Malaria Vector Status and Insecticide Susceptibility Studies in Zimbabwe
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The major problem that has evolved from malaria control progammes that utilize insecticides for vector control is the development of insecticide resistance. In Zimbabwe, despite decades of pesticide usage there is limited information on susceptibility status of major malaria vectors and possible mechanisms responsible for resistance. Susceptibility status of vectors provides useful information for making rational decisions on control strategies. The malaria vector status and their susceptibility to insecticides were studied in Gokwe, Gwave village in the Midlands province of Zimbabwe. Standard WHO bioassays, using 0.75% permethrin, 4% DDT, 5% malathion, 0.1% bendiocarb and 4% dieldrin were done on wild-collected adult anopheline mosquitoes. Susceptibility tests were done on the F1 generation of Anopheles arabiensis reared from wild-caught females using 0.75% permethrin and 4% DDT. Molecular and biochemical assays were carried out to identify kdr mutations in individual mosquitoes and to determine expression levels of non-specific esterases, monooxygenases, glutathione-S-transferases and altered acetylcholinesterase (AChE). A total of 648 anophelines were collected, with the majority (98%) being members of the An. gambiae complex. Species in the An. gambiae complex were identified by polymerase chain reaction (PCR) and An. arabiensis (72.8%) predominated all the other sibling species. Among the An. arabiensis females, 0.5% was positive for Plasmodium falciparum. WHO diagnostic tests showed that 53% of the An. arabiensis were resistant to permethrin and 32% to DDT. Insecticide susceptibility tests on F1 An. arabiensis families showed an average mortality of 85.9% (n=567) after exposure to 4% DDT and 69.8% (n=372) after exposure to 0.75% permethrin. Six families showed cross resistance to both DDT and permethrin. Biochemical assays of F1 An. arabiensis families revealed comparatively high levels of monooxygenase (48%, n = 33, p<0.05), glutathione S-transferase (26%, n = 31, p<0.05) and general esterases activity compared to the reference colony. Insensitive acetylcholinesterase was detected in 23.5% (n = 33). The kdr analysis by PCR revealed presence of both East and West Africa mutation, but was not confirmed by sequencing. The significant elevation of various enzyme systems in the F1 progeny and detection of families showing cross resistance to permethrin and DDT is suggestive of existence of multiple resistances in An. arabiensis population from Gwave. This has serious implication on malaria control. Continued use of these insecticides is likely to further select resistant vectors. Use of mosaic insecticides or rotational use of insecticides is recommended.