Vector Species and Insecticide Resistance in the Asia-Pacific
This map, from the University of Oxford’s Malaria Atlas Project shows the distribution of the different potential malaria vectors in the Asia-Pacific region.
There are many potential vectors, 16 of which are known to play a role in transmission, with 6 considered to be the most important ¹
The Asia-Pacific region can be considered to have three distinct ecological areas. The different vectors driving transmission can be characterised by these ecological areas:
- In the Greater Mekong sub-region transmission is mainly driven by the Anopheles dirus and Anopheles minimus species complexes. Transmission is greater in rural forested areas with agricultural and forest workers at particularly high risk of infection. Early outdoor biting by primary vector species makes malaria somewhat less ammenable to control by insecticide treated bednets and indoor residual spraying. The preferred breeding sites for these vectors are small, widely dispersed and often temporary bodies of water which means that larval control is not feasible in most settings.
- In the South Asia sub-region transmission is mainly driven by Anopheles culicifacies in rural areas and Anopheles stephensi in urban areas. However, the former also plays a large role in transmission in peri-urban and urban areas. These vectors bite indoors and outdoors but they often rest indoors making IRS effective in some settings. In urban settings larval control can also sometimes be useful.
- In the Pacific sub-region transmission is mainly driven by Anopheles farauti and Anopheles punctulatis complexes. Differences within the complexes and the distribution of the species, in particular in the farauti complex, mean that mosquito behaviour can vary in different places. Although biting and resting can be indoors and outdoors, LLINs and IRS have proven to be somewhat effective in these settings. Larval control is difficult given the range of breeding sites, including brackish water around coastal areas for some species.
There are important variations within sub-regions, even at local levels. The diversity of vector species that transmit malaria in the Asia-Pacific (contrast this to the 2-3 main species in sub-Saharan Africa) can cause considrable problems for control. The difference in vectors’ behaviours by location contributes to differences in epidemiology and has impacts on control. For example, in the Mekong and Pacific regions most important mosquito vector species tend to rest outdoors and bite outdoors in the early evening ². These behaviours mean that insecticide treated bed-nets and indoor residual spraying are not as effective as they might otherwise be (although both have been shown to have a significant and valuable impact even where vectors are predominantly outdoor early biters).
Innovative approaches are needed in the Asia-Pacific more than anywhere else in order to maximise the impact of vector control efforts. In developing interventions in this region, it is particularly important to identify the primary vectors and determine local breeding, feeding and resting behaviours, before planning a control programme.
This table gives an overview of the most important characteristics of vector behaviour for the six main vectors in the region. However, it is important to note that local variations can occur and basic entomological assessments should be carried out to ensure control strategies are appropriate.
Most of the Asia-Pacific countries report some level of resistance to insecticides in at least one of their major vectors, although sometimes this is quite focal. The case of India is particularly concerning since resistance is widespread and there is a high incidence of malaria. Insecticide resistance in the Greater Mekong Sub-region is also of serious concern as if it were to become more pronounced and then spread it could hamper the elimination of multidrug resistant (including ACT resistant) falciparum malaria parasites.
The operational significance of insecticide resistance on the success of malaria control interventions is still not clear. It is likely that even with low to moderate levels of resistance many control measures will still work reasonably well. It is also likely that varying levels of resistance for different insecticides will have different impacts depending on the control approach used.
¹ RBM (2013) Global Malaria Action Plan. Part III. Regional Strategies.4 Asia-Pacific. Available from: http://www.rollbackmalaria.org/microsites/gmap/3-4.html. Accessed 2nd March, 2014.
² Trung, H. D., et al., ‘Behavioural heterogeneity of Anopheles species in ecologically different localities in Southeast Asia: a challenge for vector control’, Trop Med Int. Health 10/3 (2005) 251-262.