Effects of bednet use, topography and targeted vector control on malaria transmission in the highlands of Vihiga and Kakamega counties, Western Kenya
Atieli, Harrysone Etemesi
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Insecticide-treated bed nets (ITNs) are known to be highly effective in reducing malaria morbidity and mortality. However, there is scanty information on actual usage of owned nets which varies among households. Such variations may limit the potential effect of nets and cause spatial heterogeneity on malaria transmission. Likewise there is limited information and utilization of topographic parameters such as the shape of the underlying valley ecosystem in identification of high-risk malaria foci to help enhance surveillance and targeted vector control activities in regions where they are most needed. The objective of this study was to determine the effect of ITNs use, topography and targeted vector control on malaria transmission. The study was done in; Iguhu, Mbale and Emutete in Vihiga and Kakamega counties, in malaria hypomeso-endemic highlands of western Kenya. All houses in study sites were geo-referenced and mapped. Using a randomized-repeated cross-sectional study design, residents of 300 households randomly selected from each site in Iguhu and Emutete were interviewed for ITN s ownership/usage, their houses sprayed for vector abundance and occupants screened of malaria during the dry and rainy seasons of 2009-2010. Association of topography and spatial distribution of malaria vectors and infections were determined between broad 'U'-shaped versus narrow 'V'-shaped valley ecosystems in the three sites. Baseline information from spatio-temporal data above was used to identify intense transmission areas for target vector control using Indoor Residual Spray (IRS) and Biological larviciding. Entomological and parasitological data were used for impact evaluation. Data was managed in excel spread sheets and analyzed by STATA software. Student t-test was done to determine differences in adult vectors and larval densities while Chi-square test was done to determine differences in occurrence of malaria infection prevalence. Univariate and multivariate' analysis of Turkey HSD test was done to determine the most predictive independent variable for the occurrence of larvae, adult vectors and malaria cases. Despite ITN ownership reaching more than 71%, usage was low at 56.3%. The proportion of non-adherence to ITN use was significantly higher during the dry season than rainy season, 46.6% vs. 32.9% in Iguhu (X2= 12.42, d.f = 1, P <0.001) and 53.4% vs. 41.8% in Emutete (X2 = 21.12, d.f = 1, P < 0.0001), respectively. Indoor resting female An. gambiae s.l. density was lower (43% t = 2.38, d.f = 167, P = 0.02) in houses with functional ITNs. Infection prevalence for all age groups was significantly lower by 30% among net users compared to non-net users during the rainy season (OR 0.72,95% CI 0.51-1.00, P < 0.05). Broad flat bottomed valleys had significantly high number of anopheles larvae per habitat (P=0.024 Tukey HSD test), indoor resting vector density (f-=5.7, P<0.000I) and malaria infections (t=9.96, P<0.000I) than narrow valleys ecosystem during both seasons. Targeted vector control was associated with up to 61.3% reduction in indoor resting vector densities although. the reduction/effect on malaria infection was not significant. The findings of this study highlight the wide gap between ITN ownership and usage and effect on malaria transmission. Likewise, it recognises the importance of topographic parameters on vector abundance and malaria transmission. Based on this study, there is need to sensitize households on sustained use of owned ITN s in order to optimize their role as a malaria control tool and the need to consider topographic parameters in identification of high-risk malaria foci and use of this in programmatic targeted vector control to fight malaria.