Insecticide resistance, Host preference and Plasmodium falciparum parasite rates in Anopheles mosquitoes in Mwea and Ahero rice schemes

The ability of Anopheles mosquito to transmit malaria in nature is partly enhanced by; resistance of mosquito to insecticides, feeding preference for human host and infection by Plasmodium falciparum . An assessment was conducted to determine the status of these parameters in Anopheles populations in Mwea and Ahero rice irrigation schemes in Kenya. This was important in order to understand their potential influence on local malaria transmission. A total of 1,200 female Anopheles mosquitoes (gravid and blood fed) were sampled from both sites by indoor and outdoor methods. Anopheles samples identification to their respective species in the field was done using morphological features and taxonomic keys. In Mwea scheme, all the 600 Anopheles mosquitoes collected were An. gambiae s.l out of which 195 were gravid. In Ahero, 250 An. gambiae s.l (out of which 81 were gravid) and 350 An. funestus (out of which 181 were gravid) were collected. Gravid Anopheles mosquitoes were allowed to oviposit to give F 1 generations in the insectary. These F 1 , (four replicates of 25 mosquitoes per species per insecticide) were assessed for susceptibility to permethrin, deltamethrin, dichlorodiphenyltrichloroethane (DDT), bendiocarb or fenitrothion using standard WHO protocol. Susceptible An. gambiae s.s Kisumu strain (25 mosquitoes per the 100 test mosquitoes) was used as positive control. The 1,200 field samples were further identified to their respective species using rDNA-PCR using their legs and wings. Source (s) of blood meal in 405 An. gambiae s.l from Mwea, 169 An. gambiae s.l and 269 An. funestus from Ahero were determined using blood meal Elisa. The presence of Plasmodium falciparum Welch, 1897 in the salivary glands was assessed by sporozoite Elisa in all the field collected samples. All Anopheles mosquito samples from Mwea were Anopheles arabiensis Patton, 1905, while those from Ahero were a mixed species of Anopheles arabiensis (41.7%), Anopheles funestus sensu stricto Giles , 1900 (57%), Anopheles rivulorum Leesoni, 1935 (0.66%), Anopheles leesoni Evans, 1931 (0.3%) and Anopheles parensis Gillies, 1935 (0.3%). Mosquito samples from both study sites showed reduced susceptibility to the test insecticides. An. arabiensis mosquitoes from Mwea had a human blood meal index at 0.22 (n=405).  P. falciparum circumsporozoite infection in An. arabiensis from Mwea were reported in Murinduko village at 1.5% (n=200). In Ahero, human blood meal indices were at 0.00 (n=169) and 0.17 (n=269) for An. arabiensis and An. funestus respectively. P. falciparum circumsporozoite infections in An. funestus sampled from Kamagaga and Wagai villages in Ahero were at 5% (n=147) and 2.2% (n=183) respectively.