Abstract
BackgroundOver the past 20 years, numerous studies have investigated the ecology and behaviour of malaria vectors and Plasmodium falciparum malaria transmission on the coast of Kenya. Substantial progress has been made to control vector populations and reduce high malaria prevalence and severe disease. The goal of this paper was to examine trends over the past 20 years in Anopheles species composition, density, blood-feeding behaviour, and P. falciparum sporozoite transmission along the coast of Kenya.MethodsUsing data collected from 1990 to 2010, vector density, species composition, blood-feeding patterns, and malaria transmission intensity was examined along the Kenyan coast. Mosquitoes were identified to species, based on morphological characteristics and DNA extracted from Anopheles gambiae for amplification. Using negative binomial generalized estimating equations, mosquito abundance over the period were modelled while adjusting for season. A multiple logistic regression model was used to analyse the sporozoite rates.ResultsResults show that in some areas along the Kenyan coast, Anopheles arabiensis and Anopheles merus have replaced An. gambiae sensu stricto (s.s.) and Anopheles funestus as the major mosquito species. Further, there has been a shift from human to animal feeding for both An. gambiae sensu lato (s.l.) (99% to 16%) and An. funestus (100% to 3%), and P. falciparum sporozoite rates have significantly declined over the last 20 years, with the lowest sporozoite rates being observed in 2007 (0.19%) and 2008 (0.34%). There has been, on average, a significant reduction in the abundance of An. gambiae s.l. over the years (IRR = 0.94, 95% CI 0.90–0.98), with the density standing at low levels of an average 0.006 mosquitoes/house in the year 2010.ConclusionReductions in the densities of the major malaria vectors and a shift from human to animal feeding have contributed to the decreased burden of malaria along the Kenyan coast. Vector species composition remains heterogeneous but in many areas An. arabiensis has replaced An. gambiae as the major malaria vector. This has important implications for malaria epidemiology and control given that this vector predominately rests and feeds on humans outdoors. Strategies for vector control need to continue focusing on tools for protecting residents inside houses but additionally employ outdoor control tools because these are essential for further reducing the levels of malaria transmission.
Highlights
Over the past 20 years, numerous studies have investigated the ecology and behaviour of malaria vectors and Plasmodium falciparum malaria transmission on the coast of Kenya
Strategies such as indoor spraying with residual insecticides (IRS), sleeping under long-lasting insecticide-treated bed nets (LLINs), larval habitat management (LHM), and the use of repellents and other vector control measures can be used selectively to reduce levels of transmission
Several African countries have already scaled up the delivery of key malaria control measures, notably LLINs [5] and the provision of more effective artemisinin-based combination therapy (ACT) for malaria case management [6]
Summary
Over the past 20 years, numerous studies have investigated the ecology and behaviour of malaria vectors and Plasmodium falciparum malaria transmission on the coast of Kenya. Effective vector control strategies that negatively impact the components of vectorial capacity can be developed [4] Strategies such as indoor spraying with residual insecticides (IRS), sleeping under long-lasting insecticide-treated bed nets (LLINs), larval habitat management (LHM), and the use of repellents and other vector control measures can be used selectively to reduce levels of transmission. Between 2000 and 2008, globally there was a three to 10-fold increase in ownership and use of ITNs among children under five years of age [7] and a 25-fold increase in global procurement of ACT over the last few years [7] These changes have contributed to the global decline in malaria morbidity and mortality, and may have motivated many countries to begin thinking through steps towards malaria elimination [8,9,10]. This increase in LLIN coverage along with changes in malaria treatment and management, has likely contributed to the steady decline in malaria morbidity and mortality along the Kenyan coast [8,9,10]
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