Malaria in the western highlands of Kenya. Transmission patterns and intervention strategies
Abstract
Background: The impact of insecticide treated nets (ITNs) on reducing malaria incidence is shown mainly through data
collection from health facilities. Routine evaluation of long-term epidemiological and entomological dynamics is currently
unavailable. In Kenya, new policies supporting the provision of free ITNs were implemented nationwide in June 2006. To
evaluate the impacts of ITNs on malaria transmission, we conducted monthly surveys in three sentinel sites with different
transmission intensities in western Kenya from 2002 to 2010.
Methods and Findings: Longitudinal samplings of malaria parasite prevalence in asymptomatic school children and vector
abundance in randomly selected houses were undertaken monthly from February 2002. ITN ownership and usage surveys
were conducted annually from 2004 to 2010. Asymptomatic malaria parasite prevalence and vector abundances gradually
decreased in all three sites from 2002 to 2006, and parasite prevalence reached its lowest level from late 2006 to early 2007.
The abundance of the major malaria vectors, Anopheles funestus and An. gambiae, increased about 5–10 folds in all study
sites after 2007. However, the resurgence of vectors was highly variable between sites and species. By 2010, asymptomatic
parasite prevalence in Kombewa had resurged to levels recorded in 2004/2005, but the resurgence was smaller in
magnitude in the other sites. Household ITN ownership was at 50–70% in 2009, but the functional and effective bed net
coverage in the population was estimated at 40.3%, 49.4% and 28.2% in 2010 in Iguhu, Kombewa, and Marani, respectively.
Conclusion: The resurgence in parasite prevalence and malaria vectors has been observed in two out of three sentinel sites
in western Kenya despite a high ownership of ITNs. The likely factors contributing to malaria resurgence include reduced
efficacy of ITNs, insecticide resistance in mosquitoes and lack of proper use of ITNs. These factors should be targeted to
avoid further resurgence of malaria transmission.
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