Large impacts of vector control on the effective population size (Ne) of malaria vector mosquitoes in Equatorial Guinea

Control of malaria mosquitoes in sub-Saharan Africa is currently achieved using two main approaches: indoor residual spraying of insecticides (IRS) and long-lasting insecticidal net (LLIN) campaigns. Although many programs have been successful in reducing malaria infection, demonstrating the impact of these programs on vector populations is complicated by numerous variables associated with variation in data collection methods. Without accurate ways of measuring the impacts of vector control, it is also difficult to evaluate whether the frequency of insecticide spraying is optimized to keep transmission rates low. Here, we analyzed more than 2,200 samples of three important malaria vectors—Anopheles gambiae, An. melas, and An. moucheti—from seven sites in Equatorial Guinea that were collected over the course of anti-vector programs in that country (2004–2010). Taking advantage of recently developed coalescent genetic approaches, we addressed two main questions: a) what is the impact of vector control programs on effective population size, and b) how is the effective population size effected by single insecticide spray? We are able to show convincingly for the first time that both IRS– and LLIN–based control resulted in dramatically lowered effective population sizes (between 55%–87%) in all populations, with the exception of a single population of An. melas. In contrast, we did not observe population size reduction in negative control populations. We also found that mosquito populations are dramatically reduced following single IRS rounds (65-92%), but rebounded (2,818% increase) between 3-5 months after spraying, indicating that increased spray frequency is likely to greatly improve the impact of IRS on malaria transmission. Our findings are especially important to malaria control because we were able to conclusively link anti-vector interventions to genetic impacts, a linkage that has been difficult to establish in the past.