Anopheles Mosquitoes as Vectors of Malaria in East Africa: Bed Nets and Beyond
By John P. Roche
Malaria is a devastating disease, with almost two hundred million cases and half a million deaths per year worldwide. The World Health Organization estimates that the rate of mortality from malaria has decreased 47% since the year 2000, and the reduction in the rate of mortality has been even more impressive in Africa, decreasing 58% since 2000. But the disease remains a leading killer in the world, and further reductions in malaria transmission are needed. The theme of the 2015 World Malaria Day (April 25, 2015) is “Defeat Malaria.” This goal is particularly critical in sub-Saharan Africa, where 80% of malaria cases occur. This article examines some of the recent research in East Africa that is steering efforts at eradicating malaria.
Malaria is caused by the Plasmodium parasite, which is spread to humans by bites from Anopheles mosquitoes. In Kenya, the three species of Anopheles that are the most abundant, and which have the highest rates of transmitting malaria, are Anopheles funestus, Anopheles gambiae, and Anopheles arabiensis. In the 1990s, insecticide-treated bed nets (ITNs) were introduced in western Kenya, and this led to a dramatic decline in Anopheles funestus mosquitoes, a species that is often found inside homes. Edward Walker and colleagues conducted a recent study of Anopheles funestus in western Kenya and found that Anopheles funestus mosquitoes were the most abundant malaria vector sampled there in 2010 and 2011. They also found that Anopheles funestus had high rates of transmission of the malaria parasite, and that it had low mortality rates in bioassays of insecticide effectiveness.
Walker’s data support the hypothesis that Anopheles funestus increased in western Kenya as a result of developing resistance to insecticides in ITNs. In this location in Kenya, ITNs appear to have provided good short-term reduction in malaria transmission, but poor longer-term reduction because genes for resistance to insecticides increased in the population. Data from numerous studies have shown that insecticide resistance can develop in response to ITNs. The problem of insecticide resistance is heightened because Anopheles species can be highly resistant to multiple insecticides. For example, a study in Tanzania by Dickson Lwetoijera and colleagues found that Anopheles funestus had high levels of resistance to the insecticides bendiocarb, DDT, deltamethrin, lambda cyhalothrin, and permethrin.
Development of insecticide resistance could be a significant factor in the resurgence of Anopheles funestus, but other factors could have contributed as well, such as changes in the use of bed nets, changes in land use, and changes in patterns of land cover. The 2012 World Malaria Report estimated that about 90% of people with an ITN use it, but usage is not 100%, and it may vary for individuals over time. Because multiple factors affect the abundance of, and transmission rates for, Anopheles vectors, malaria-control experts carry out multiple reduction strategies at the same time to increase effectiveness, an approach entomologists call integrated vector control. For example, within the home, a combination of ITNs and indoor residual spraying of insecticide to walls is more effective than using only one of these control tactics. The 2012 World Malaria Report estimated that the use of ITNs in Africa increased from 3% in 2000 to 53% in 2011, but the use of indoor residual spraying only increased from less than 5% in 2005 to 11% in 2010. Therefore, the rate of use of indoor residual spraying could be increased dramatically.
Another category of malaria vector control focuses on land use and land cover. For example, standing water in which mosquitoes breed can be drained or treated to kill mosquito larvae. Louise Kelly-Hope and colleagues conducted a statistical study looking at variation in environmental factors in southeastern Kenya and found that different vector species may respond to changes in environmental conditions in different ways. For example, they found that for Anopheles gambiae and Anopheles arabiensis, higher levels of precipitation correlated with higher malaria transmission, but this was not the case for Anopheles funestus. They also found that correlations for both precipitation and temperature with the rate of malaria transmission varied between Anopheles arabiensis and Anopheles funestus. Their data suggest that to be more effective at reducing vector abundance and transmission, environmentally-focused vector-control measures could be tailored to individual Anopheles species.
Given the complex interplay of factors that influence vector abundance and rates of transmission, one of the biggest challenges facing attempts at malaria eradication in Africa is gathering key data that can inform mosquito control in specific regions. Additional data on vector abundance and behavior, transmission rates, insecticide resistance, patterns of use of protective measures such as ITNs, and environmental conditions are all of pressing importance. Given the dramatic reductions achieved in malaria mortality in the past decade, with ongoing data collection and further development of integrated vector control, we can hope that the goal of the 2015 World Malaria Day — to defeat malaria — will become a reality.
John P. Roche is a science writer and author with a PhD and a postdoctoral fellowship in the biological sciences. He has served as editor-in-chief of university research periodicals at Indiana University and Boston College, has published more than 150 articles, and has written and taught extensively about science and science writing. Dr. Roche also directs Science View Productions™, which provides technical writing and developmental editing for clients in academia and industry.