Population structure and gene flow of Anopheles farauti s.s. (Diptera: Culicidae) among ten sites on five islands of Vanuatu: implications for malaria control

Plasmodium vivax and Plasmodium falciparum at the crossroads of exchange among islands in Vanuatu: implications for malaria elimination strategies

Understanding the transmission and movement of Plasmodium parasites is crucial for malaria elimination and prevention of resurgence. Located at the limit of malaria transmission in the Pacific, Vanuatu is an ideal candidate for elimination programs due to low endemicity and the isolated nature of its island setting. We analyzed the variation in the merozoite surface protein 1 (msp1) and the circumsporozoite protein (csp) of P. falciparum and P. vivax populations to examine the patterns of gene flow and population structures among seven sites on five islands in Vanuatu. Genetic diversity was in general higher in P. vivax than P. falciparum from the same site. In P. vivax, high genetic diversity was likely maintained by greater extent of gene flow among sites and among islands. Consistent with the different patterns of gene flow, the proportion of genetic variance found among islands was substantially higher in P. falciparum (28.81–31.23%) than in P. vivax (-0.53–3.99%). Our data suggest that the current island-by-island malaria elimination strategy in Vanuatu, while adequate for P. falciparum elimination, might need to be complemented with more centrally integrated measures to control P. vivax movement across islands.

Anopheles punctulatus group: evolution, distribution, and control

The major malaria vectors of the Southwest Pacific belong to a group of closely related mosquitoes known as the Anopheles punctulatus group. The group comprises 13 co-occurring species that either are isomorphic or carry overlapping morphological features, and today several species remain informally named. The advent of species-diagnostic molecular tools in the 1990s permitted a new raft of studies into the newly differentiated mosquitoes of this group, and these have revealed five species as the region's primary malaria vectors: An. farauti, An. hinesorum, An. farauti 4, An. koliensis, and An. punctulatus. Species' distributions are now well established across Papua New Guinea, northern Australia, and the Solomon Archipelago, but little has been documented thus far in eastern Indonesia. As each species reveals significant differences in distribution and biology, the relative paucity of knowledge of their biology or ecology in relation to malaria transmission is brought into clearer focus. Only three of the species have undergone some form of spatial or population genetics analyses, and this has revealed striking differences in their genetic signatures throughout the region. This review compiles and dissects the key findings for this important mosquito group and points to where future research should focus to maximize the output of field studies in developing relevant knowledge on these malaria vectors.

The dominant Anopheles vectors of human malaria in the Asia-Pacific region: occurrence data, distribution maps and bionomic précis

BACKGROUND

The final article in a series of three publications examining the global distribution of 41 dominant vector species (DVS) of malaria is presented here. The first publication examined the DVS from the Americas, with the second covering those species present in Africa, Europe and the Middle East. Here we discuss the 19 DVS of the Asian-Pacific region. This region experiences a high diversity of vector species, many occurring sympatrically, which, combined with the occurrence of a high number of species complexes and suspected species complexes, and behavioural plasticity of many of these major vectors, adds a level of entomological complexity not comparable elsewhere globally. To try and untangle the intricacy of the vectors of this region and to increase the effectiveness of vector control interventions, an understanding of the contemporary distribution of each species, combined with a synthesis of the current knowledge of their behaviour and ecology is needed.

RESULTS

Expert opinion (EO) range maps, created with the most up-to-date expert knowledge of each DVS distribution, were combined with a contemporary database of occurrence data and a suite of open access, environmental and climatic variables. Using the Boosted Regression Tree (BRT) modelling method, distribution maps of each DVS were produced. The occurrence data were abstracted from the formal, published literature, plus other relevant sources, resulting in the collation of DVS occurrence at 10116 locations across 31 countries, of which 8853 were successfully geo-referenced and 7430 were resolved to spatial areas that could be included in the BRT model. A detailed summary of the information on the bionomics of each species and species complex is also presented.

CONCLUSIONS

This article concludes a project aimed to establish the contemporary global distribution of the DVS of malaria. The three articles produced are intended as a detailed reference for scientists continuing research into the aspects of taxonomy, biology and ecology relevant to species-specific vector control. This research is particularly relevant to help unravel the complicated taxonomic status, ecology and epidemiology of the vectors of the Asia-Pacific region. All the occurrence data, predictive maps and EO-shape files generated during the production of these publications will be made available in the public domain. We hope that this will encourage data sharing to improve future iterations of the distribution maps.

Implementation of a novel PCR based method for detecting malaria parasites from naturally infected mosquitoes in Papua New Guinea

Background

Detection of Plasmodium species in mosquitoes is important for designing vector control studies. However, most of the PCR-based detection methods show some potential limitations. The objective of this study was to introduce an effective PCR-based method for detecting Plasmodium vivax and Plasmodium falciparum from the field-caught mosquitoes of Papua New Guinea.

Methods

A method has been developed to concurrently detect mitochondrial cytochrome b (Cyt b) of four human Plasmodium species using PCR (Cytb-PCR). To particularly discriminate P. falciparum from P. vivax, Plasmodium ovale and Plasmodium malariae, a polymerase chain reaction-repeated fragment length polymorphism (PCR-RFLP) has further been developed to use with this method. However, due to limited samples number of P. ovale and P. malariae; this study was mainly confined to P. vivax and P. falciparum. The efficiency of Cytb-PCR was evaluated by comparing it with two 'gold standards' enzyme linked immunosorbent assay specific for circumsporozoite protein (CS-ELISA) using artificially infected mosquitoes; and nested PCR specific for small subunit ribosomal RNA (SSUrRNA) using field caught mosquitoes collected from three areas (Kaboibus, Wingei, and Jawia) of the East Sepic Province of Papua New Guinea.

Results

A total of 90 mosquitoes were artificially infected with three strains of Plasmodium: P. vivax-210 (n = 30), P. vivax-247 (n = 30) and P. falciparum (n = 30). These infected mosquitoes along with another 32 unfed mosquitoes were first checked for the presence of Plasmodium infection by CS-ELISA, and later the same samples were compared with the Cytb-PCR. CS-ELISA for P. vivax-210, P. vivax-247 and P. falciparum detected positive infection in 30, 19 and 18 mosquitoes respectively; whereas Cytb-PCR detected 27, 16 and 16 infections, respectively. The comparison revealed a close agreement between the two assays (κ = 0.862, 0.842 and 0.894, respectively for Pv-210, Pv-247 and P. falciparum groups). It was found that the eight CS-ELISA-positive mosquitoes detected negative by Cytb-PCR were false-positive results. The lowest detection limit of this Cytb-PCR was 10 sporozoites. A highly concordance result was also found between nested PCR and Cytb-PCR using 107 field caught mosquitoes, and both tests concordantly detected P. falciparum in an Anopheles punctulatus mosquito collected from Kaboibus. Both tests thus suggested an overall sporozoite rate of 0.9% (1/107) in the study areas. Subsequently, PCR-RFLP efficiently discriminated P. falciparum from P. vivax for all of the Cytb-PCR positive samples.

Conclusion

A single step PCR based method has been introduced here that is highly sensitive, efficient and reliable for identifying P. vivax and P. falciparum from mosquitoes. The reliability of the technique was confirmed by its ability to detect Plasmodium as efficiently as those of CS-ELISA and nested PCR. Application of the assay offers the opportunity to detect vector species of Papua New Guinea and may contribute for designing further vector control programmes.