Spatial-temporal vector abundance and malaria transmission dynamics in Nchelenge and Lake Mweru islands, a region with high burden of malaria in northern Zambia

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Abstract Background Over a decade of vector control by indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) distribution on mainland and only LLINs on islands, has had minimal impact on disease burden in Nchelenge district, northern Zambia. Anopheles funestus and An. gambiae are vectors known only from the mainland. Understanding vector bionomics in the district is necessary for planning and targeting effective vector control. This study aimed to provide information on malaria vectors’ abundance, seasonality and Plasmodium falciparum sporozoite infectivity in Nchelenge including islands.Methods Mosquitoes were collected in 192 CDC indoor light traps set in 56 households between January 2015 and January 2016. Morphological and molecular species identifications, and P. falciparum circumsporoites by ELISA were performed. Mosquito counts and relative abundances from the islands and mainland were compared and household factors associated with vector counts determined.Results A total of 5888 anophelines were collected during the study. Of these 5,704 were female An. funestus s.l. and 248 female An. gambiae s.l. The highest proportion of An. funestus (n = 4030) was from Chisenga Island and that of An. gambiae (n = 174) was from Kilwa Island. The dorminance of An. funestus s.l. over An. gambiae s.l. was significant on Chisenga island (z = 6.66, p < 0.001) and mainland (z = 4.83, p < 001) but not on Kilwa island (z = 1.95, p = 0.052). Highest counts of An. funestus were associated with dry season (p < 0.001) whilst abundance of An. gambiae was associated with rainy season (p = 0.404). At household level vector counts were associated with high household occupancy and high bed net use. The highest estimated annual entomological inoculation rate was from Chisenga island with 136.51ib/p/y followed by Kilwa island with 55.05 ib/p/yr then Mainland with 30.31ib/p/yr.Conclusions There was varied species abundance and malaria transmission risk across sites and seasons. Risk for malaria transmission was perennial and higher on the islands. The minimal impact of vector control efforts on mainland was evident but limited overall. Vector control intervention coverage with effective tools need to be extended to the islands to effectively control malaria transmission in Nchelenge district.

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