Abstract
Malaria vectors in sub-Saharan Africa have proven themselves very difficult adversaries in the global struggle against malaria. Decades of anti-vector interventions have yielded mixed results—with successful reductions in transmission in some areas and limited impacts in others. These varying successes can be ascribed to a lack of universally effective vector control tools, as well as the development of insecticide resistance in mosquito populations. Understanding the impact of vector control on mosquito populations is crucial for planning new interventions and evaluating existing ones. However, estimates of population size changes in response to control efforts are often inaccurate because of limitations and biases in collection methods. Attempts to evaluate the impact of vector control on mosquito effective population size (Ne) have produced inconclusive results thus far. Therefore, we obtained data for 13–15 microsatellite markers for more than 1,500 mosquitoes representing multiple time points for seven populations of three important vector species—Anopheles gambiae, An. melas, and An. moucheti—in Equatorial Guinea. These populations were exposed to indoor residual spraying or long-lasting insecticidal nets in recent years. For comparison, we also analyzed data from two populations that have no history of organized vector control. We used Approximate Bayesian Computation to reconstruct their demographic history, allowing us to evaluate the impact of these interventions on the effective population size. In six of the seven study populations, vector control had a dramatic impact on the effective population size, reducing Ne between 55%–87%, the exception being a single An. melas population. In contrast, the two negative control populations did not experience a reduction in effective population size. This study is the first to conclusively link anti-vector intervention programs in Africa to sharply reduced effective population sizes of malaria vectors.
Highlights
Throughout much of sub-Saharan Africa, Anopheles gambiae s.s. is the most important vector of malaria, a disease that claims over 780,000 lives every year [1]
The battle against malaria mosquitoes in sub-Saharan Africa is being fought with two main weapons: indoor residual spraying of insecticides (IRS) and long-lasting insecticidal net (LLIN) campaigns
Many programs have been successful in reducing malaria infection, demonstrating the impact of these programs on vector populations is typically confounded by numerous variables associated with collection methods
Summary
Throughout much of sub-Saharan Africa, Anopheles gambiae s.s. is the most important vector of malaria, a disease that claims over 780,000 lives every year [1]. Its effectiveness as a vector stems from its close association with human habitat, its habit of readily entering houses at night to feed, and its preference for human blood meals. This species is comprised of two molecular forms; the M and S form [2,3] which are considered incipient species. An. gambiae s.s. has six, morphologically nearly-identical sibling species Together these make up the An. gambiae species complex, known as An. gambiae s.l. One of these, Anopheles melas, is a dominant vector in locations along the West African coast, where it breeds within mangrove belts and tidal swamps [4]. Besides the Anopheles gambiae complex, various other species contribute to transmission as well, and in the equatorial rainforests of Central Africa the anthropophilic and endophilic Anopheles moucheti is an important vector [4]
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