The major foci of pyrethroid resistance in 1990 to 2010 were in West and Central African populations of Anopheles gambiae s.s. Pyrethroid resistance in Anopheles arabiensis has been reported in several countries of East and Central Africa. Four cross-sectional surveys of A. arabiensis in Lower Moshi were conducted in 2009, 2011, 2012 and 2013 to determine levels of resistance to pyrethroids, organochlorines, organophosphates and carbamates using World Health Organization (WHO) standard diagnostic dosages. Mosquitoes were identified to species level and genotyped for both L1014F and L1014S mutations by hydrolysis probe assays. A. arabiensis remains the dominant malaria vector in the area. Full susceptibility to dichlorodiphenyltrichloroethane (DDT), organophosphates and carbamates was recorded in all eight villages. Following the current WHO guidelines, resistance to permethrin and lambdacyhalothrin was observed in 2009 with mortality rates ranging from 80 to 90% and from 66 to 97% for lambdacyhalothrin. Reduced susceptibility to deltamethrin was observed (87 to 97% mortality). The percentage mortality to permethrin, deltamethrin, lambdacyhalothrin was less than 90% in 2013 in all villages except in one village where mortality rate for deltamethrin was found to be 99%. These results clearly demonstrate the presence of pyrethroid resistance in A. arabiensis in Lower Moshi. The L1014F resistant allele was detected in one mosquito out of 642 that were screened for kdr mutation (allele frequency of 0.08%). The lack of DDT resistance coupled with previous studies showing very low frequency kdr suggests that enzyme-based mechanisms are responsible for resistance in A. arabiensis. Further studies are needed to investigate operational impact of observed resistance on malaria vector control interventions in the area.
Key words: Anopheles, resistance, mortality, dynamics, mutation, insecticide.
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