Anopheles diversity, biting behaviour and transmission potential in forest and farm environments of Gia Lai province, Vietnam.

BACKGROUND: Despite recent reductions in Vietnam, malaria transmission persists in some areas in forests and farmlands where a high density of Anopheles mosquitoes relative to other environments occurs. To inform effective malaria control measures, it is important to understand vector bionomics and the malaria transmission role of Anopheles spp. in the highland regions of Vietnam. This study was conducted to quantify the abundance, composition and biting behaviour of the Anopheles mosquito population, and the proportion of Plasmodium spp. infected mosquitoes collected from forest and agricultural farm sites in Gia Lai province, Vietnam. METHODS: Forest and agricultural farm sites in Gia Lai province were selected for mosquito collections (total eight sites). Mosquito collection was performed by Human-baited Double Net Trap (HDNT), animal-baited traps (ABT) using cattle, and CDC light traps. Captured mosquitoes were identified morphologically, and salivary glands of Anopheles mosquitoes were examined for sporozoites using microscopy. Plasmodium infection was determined by Polymerase Chain Reaction (PCR), and identification of blood meal type was determined by PCR and diffuse serum agglutination assay. RESULTS: A total of 1815 Anopheles mosquitoes belonging to 19 species were collected by ABT (n = 1169), HDNT (n = 471) and CDC light trap (n = 175). Anopheles abundance and diversity varied by district and environment. Capture by HDNT of Anopheles of vectorial concern was observed between early evening and early morning. Plasmodium vivax infection was determined by PCR in two Anopheles dirus specimens captured by HDNT in forest sites. Blood from a range of hosts could, including human blood, could be detected in species considered primary and secondary vectors An. dirus, and Anopheles aconitus, and Anopheles maculatus, respectively. CONCLUSIONS: A low number of Anopheles spp. considered primary vectors of concern and very low numbers of Plasmodium spp. infected Anopheles mosquitoes were captured at the end of the rainy season in the Central Highlands of Vietnam. However, capture species of vectorial concern by HDNT throughout the early to late evening demonstrates that use of additional personal protective measures could supplement current preventative measures, such as bed nets to prevent exposure to vectors of concern in this region.

Characterising residual malaria transmission in forested areas with low coverage of core vector control in central Viet Nam.

BACKGROUND: Despite great success in significantly reducing the malaria burden in Viet Nam over recent years, the ongoing presence of malaria vectors and Plasmodium infection in remote forest areas and among marginalised groups presents a challenge to reaching elimination and a threat to re-emergence of transmission. Often transmission persists in a population despite high reported coverage of long-lasting insecticidal nets (LLINs), the mainstay control method for malaria. To investigate what factors may contribute to this, a mixed-methods study was conducted in Son Thai commune, a community in south-central Viet Nam that has ongoing malaria cases despite universal LLIN coverage. A cross-sectional behavioural and net-coverage survey was conducted along with observations of net use and entomological collections in the village, farm huts and forest sites used by members of the community. RESULTS: Most community members owned a farm hut plot and 71.9% of adults aged 18+ years sometimes slept overnight in the farm hut, while one-third slept overnight in the forest. Ownership and use of nets in the village households was high but in the farm huts and forest was much lower; only 44.4% reported regularly using a bednet in the farm and 12.1% in the forest. No primary anopheline species were captured in the village, but Anopheles dirus (s.l.) (n = 271) and An. maculatus (s.l.) (n = 14) were captured as far as 4.5 km away in farm huts and forest. A high proportion of biting was conducted in the early evening before people were under nets. Entomological inoculation rates (EIR) of An. dirus (s.l.) were 17.8 and 25.3 infectious bites per person per year in the outdoor farm hut sites and forest, respectively, for Plasmodium falciparum and 25.3 in the forest sites for P. vivax. CONCLUSIONS: Despite high net coverage in the village, gaps in coverage and access appear in the farm huts and forest where risk of anopheline biting and parasite transmission is much greater. Since subsistence farming and forest activities are integral to these communities, new personal protection methods need to be explored for use in these areas that can ideally engage with the community, be durable, portable and require minimal behavioural change.