Abstract
BackgroundThe 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 FindingsLongitudinal 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.ConclusionThe 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.
Highlights
Malaria is the leading cause of child mortality and morbidity in Africa
The likely factors contributing to malaria resurgence include reduced efficacy of insecticide treated nets (ITNs), insecticide resistance in mosquitoes and lack of proper use of ITNs
This can be attributed to malaria control measures, predominantly to the use of insecticide treated nets (ITNs), indoor residual spray (IRS) and artemisinin-based combination therapies (ACT), which have been implemented on a wide scale [1,2,3,4,5,6]
Summary
There is growing evidence documenting a substantial decline in clinical malaria morbidity and mortality in a number of African countries, including Kenya This can be attributed to malaria control measures, predominantly to the use of insecticide treated nets (ITNs), indoor residual spray (IRS) and artemisinin-based combination therapies (ACT), which have been implemented on a wide scale [1,2,3,4,5,6]. The changing trend in malaria incidence has been shown mainly through data or surveys from health facilities, where most records are based on presumptive treatment with only a small portion based on laboratory confirmed diagnoses They are not reliable estimates of the true status of malaria transmission in the general population because any improvement in diagnosis and treatment will appear to make a great difference [24,25]. 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
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