This contribution was provided by Dr. Christen Fornadel, Senior Malaria Vector Control Specialist at PMI.
The President’s Malaria Initiative (PMI) has made increasing investments in entomological monitoring across all 19 program countries in order to monitor the effects of two of PMI’s four main interventions, distribution of long-lasting, insecticide treated bed nets (LLINs) and indoor residual spraying (IRS), both of which are aimed at controlling mosquito populations. Both of these interventions are insecticide based, so as they are scaled up, one can expect to see changes in the species composition of the vector population and possibly changes in malaria mosquito behavior. But most importantly, we have already seen and are likely to continue to see changes in mosquito susceptibility to the insecticides used on LLINs and for IRS. As malaria vector control has escalated across Africa, so have the number of reports of pyrethroid resistance in both major vector groups, Anopheles gambiae s.l. and An funestus s.l., and today it is rare to find sites in Africa where one or both these vectors do not show some level of pyrethroid resistance. The global community spends hundreds of millions of dollars on malaria control, so it is important to make sure that we are doing entomological monitoring to see that our investments are making an impact, and that those resources are not wasted.
This report (attached) summarizes the insecticide susceptibility data that has been collected by PMI as of the end of January 2015. The report includes data collected by PMI entomologists and contractors, as well as data collected in collaboration with partner-country universities and/or National Malaria Control Programs. In some cases data collected by other partners is also cited. For each PMI country, background information on vector control interventions, particularly PMI-supported IRS, is summarized. A note on the data collected and conclusions follow. Summary tables of mosquito mortality data are also included for each country. Unless otherwise noted, WHO tube bioassays were conducted, and percent mortalities 24 hours after exposure were recorded for 2- to 5-day-old female mosquitoes reared from field- collected larvae.
From the insecticide resistance data collected by PMI, there are some broad trends across Africa that can be summarized, although every country context differs, and there is a range of resistance within countries. An. gambiae s.l. in West Africa is generally highly resistant to pyrethroids and DDT, while resistance to carbamates ranges from 100% susceptible to moderate resistance. There is also probable to moderate resistance developing to fenitrothion and malathion, but generally 100% susceptibility to pirimiphos-methyl. There is a report of resistance to pirimiphos-methyl across multiple sites in Nigeria, which requires confirmation, as this is the first such report in a PMI country.
In East Africa, An. arabiensis and An. gambiae s.s. are both major vectors and not always distinguished by molecular methods, so there is a large variation in resistance reports. In general, resistance is lower than in West Africa. Pyrethroid and DDT resistance ranges from none to very high. Where species are differentiated (Kenya), An. arabiensis has lower pyrethroid resistance and is susceptible to DDT. Carbamate resistance varies from susceptible to moderate resistance. An. gambiae s.l. is mainly 100% susceptible to organophosphates with the exception of moderate resistance to malathion or fenitrothion in a few sites. There is no resistance to pirimiphos-methyl. In Central Africa there are reports of very high pyrethroid and DDT resistance across all countries for An.gambiae s.l, although there are some sites that have lower or only probable resistance. There is emerging carbamate resistance, although many sites still have susceptibility. There is 100% susceptibility to organophosphates. Within most countries in Southern Africa, resistance to pyrethroids and DDT for An. gambiae s.l. ranges from 100% susceptibility to very high resistance. This could be due to differences in species composition, as in East Africa. There is emerging or moderate carbamate resistance in most countries. There is 100% susceptibility to organophosphates, with the exception of possible resistance to fenitrothion at a few sites in Madagascar. In general, An. funestus in East and Southern Africa are highly resistant to pyrethroids (with the exception of Madagascar), moderately to highly resistant to carbamates, and susceptible to DDT and organophosphates.
PMI will continue to monitor insecticide resistance across the program’s 19 sub-Saharan countries, and will expand support to insecticide resistance mechanism testing, along with insecticide resistance intensity testing, in order to better understand the impact of the levels of resistance currently being seen on our vector control interventions.