Plasmodium knowlesi: the game changer for malaria eradication

Plasmodium knowlesi is a zoonotic malaria parasite that has gained increasing medical interest over the past two decades. This zoonotic parasitic infection is prevalent in Southeast Asia and causes many cases with fulminant pathology. Despite several biogeographical restrictions that limit its distribution, knowlesi malaria cases have been reported in different parts of the world due to travelling and tourism activities. Here, breakthroughs and key information generated from recent (over the past five years, but not limited to) studies conducted on P. knowlesi were reviewed, and the knowledge gap in various research aspects that need to be filled was discussed. Besides, challenges and strategies required to control and eradicate human malaria with this emerging and potentially fatal zoonosis were described.

Zoonotic malaria transmission and land use change in Southeast Asia: what is known about the vectors

Zoonotic Plasmodium infections in humans in many Southeast Asian countries have been increasing, including in countries approaching elimination of human-only malaria transmission. Most simian malarias in humans are caused by Plasmodium knowlesi, but recent research shows that humans are at risk of many different simian Plasmodium species. In Southeast Asia, simian Plasmodium species are mainly transmitted by mosquitoes in the Anopheles leucosphyrus and Anopheles dirus complexes. Although there is some evidence of species outside the Leucosphyrus Group transmitting simian Plasmodium species, these await confirmation of transmission to humans. The vectors of monkey malarias are mostly found in forests and forest fringes, where they readily bite long-tailed and pig-tailed macaques (the natural reservoir hosts) and humans. How changing land-uses influence zoonotic malaria vectors is still poorly understood. Fragmentation of forests from logging, agriculture and other human activities is associated with increased zoonotic Plasmodium vector exposure. This is thought to occur through altered macaque and mosquito distributions and behaviours, and importantly, increased proximity of humans, macaques, and mosquito vectors. Underlying the increase in vector densities is the issue that the land-use change and human activities create more oviposition sites and, in correlation, increases availably of human blood hosts. The current understanding of zoonotic malaria vector species is largely based on a small number of studies in geographically restricted areas. What is known about the vectors is limited: the data is strongest for distribution and density with only weak evidence for a limited number of species in the Leucosphyrus Group for resting habits, insecticide resistance, blood feeding habits and larval habitats. More data are needed on vector diversity and bionomics in additional geographic areas to understand both the impacts on transmission of anthropogenic land-use change and how this significant disease in humans might be controlled.

Is there evidence of sustained human-mosquito-human transmission of the zoonotic malaria Plasmodium knowlesi? A systematic literature review

Background

The zoonotic malaria parasite Plasmodium knowlesi has emerged across Southeast Asia and is now the main cause of malaria in humans in Malaysia. A critical priority for P. knowlesi surveillance and control is understanding whether transmission is entirely zoonotic or is also occurring through human-mosquito-human transmission.

Methods

A systematic literature review was performed to evaluate existing evidence which refutes or supports the occurrence of sustained human-mosquito-human transmission of P. knowlesi. Possible evidence categories and study types which would support or refute non-zoonotic transmission were identified and ranked. A literature search was conducted on Medline, EMBASE and Web of Science using a broad search strategy to identify any possible published literature. Results were synthesized using the Synthesis Without Meta-analysis (SWiM) framework, using vote counting to combine the evidence within specific categories.

Results

Of an initial 7,299 studies screened, 131 studies were included within this review: 87 studies of P. knowlesi prevalence in humans, 14 studies in non-human primates, 13 studies in mosquitoes, and 29 studies with direct evidence refuting or supporting non-zoonotic transmission. Overall, the evidence showed that human-mosquito-human transmission is biologically possible, but there is limited evidence of widespread occurrence in endemic areas. Specific areas of research were identified that require further attention, notably quantitative analyses of potential transmission dynamics, epidemiological and entomological surveys, and ecological studies into the sylvatic cycle of the disease.

Conclusion

There are key questions about P. knowlesi that remain within the areas of research that require more attention. These questions have significant implications for malaria elimination and eradication programs. This paper considers limited but varied research and provides a methodological framework for assessing the likelihood of different transmission patterns for emerging zoonotic diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04110-z.

Multiplex PCR Assay for the Identification of Four Species of the Anopheles Leucosphyrus Sub-Group in Malaysia

Simple Summary

Plasmodium parasites cause malaria. The bites of infected female Anopheles mosquitoes, known as “malaria vectors,” transmit the parasites to people. To prevent the spread of malaria, precise mosquito species identification is essential. This study aims to develop a quick and accurate method for identifying the Anopheles species (An. introlatus, An. latens, An. cracens, and An. balabacensis), which have been incriminated as vectors for simian malaria in Malaysia. Overall, six primers targeting the internal transcribed spacer 2 (ITS2) region of each species were designed for this assay. This study is helpful for the researchers or vector-related field workers to correctly identify the mosquitoes for control activities.

Abstract

The Leucosphyrus Group of mosquitoes are the major simian malaria vectors in Malaysia. Accurate species identification is required to help in curbing the spread of simian malaria. The aim of the study is to provide an accurate molecular method for identifying the four important Anopheles vector species found in Malaysia. Mosquito specimens were collected from various localities in Malaysia, where simian malaria cases were reported. DNA from 122 mosquito specimens was tested to develop a multiplex polymerase chain reaction (PCR) assay. The specificity of this assay was tested against other mosquito species. Molecular identification of the species was further confirmed by analysing the internal transcribed spacer 2 (ITS2) DNA region of the specimens. Anopheles balabacensis and An. latens showed two distinct clades in the phylogenetic tree. The multiplex PCR assay was developed based on the ITS2 region for the identification of Anopheles introlatus (298–299 bp), Anopheles latens (197–198 bp), Anopheles cracens (421–426 bp), and Anopheles balabacensis (224–228 bp). This method will be useful to accurately identify the major Anopheles Leucosphyrus Group species in Malaysia, which are difficult to identify morphologically, to determine the correct vector as well as its geographical distribution.

Spatial distribution of Plasmodium knowlesi cases and their vectors in Johor, Malaysia: in light of human malaria elimination

Background

Plasmodium knowlesi, a simian malaria parasite infection, increases as Plasmodium falciparum and Plasmodium vivax infections decrease in Johor, Malaysia. Therefore, this study aimed to identify the distribution of vectors involved in knowlesi malaria transmission in Johor. This finding is vital in estimating hotspot areas for targeted control strategies.

Methods

Anopheles mosquitoes were collected from the location where P. knowlesi cases were reported. Cases of knowlesi malaria from 2011 to 2019 in Johor were analyzed. Internal transcribed spacers 2 (ITS2) and cytochrome c oxidase subunit I (COI) genes were used to identify the Leucosphyrus Group of Anopheles mosquitoes. In addition, spatial analysis was carried out on the knowlesi cases and vectors in Johor.

Results

One hundred and eighty-nine cases of P. knowlesi were reported in Johor over 10 years. Young adults between the ages of 20–39 years comprised 65% of the cases. Most infected individuals were involved in agriculture and army-related occupations (22% and 32%, respectively). Four hundred and eighteen Leucosphyrus Group Anopheles mosquitoes were captured during the study. Anopheles introlatus was the predominant species, followed by Anopheles latens. Spatial analysis by Kriging interpolation found that hotspot regions of P. knowlesi overlapped or were close to the areas where An. introlatus and An. latens were found. A significantly high number of vectors and P. knowlesi cases were found near the road within 0–5 km.

Conclusions

This study describes the distribution of P. knowlesi cases and Anopheles species in malaria-endemic transmission areas in Johor. Geospatial analysis is a valuable tool for studying the relationship between vectors and P. knowlesi cases. This study further supports that the Leucosphyrus Group of mosquitoes might be involved in transmitting knowlesi malaria cases in Johor. These findings may provide initial evidence to prioritize diseases and vector surveillance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03963-0.

Epidemiology of the zoonotic malaria Plasmodium knowlesi in changing landscapes

Within the past two decades, incidence of human cases of the zoonotic malaria Plasmodium knowlesi has increased markedly. P. knowlesi is now the most common cause of human malaria in Malaysia and threatens to undermine malaria control programmes across Southeast Asia. The emergence of zoonotic malaria corresponds to a period of rapid deforestation within this region. These environmental changes impact the distribution and behaviour of the simian hosts, mosquito vector species and human populations, creating new opportunities for P. knowlesi transmission. Here, we review how landscape changes can drive zoonotic disease emergence, examine the extent and causes of these changes across Southeast and identify how these mechanisms may be impacting P. knowlesi dynamics. We review the current spatial epidemiology of reported P. knowlesi infections in people and assess how these demographic and environmental changes may lead to changes in transmission patterns. Finally, we identify opportunities to improve P. knowlesi surveillance and develop targeted ecological interventions within these landscapes.

The vectors of Plasmodium knowlesi and other simian malarias Southeast Asia: challenges in malaria elimination

Plasmodium knowlesi, a simian malaria parasite of great public health concern has been reported from most countries in Southeast Asia and exported to various countries around the world. Currently P. knowlesi is the predominant species infecting humans in Malaysia. Besides this species, other simian malaria parasites such as P. cynomolgi and P. inui are also infecting humans in the region. The vectors of P. knowlesi and other Asian simian malarias belong to the Leucosphyrus Group of Anopheles mosquitoes which are generally forest dwelling species. Continual deforestation has resulted in these species moving into forest fringes, farms, plantations and human settlements along with their macaque hosts. Limited studies have shown that mosquito vectors are attracted to both humans and macaque hosts, preferring to bite outdoors and in the early part of the night. We here review the current status of simian malaria vectors and their parasites, knowledge of vector competence from experimental infections and discuss possible vector control measures. The challenges encountered in simian malaria elimination are also discussed. We highlight key knowledge gaps on vector distribution and ecology that may impede effective control strategies.

Plasmodium knowlesi detection methods for human infections-Diagnosis and surveillance

Within the overlapping geographical ranges of P. knowlesi monkey hosts and vectors in Southeast Asia, an estimated 1.5 billion people are considered at risk of infection. P. knowlesi can cause severe disease and death, the latter associated with delayed treatment occurring from misdiagnosis. Although microscopy is a sufficiently sensitive first-line tool for P. knowlesi detection for most low-level symptomatic infections, misdiagnosis as other Plasmodium species is common, and the majority of asymptomatic infections remain undetected. Current point-of-care rapid diagnostic tests demonstrate insufficient sensitivity and poor specificity for differentiating P. knowlesi from other Plasmodium species. Molecular tools including nested, real-time, and single-step PCR, and loop-mediated isothermal amplification (LAMP), are sensitive for P. knowlesi detection. However, higher cost and inability to provide the timely point-of-care diagnosis needed to guide appropriate clinical management has limited their routine use in most endemic clinical settings. P. knowlesi is likely underdiagnosed across the region, and improved diagnostic and surveillance tools are required. Reference laboratory molecular testing of malaria cases for both zoonotic and non-zoonotic Plasmodium species needs to be more widely implemented by National Malaria Control Programs across Southeast Asia to accurately identify the burden of zoonotic malaria and more precisely monitor the success of human-only malaria elimination programs. The implementation of specific serological tools for P. knowlesi would assist in determining the prevalence and distribution of asymptomatic and submicroscopic infections, the absence of transmission in certain areas, and associations with underlying land use change for future spatially targeted interventions.

Molecular epidemiology and population genomics of Plasmodium knowlesi

Molecular epidemiology has been central to uncovering P. knowlesi as an important cause of human malaria in Southeast Asia, and to understanding the complex nature of this zoonosis. Species-specific parasite detection and characterization of sequences were vital to show that P. knowlesi was distinct from the human parasite species that had been presumed to cause all malaria. With established sensitive and specific molecular detection tools, surveys subsequently indicated the distribution of P. knowlesi infections in humans, wild primate reservoir host species, and mosquito vector species. The importance of studying P. knowlesi genetic polymorphism was indicated initially by analysing a few nuclear gene loci as well as the mitochondrial genome, and subsequently by multi-locus microsatellite analyses and whole-genome sequencing. Different human infections generally have unrelated P. knowlesi genotypes, acquired from the diverse local parasite reservoirs in macaques. However, individual human infections are usually less genetically complex than those of wild macaques which experience more frequent superinfection with different P. knowlesi genotypes. Multi-locus analyses have revealed deep population subdivisions within P. knowlesi, which are structured both geographically and in relation to different macaque reservoir host species. Simplified genotypic discrimination assays now enable efficient large-scale surveillance of the sympatric P. knowlesi subpopulations within Malaysian Borneo. The whole-genome sequence analyses have also identified loci under recent positive natural selection in the P. knowlesi genome, with evidence that different loci are affected in different populations. These provide a foundation to understand recent adaptation of the zoonotic parasite populations, and to track and interpret future changes as they emerge.

Developing a Predictive Model for Plasmodium knowlesi-Susceptible Areas in Malaysia Using Geospatial Data and Artificial Neural Networks

Plasmodium knowlesi is an emerging species for malaria in Malaysia, particularly in East Malaysia. This infection contributes to almost half of all malaria cases and deaths in Malaysia and poses a challenge in eradicating malaria. The aim of this study was to develop a predictive model for P. knowlesi susceptibility areas in Sabah, Malaysia, using geospatial data and artificial neural networks (ANNs). Weekly malaria cases from 2013 to 2014 were used to identify the malaria hotspot areas. The association of malaria cases with environmental factors (elevation, water bodies, and population density, and satellite images providing rainfall, land surface temperature, and normalized difference vegetation indices) were statistically determined. The significant environmental factors were used as input for the ANN analysis to predict malaria cases. Finally, the malaria susceptibility index and zones were mapped out. The results suggested integrating geospatial data and ANNs to predict malaria cases, with overall correlation coefficient of 0.70 and overall accuracy of 91.04%. From the malaria susceptibility index and zoning analyses, it was found that areas located along the Crocker Range of Sabah and the East part of Sabah were highly susceptible to P. knowlesi infections. Following this analysis, targetted entomological mapping and malaria control programs can be initiated.

Risk factor of plasmodium knowlesi infection in Sabah Borneo Malaysia, 2020: A population-based case-control study

BACKGROUND

In the Malaysian state of Sabah, P. knowlesi notifications increased from 2% (59/2,741) of total malaria notifications in 2004 to 98% (2030/2,078) in 2017. There was a gap regarding P. knowlesi acquisition risk factors related to practice specifically in working age group. The main objective of this study was to identify the risk factors for acquiring P. knowlesi infection in Sabah among the working age group.

METHODS AND METHODS

This retrospective population-based case-control study was conducted in Ranau district to assess sociodemographic, behavioural and medical history risk factors using a pretested questionnaire. The data were entered and analyzed using IBM SPSS version 23. Bivariate analysis was conducted using binary logistic regression whereas multivariate analysis was conducted using multivariable logistic regression. We set a statistical significance at p-value less than or equal to 0.05.

RESULTS

A total of 266 cases and 532 controls were included in the study. Male gender (AOR = 2.71; 95% CI: 1.63-4.50), spending overnight in forest (AOR = 1.92; 95% CI: 1.20-3.06), not using mosquito repellent (AOR = 2.49; 95% CI: 1.36-4.56) and history of previous malaria infection (AOR = 49.34; 95% CI: 39.09-78.32) were found to be independent predictors of P. knowlesi infection.

CONCLUSIONS

This study showed the need to strengthen the strategies in preventing and controlling P. knowlesi infection specifically in changing the practice of spending overnight in forest and increasing the usage of personal mosquito repellent.

The seasonal dynamics and biting behavior of potential Anopheles vectors of Plasmodium knowlesi in Palawan, Philippines

BACKGROUND

A small number of human cases of the zoonotic malaria Plasmodium knowlesi have been reported in Palawan Island, the Philippines. Identification of potential vector species and their bionomics is crucial for understanding human exposure risk in this setting. Here, we combined longitudinal surveillance with a trap-evaluation study to address knowledge gaps about the ecology and potential for zoonotic spillover of this macaque malaria in Palawan Island.

METHODS

The abundance, diversity and biting behavior of human-biting Anopheles mosquitoes were assessed through monthly outdoor human landing catches (HLC) in three ecotypes representing different land use (forest edge, forest and agricultural area) across 8 months. Additionally, the host preference and biting activity of potential Anopheles vectors were assessed through comparison of their abundance and capture time in traps baited with humans (HLC, human-baited electrocuting net-HEN) or macaques (monkey-baited trap-MBT, monkey-baited electrocuting net-MEN). All female Anopheles mosquitoes were tested for the presence of Plasmodium parasites by PCR.

RESULTS

Previously incriminated vectors Anopheles balabacensis and An. flavirostris accounted for > 95% of anophelines caught in longitudinal surveillance. However, human biting densities were relatively low (An. balabacensis: 0.34-1.20 per night, An. flavirostris: 0-2 bites per night). Biting densities of An. balabacensis were highest in the forest edge, while An. flavirostris was most abundant in the agricultural area. The abundance of An. balabacensis and An. flavirostris was significantly higher in HLC than in MBT. None of the 357 female Anopheles mosquitoes tested for Plasmodium infection were positive.

CONCLUSIONS

The relatively low density and lack of malaria infection in Anopheles mosquitoes sampled here indicates that exposure to P. knowlesi in this setting is considerably lower than in neighboring countries (i.e. Malaysia), where it is now the primary cause of malaria in humans. Although anophelines had lower abundance in MBTs than in HLCs, An. balabacensis and An. flavirostris were caught by both methods, suggesting they could act as bridge vectors between humans and macaques. These species bite primarily outdoors during the early evening, confirming that insecticide-treated nets are unlikely to provide protection against P. knowlesi vectors.

Environmental and spatial risk factors for the larval habitats of Plasmodium knowlesi vectors in Sabah, Malaysian Borneo

Land-use changes, such as deforestation and agriculture, can influence mosquito vector populations and malaria transmission. These land-use changes have been linked to increased incidence in human cases of the zoonotic malaria Plasmodium knowlesi in Sabah, Malaysian Borneo. This study investigates whether these associations are partially driven by fine-scale land-use changes creating more favourable aquatic breeding habitats for P. knowlesi anopheline vectors. Using aerial remote sensing data, we developed a sampling frame representative of all land use types within a major focus of P. knowlesi transmission. From 2015 to 2016 monthly longitudinal surveys of larval habitats were collected in randomly selected areas stratified by land use type. Additional remote sensing data on environmental variables, land cover and landscape configuration were assembled for the study site. Risk factor analyses were performed over multiple spatial scales to determine associations between environmental and spatial variables and anopheline larval presence. Habitat fragmentation (300 m), aspect (350 m), distance to rubber plantations (100 m) and Culex larval presence were identified as risk factors for Anopheles breeding. Additionally, models were fit to determine the presence of potential larval habitats within the areas surveyed and used to generate a time-series of monthly predictive maps. These results indicate that land-use change and topography influence the suitability of larval habitats, and may partially explain the link between P. knowlesi incidence and deforestation. The predictive maps, and identification of the spatial scales at which risk factors are most influential may aid spatio-temporally targeted vector control interventions.

Residual Malaria Transmission in Select Countries of Asia-Pacific Region: Old Wine in a New Barrel

Background

Despite substantial reductions in malaria burden and improvement in case management, malaria remains a major public health challenge in the Asia-Pacific region. Residual malaria transmission (RMT) is the fraction of total transmission that persists after achievement of full operational coverage with effective insecticide-treated bed nets (ITNs)/long-lasting insecticidal nets (LLINs) and/or indoor residual spray interventions. There is a critical need to standardize and share best practices for entomological, anthropological, and product development investigative protocols to meet the challenges of RMT and elimination goals.

Methods

A systematic review was conducted to describe when and where RMT is occurring, while specifically targeting ownership and usage of ITN/LLINs, indoor residual spray application, insecticide susceptibility of vectors, and human and vector biting behavior, with a focus on nighttime activities.

Results

Sixty-six publications from 1995 to present met the inclusion criteria for closer review. Associations between local vector control coverage and use with behaviors of human and mosquito vectors varied by locality and circumstance. Consequently, the magnitude of RMT is insufficiently studied and analyzed with sparse estimates of individual exposure in communities, insufficient or incomplete observations of ITN/LLIN use, and the local human population movement into and from high-risk areas.

Conclusions

This review identified significant gaps or deficiencies that require urgent attention, namely, developing standardized procedures and methods to estimate risk exposure beyond the peridomestic setting, analytical approaches to measure key human-vector interactions, and seasonal location-specific agricultural or forest use calendars, and establishing the collection of longitudinal human and vector data close in time and location.

New vectors that are early feeders for Plasmodium knowlesi and other simian malaria parasites in Sarawak, Malaysian Borneo

Plasmodium knowlesi is the main cause of malaria in Sarawak, where studies on vectors of P. knowlesi have been conducted in only two districts. Anopheles balabacensis and An. donaldi were incriminated as vectors in Lawas and An. latens in Kapit. We studied a third location in Sarawak, Betong, where of 2169 mosquitoes collected over 36 days using human-landing catches, 169 (7.8%) were Anopheles spp. PCR and phylogenetic analyses identified P. knowlesi and/or P. cynomolgi, P. fieldi, P. inui, P. coatneyi and possibly novel Plasmodium spp. in salivary glands of An. latens and An. introlatus from the Leucosphyrus Group and in An. collessi and An. roperi from the Umbrosus Group. Phylogenetic analyses of cytochrome oxidase subunit I sequences indicated three P. knowlesi-positive An. introlatus had been misidentified morphologically as An. latens, while An. collessi and An. roperi could not be delineated using the region sequenced. Almost all vectors from the Leucosphyrus Group were biting after 1800 h but those belonging to the Umbrosus Group were also biting between 0700 and 1100 h. Our study incriminated new vectors of knowlesi malaria in Sarawak and underscores the importance of including entomological studies during the daytime to obtain a comprehensive understanding of the transmission dynamics of malaria.

Plasmodium knowlesi Malaria in Sabah, Malaysia, 2015-2017: Ongoing Increase in Incidence Despite Near-elimination of the Human-only Plasmodium Species

BACKGROUND

Malaysia aims to eliminate malaria by 2020. However, while cases of Plasmodium falciparum and Plasmodium vivax have decreased substantially, the incidence of zoonotic malaria from Plasmodium knowlesi continues to increase, presenting a major challenge to regional malaria control efforts. Here we report incidence of all Plasmodium species in Sabah, including zoonotic P. knowlesi, during 2015-2017.

METHODS

Microscopy-based malaria notification data and polymerase chain reaction (PCR) results were obtained from the Sabah Department of Health and State Public Health Laboratory, respectively, from January 2015 to December 2017. From January 2016 this was complemented by a statewide prospective hospital surveillance study. Databases were matched, and species was determined by PCR, or microscopy if PCR was not available.

RESULTS

A total of 3867 malaria cases were recorded between 2015 and 2017, with PCR performed in 93%. Using PCR results, and microscopy if PCR was unavailable, P. knowlesi accounted for 817 (80%), 677 (88%), and 2030 (98%) malaria cases in 2015, 2016, and 2017, respectively. P. falciparum accounted for 110 (11%), 45 (6%), and 23 (1%) cases and P. vivax accounted for 61 (6%), 17 (2%), and 8 (0.4%) cases, respectively. Of those with P. knowlesi, the median age was 35 (interquartile range: 24-47) years, and 85% were male.

CONCLUSIONS

Malaysia is approaching elimination of the human-only Plasmodium species. However, the ongoing increase in P. knowlesi incidence presents a major challenge to malaria control and warrants increased focus on knowlesi-specific prevention activities. Wider molecular surveillance in surrounding countries is required.

Prevalence of simian malaria parasites in macaques of Singapore

Plasmodium knowlesi is a simian malaria parasite currently recognized as the fifth causative agent of human malaria. Recently, naturally acquired P. cynomolgi infection in humans was also detected in Southeast Asia. The main reservoir of both parasites is the long-tailed and pig-tailed macaques, which are indigenous in this region. Due to increased urbanization and changes in land use, there has been greater proximity and interaction between the long-tailed macaques and the general population in Singapore. As such, this study aims to determine the prevalence of simian malaria parasites in local macaques to assess the risk of zoonosis to the general human population. Screening for the presence of malaria parasites was conducted on blood samples from 660 peridomestic macaques collected between Jan 2008 and Mar 2017, and 379 wild macaques collected between Mar 2009 and Mar 2017, using a Pan-Plasmodium-genus specific PCR. Positive samples were then screened using a simian Plasmodium species-specific nested PCR assay to identify the species of parasites (P. knowlesi, P. coatneyi, P. fieldi, P. cynomolgi, and P. inui) present. All the peridomestic macaques sampled were tested negative for malaria, while 80.5% of the 379 wild macaques were infected. All five simian Plasmodium species were detected; P. cynomolgi being the most prevalent (71.5%), followed by P. knowlesi (47.5%), P. inui (42.0%), P. fieldi (32.5%), and P. coatneyi (28.5%). Co-infection with multiple species of Plasmodium parasites was also observed. The study revealed that Singapore's wild long-tailed macaques are natural hosts of the five simian malaria parasite species, while no malaria was detected in all peridomestic macaques tested. Therefore, the risk of simian malaria transmission to the general human population is concluded to be low. However, this can be better demonstrated with the incrimination of the vectors of simian malaria parasites in Singapore.

Plasmodium knowlesi infecting humans in Southeast Asia: What's next?

Plasmodium knowlesi, a simian malaria parasite, has been in the limelight since a large focus of human P. knowlesi infection was reported from Sarawak (Malaysian Borneo) in 2004. Although this infection is transmitted across Southeast Asia, the largest number of cases has been reported from Malaysia. The increasing number of knowlesi malaria cases has been attributed to the use of molecular tools for detection, but environmental changes including deforestation likely play a major role by increasing human exposure to vector mosquitoes, which coexist with the macaque host. In addition, with the reduction in human malaria transmission in Southeast Asia, it is possible that human populations are at a greater risk of P. knowlesi infection due to diminishing cross-species immunity. Furthermore, the possibility of increasing exposure of humans to other simian Plasmodium parasites such as Plasmodium cynomolgi and Plasmodium inui should not be ignored. We here review the current status of these parasites in humans, macaques, and mosquitoes to support necessary reorientation of malaria control and elimination in the affected areas.

Proposed Integrated Control of Zoonotic Plasmodium knowlesi in Southeast Asia Using Themes of One Health

Zoonotic malaria, Plasmodium knowlesi, threatens the global progression of malaria elimination. Southeast Asian regions are fronting increased zoonotic malaria rates despite the control measures currently implemented-conventional measures to control human-malaria neglect P. knowlesi's residual transmission between the natural macaque host and vector. Initiatives to control P. knowlesi should adopt themes of the One Health approach, which details that the management of an infectious disease agent should be scrutinized at the human-animal-ecosystem interface. This review describes factors that have conceivably permitted the emergence and increased transmission rates of P. knowlesi to humans, from the understanding of genetic exchange events between subpopulations of P. knowlesi to the downstream effects of environmental disruption and simian and vector behavioral adaptations. These factors are considered to advise an integrative control strategy that aligns with the One Health approach. It is proposed that surveillance systems address the geographical distribution and transmission clusters of P. knowlesi and enforce ecological regulations that limit forest conversion and promote ecosystem regeneration. Furthermore, combining individual protective measures, mosquito-based feeding trapping tools and biocontrol strategies in synergy with current control methods may reduce mosquito population density or transmission capacity.

New vectors in northern Sarawak, Malaysian Borneo, for the zoonotic malaria parasite, Plasmodium knowlesi

Background

Plasmodium knowlesi is a significant cause of human malaria in Sarawak, Malaysian Borneo. Only one study has been previously undertaken in Sarawak to identify vectors of P. knowlesi, where Anopheles latens was incriminated as the vector in Kapit, central Sarawak. A study was therefore undertaken to identify malaria vectors in a different location in Sarawak.

Methods

Mosquitoes found landing on humans and resting on leaves over a 5-day period at two sites in the Lawas District of northern Sarawak were collected and identified. DNA samples extracted from salivary glands of Anopheles mosquitoes were subjected to nested PCR malaria-detection assays. The small subunit ribosomal RNA (SSU rRNA) gene of Plasmodium was sequenced, and the internal transcribed spacer 2 (ITS2) and mitochondrial cytochrome c oxidase subunit 1 (cox1) gene of the mosquitoes were sequenced from the Plasmodium-positive samples for phylogenetic analysis.

Results

Totals of 65 anophelines and 127 culicines were collected. By PCR, 6 An. balabacensis and 5 An. donaldi were found to have single P. knowlesi infections while 3 other An. balabacensis had either single, double or triple infections with P. inui, P. fieldi, P. cynomolgi and P. knowlesi. Phylogenetic analysis of the Plasmodium SSU rRNA gene confirmed 3 An. donaldi and 3 An. balabacensis with single P. knowlesi infections, while 3 other An. balabacensis had two or more Plasmodium species of P. inui, P. knowlesi, P. cynomolgi and some species of Plasmodium that could not be conclusively identified. Phylogenies inferred from the ITS2 and/or cox1 sequences of An. balabacensis and An. donaldi indicate that they are genetically indistinguishable from An. balabacensis and An. donaldi, respectively, found in Sabah, Malaysian Borneo.

Conclusions

Previously An. latens was identified as the vector for P. knowlesi in Kapit, central Sarawak, Malaysian Borneo, and now An. balabacensis and An. donaldi have been incriminated as vectors for zoonotic malaria in Lawas, northern Sarawak.

Human exposure to zoonotic malaria vectors in village, farm and forest habitats in Sabah, Malaysian Borneo

The zoonotic malaria parasite, Plasmodium knowlesi, is now a substantial public health problem in Malaysian Borneo. Current understanding of P. knowlesi vector bionomics and ecology in Sabah comes from a few studies near the epicentre of human cases in one district, Kudat. These have incriminated Anopheles balabacensis as the primary vector, and suggest that human exposure to vector biting is peri-domestic as well as in forest environments. To address the limited understanding of vector ecology and human exposure risk outside of Kudat, we performed wider scale surveillance across four districts in Sabah with confirmed transmission to investigate spatial heterogeneity in vector abundance, diversity and infection rate. Entomological surveillance was carried out six months after a cross-sectional survey of P. knowlesi prevalence in humans throughout the study area; providing an opportunity to investigate associations between entomological indicators and infection. Human-landing catches were performed in peri-domestic, farm and forest sites in 11 villages (3-4 per district) and paired with estimates of human P. knowlesi exposure based on sero-prevalence. Anopheles balabacensis was present in all districts but only 6/11 villages. The mean density of An. balabacensis was relatively low, but significantly higher in farm (0.094/night) and forest (0.082/night) than peri-domestic areas (0.007/night). Only one An. balabacensis (n = 32) was infected with P. knowlesi. Plasmodium knowlesi sero-positivity in people was not associated with An. balabacensis density at the village-level however post hoc analyses indicated the study had limited power to detect a statistical association due low vector density. Wider scale sampling revealed substantial heterogeneity in vector density and distribution between villages and districts. Vector-habitat associations predicted from this larger-scale surveillance differed from those inferred from smaller-scale studies in Kudat; highlighting the importance of local ecological context. Findings highlight potential trade-offs between maximizing temporal versus spatial breadth when designing entomological surveillance; and provide baseline entomological and epidemiological data to inform future studies of entomological risk factors for human P. knowlesi infection.

Natural Wolbachia infection in field-collected Anopheles and other mosquito species from Malaysia

BACKGROUND

The endosymbiont bacterium Wolbachia is maternally inherited and naturally infects some filarial nematodes and a diverse range of arthropods, including mosquito vectors responsible for disease transmission in humans. Previously, it has been found infecting most mosquito species but absent in Anopheles and Aedes aegypti. However, recently these two mosquito species were found to be naturally infected with Wolbachia. We report here the extent of Wolbachia infections in field-collected mosquitoes from Malaysia based on PCR amplification of the Wolbachia wsp and 16S rRNA genes.

METHODS

The prevalence of Wolbachia in Culicinae mosquitoes was assessed via PCR with wsp primers. For some of the mosquitoes, in which the wsp primers failed to amplify a product, Wolbachia screening was performed using nested PCR targeting the 16S rRNA gene. Wolbachia sequences were aligned using Geneious 9.1.6 software, analyzed with BLAST, and the most similar sequences were downloaded. Phylogenetic analyses were carried out with MEGA 7.0 software. Graphs were drawn with GraphPad Prism 8.0 software.

RESULTS

A total of 217 adult mosquitoes representing 26 mosquito species were screened. Of these, infections with Wolbachia were detected in 4 and 15 mosquito species using wsp and 16S rRNA primers, respectively. To our knowledge, this is the first time Wolbachia was detected using 16S rRNA gene amplification, in some Anopheles species (some infected with Plasmodium), Culex sinensis, Culex vishnui, Culex pseudovishnui, Mansonia bonneae and Mansonia annulifera. Phylogenetic analysis based on wsp revealed Wolbachia from most of the mosquitoes belonged to Wolbachia Supergroup B. Based on 16S rRNA phylogenetic analysis, the Wolbachia strain from Anopheles mosquitoes were more closely related to Wolbachia infecting Anopheles from Africa than from Myanmar.

CONCLUSIONS

Wolbachia was found infecting Anopheles and other important disease vectors such as Mansonia. Since Wolbachia can affect its host by reducing the life span and provide resistance to pathogen infection, several studies have suggested it as a potential innovative tool for vector/vector-borne disease control. Therefore, it is important to carry out further studies on natural Wolbachia infection in vector mosquitoes' populations as well as their long-term effects in new hosts and pathogen suppression.

Natural Human Infections With Plasmodium cynomolgi and Other Malaria Species in an Elimination Setting in Sabah, Malaysia

To determine the presence and species composition of malaria infections, we screened a subset of samples collected during a cross-sectional survey in Northern Sabah, Malaysia using highly sensitive molecular techniques. Results identified 54 asymptomatic submicroscopic malaria infections, including a large cluster of Plasmodium falciparum and 3 P. knowlesi infections. We additionally identified 2 monoinfections with the zoonotic malaria Plasmodium cynomolgi, both in individuals reporting no history of forest activities or contact with macaques. Results highlight the need for improved surveillance strategies to detect these infections and determine public health impacts.

Indoor and outdoor residual spraying of a novel formulation of deltamethrin K-Othrine® (Polyzone) for the control of simian malaria in Sabah, Malaysia

Since 2000, human malaria cases in Malaysia were rapidly reduced with the use of insecticides in Indoor Residual Spray (IRS) and Long-Lasting Insecticide Net (LLIN). Unfortunately, monkey malaria in humans has shown an increase especially in Sabah and Sarawak. The insecticide currently used in IRS is deltamethrin K-Othrine® WG 250 wettable granule, targeting mosquitoes that rest and feed indoor. In Sabah, the primary vector for knowlesi malaria is An. balabacensis a species known to bite outdoor. This study evaluates an alternative method, the Outdoor Residual Spray (ORS) using a novel formulation of deltamethrin K-Othrine® (PolyZone) to examine it suitability to control knowlesi malaria vector in Sabah, compared to the current method. The study was performed at seven villages in Sabah having similar type of houses (wood, bamboo and concrete). Houses were sprayed with deltamethrin K-Othrine® (PolyZone) at two different dosages, 25 mg/m2 and 30 mg/m2 and deltamethrin K-Othrine® WG 250 wettable granule at 25 mg/m2, sprayed indoor and outdoor. Residual activity on different walls was assessed using standard cone bioassay techniques. For larval surveillances, potential breeding sites were surveyed. Larvae were collected and identified, pre and post spraying. Adult survey was done using Human Landing Catch (HLC) performed outdoor and indoor. Detection of malaria parasite in adults was conducted via microscopy and molecular methods. Deltamethrin K-Othrine® (PolyZone) showed higher efficacy when sprayed outdoor. The efficacy was found varied when sprayed on different types of wall surfaces. Deltamethrin K-Othrine® (PolyZone) at 25 mg/m2 was the most effective with regards to ability to high mortality and effective knock down (KD). The vector population was reduced significantly post-spraying and reduction in breeding sites as well. The number of simian malaria infected vector, human and simian malaria transmission were also greatly reduced.

A Review of Published Literature Regarding Health Issues of Coastal Communities in Sabah, Malaysia

Several of the coastal zones in Sabah, Malaysia, are isolated and inaccessible. This study aimed to review the published literature on the health status of the coastal communities in Sabah. The following four main health issues were found: (i) malaria, (ii) tuberculosis (TB), (iii) seafood poisoning, and (iv) antenatal problems. Factors associated with the risk of acquiring malarial infection in the studied coastal area were advanced age, male sex, farming as an occupation, history of travel outside the village, and rainy seasons. TB infection was primarily observed in adult men. Seafood poisoning was significantly common in Sabah. Studies have reported that tetrodotoxin and paralytic shellfish poisoning were commonly reported (30-60 cases annually). Several pregnant women in the coastal community had insufficient knowledge of the national antenatal care programme. Nonetheless, 99% of them received antenatal care at public healthcare facilities with 92% of them undergoing safe delivery. Nevertheless, a majority of the pregnant women had iodine deficiency due to low iodised salt intake. Findings from this review highlighted that the coastal communities in Sabah are experiencing significant health problems. Specific attention is required to significantly enhance the health and well-being of the individuals living in the coastal communities in Sabah.

Updates on malaria incidence and profile in Malaysia from 2013 to 2017

Background

To date, most of the recent publications on malaria in Malaysia were conducted in Sabah, East Malaysia focusing on the emergence of Plasmodium knowlesi. This analysis aims to describe the incidence, mortality and case fatality rate of malaria caused by all Plasmodium species between Peninsular Malaysia and East Malaysia (Sabah and Sarawak) over a 5-year period (2013–2017).

Methods

This is a secondary data review of all diagnosed and reported malaria confirmed cases notified to the Ministry of Health, Malaysia between January 2013 and December 2017.

Results

From 2013 to 2017, a total of 16,500 malaria cases were notified in Malaysia. The cases were mainly contributed from Sabah (7150; 43.3%) and Sarawak (5684; 34.4%). Majority of the patients were male (13,552; 82.1%). The most common age group in Peninsular Malaysia was 20 to 29 years (1286; 35.1%), while Sabah and Sarawak reported highest number of malaria cases in age group of 30 to 39 years (2776; 21.6%). The top two races with malaria in Sabah and Sarawak were Bumiputera Sabah (5613; 43.7%) and Bumiputera Sarawak (4512; 35.1%), whereas other ethnic group (1232; 33.6%) and Malays (1025; 28.0%) were the two most common races in Peninsular Malaysia. Plasmodium knowlesi was the commonest species in Sabah and Sarawak (9902; 77.1%), while there were more Plasmodium vivax cases (1548; 42.2%) in Peninsular Malaysia. The overall average incidence rate, mortality rate and case fatality rates for malaria from 2013 to 2017 in Malaysia were 0.106/1000, 0.030/100,000 and 0.27%, respectively. Sarawak reported the highest average incidence rate of 0.420/1000 population followed by Sabah (0.383/1000). Other states in Peninsular Malaysia reported below the national average incidence rate with less than 0.100/1000.

Conclusions

There were different trends and characteristics of notified malaria cases in Peninsular Malaysia and Sabah and Sarawak. They provide useful information to modify current prevention and control measures so that they are customised to the peculiarities of disease patterns in the two regions in order to successfully achieve the pre-elimination of human-only species in the near future.

Anopheles sundaicus Mosquitoes as Vector for Plasmodium knowlesi, Andaman and Nicobar Islands, India

Using PCR and sequencing, we found Plasmodium knowlesi in the malaria vector Anopheles sundaicus mosquito collected from Katchal Island in the Andaman and Nicobar Islands, India. We cannot rule out natural transmission of this parasite to humans through this mosquito species. An in-depth investigation is needed to prevent disease outbreaks.

The Role of Ecological Linkage Mechanisms in Plasmodium knowlesi Transmission and Spread

Defining the linkages between landscape change, disease ecology and human health is essential to explain and predict the emergence of Plasmodium knowlesi malaria, a zoonotic parasite residing in Southeast Asian macaques, and transmitted by species of Anopheles mosquitos. Changing patterns of land use throughout Southeast Asia, particularly deforestation, are suggested to be the primary drivers behind the recent spread of this zoonotic parasite in humans. Local ecological changes at the landscape scale appear to be increasing the risk of disease in humans by altering the dynamics of transmission between the parasite and its primary hosts. This paper will focus on the emergence of P. knowlesi in humans in Malaysian Borneo and the ecological linkage mechanisms suggested to be playing an important role.

Long-Tailed Macaque Response to Deforestation in a Plasmodium knowlesi-Endemic Area

Land-use changes can impact infectious disease transmission by increasing spatial overlap between people and wildlife disease reservoirs. In Malaysian Borneo, increases in human infections by the zoonotic malaria Plasmodium knowlesi are hypothesised to be due to increasing contact between people and macaques due to deforestation. To explore how macaque responses to environmental change impact disease risks, we analysed movement of a GPS-collared long-tailed macaque in a knowlesi-endemic area in Sabah, Malaysia, during a deforestation event. Land-cover maps were derived from satellite-based and aerial remote sensing data and models of macaque occurrence were developed to evaluate how macaque habitat use was influenced by land-use change. During deforestation, changes were observed in macaque troop home range size, movement speeds and use of different habitat types. Results of models were consistent with the hypothesis that macaque ranging behaviour is disturbed by deforestation events but begins to equilibrate after seeking and occupying a new habitat, potentially impacting human disease risks. Further research is required to explore how these changes in macaque movement affect knowlesi epidemiology on a wider spatial scale.

Local human movement patterns and land use impact exposure to zoonotic malaria in Malaysian Borneo

Human movement into insect vector and wildlife reservoir habitats determines zoonotic disease risks; however, few data are available to quantify the impact of land use on pathogen transmission. Here, we utilise GPS tracking devices and novel applications of ecological methods to develop fine-scale models of human space use relative to land cover to assess exposure to the zoonotic malaria Plasmodium knowlesi in Malaysian Borneo. Combining data with spatially explicit models of mosquito biting rates, we demonstrate the role of individual heterogeneities in local space use in disease exposure. At a community level, our data indicate that areas close to both secondary forest and houses have the highest probability of human P. knowlesi exposure, providing quantitative evidence for the importance of ecotones. Despite higher biting rates in forests, incorporating human movement and space use into exposure estimates illustrates the importance of intensified interactions between pathogens, insect vectors and people around habitat edges.

Vector compositions change across forested to deforested ecotones in emerging areas of zoonotic malaria transmission in Malaysia

In lowland areas of Malaysia, Plasmodium knowlesi infection is associated with land use change and high proportions of the vector Anopheles balabacensis. We conducted a 15-month study in two Malaysian villages to determine the effect of habitat on vector populations in understudied high-altitude, high-incidence districts. Anopheles mosquitoes were sampled in human settlements, plantations and forest edges, and screened for Plasmodium species by PCR. We report the first An. donaldi positive for P. knowlesi. This potential vector was associated with habitat fragmentation measured as disturbed forest edge:area ratio, while An. balabacensis was not, indicating fragmented land use could favour An. donaldi. Anopheline species richness and diversity decreased from forest edge, to plantation, to human settlement. Greater numbers of An. balabacensis and An. donaldi were found in forest edges compared to human settlements, suggesting exposure to vectors and associated zoonoses may be greater for people entering this habitat.

Genetic characterisation of the erythrocyte-binding protein (PkβII) of Plasmodium knowlesi isolates from Malaysia

Plasmodium knowlesi contributes to the majority of human malaria incidences in Malaysia. Its uncontrollable passage among the natural monkey hosts can potentially lead to zoonotic outbreaks. The merozoite of this parasite invades host erythrocytes through interaction between its erythrocyte-binding proteins (EBPs) and their respective receptor on the erythrocytes. The regionII of P. knowlesi EBP, P. knowlesi beta (PkβII) protein is found to be mediating merozoite invasion into monkey erythrocytes by interacting with sialic acid receptors. Hence, the objective of this study was to investigate the genetic diversity, natural selection and haplotype grouping of PkβII of P. knowlesi isolates in Malaysia. Polymerase chain reaction amplifications of PkβII were performed on archived blood samples from Malaysia and 64 PkβII sequences were obtained. Sequence analysis revealed length polymorphism, and its amino acids at critical residues indicate the ability of PkβII to mediate P. knowlesi invasion into monkey erythrocytes. Low genetic diversity (π = 0.007) was observed in the PkβII of Malaysia Borneo compared to Peninsular Malaysia (π = 0.015). The PkβII was found to be under strong purifying selection to retain infectivity in monkeys and it plays a limited role in the zoonotic potential of P. knowlesi. Its haplotypes could be clustered into Peninsular Malaysia and Malaysia Borneo groups, indicating the existence of two distinct P. knowlesi parasites in Malaysia as reported in an earlier study.

Effect of different habitat types on abundance and biting times of Anopheles balabacensis Baisas (Diptera: Culicidae) in Kudat district of Sabah, Malaysia

BACKGROUND

We investigated the effect of five common habitat types on the diversity and abundance of Anopheles spp. and on the biting rate and time of Anopheles balabacensis (currently the only known vector for Plasmodium knowlesi in Sabah) at Paradason village, Kudat, Sabah. The habitats were forest edge, playground area, longhouse, oil palm plantation and shrub-bushes area. Sampling of Anopheles was done monthly using the human landing catch method in all habitat types for 14 months (October 2013 to December 2014, excluding June 2014). The Anopheles species were morphologically identified and subjected to PCR assay for the detection of Plasmodium parasites. Generalised linear mixed models (GLMM) were applied to test the variation in abundance and biting rates of An. balabacensis in different habitat types.

RESULTS

A total of 1599 Anopheles specimens were collected in the village, of which about 90% were An. balabacensis. Anopheles balabacensis was present throughout the year and was the dominant Anopheles species in all habitat types. The shrub bushes habitat had the highest Anopheles species diversity while forest edge had the greatest number of Anopheles individuals caught. GLMM analysis indicated that An. balabacensis abundance was not affected by the type of habitats, and it was more active during the early and late night compared to predawn and dawn. PCR assay showed that 1.61% of the tested An. balabacensis were positive for malaria parasites, most of which were caught in oil palm estates and infected with one to two Plasmodium species.

CONCLUSIONS

The identification of infected vectors in a range of habitats, including agricultural and farming areas, illustrates the potential for humans to be exposed to P. knowlesi outside forested areas. This finding contributes to a growing body of evidence implicating environmental changes due to deforestation, expansion of agricultural and farming areas, and development of human settlements near to forest fringes in the emergence of P. knowlesi in Sabah.