Pyrethroid resistance in the major malaria vector Anopheles arabiensis from Gwave, a malaria-endemic area in Zimbabwe

Background: Insecticide resistance can present a major obstacle to malaria control programmes. Following the recent detection of DDT resistance in Anopheles arabiensis in Gokwe, Zimbabwe, the underlying resistance mechanisms in this population were studied. Methods: Standard WHO bioassays, using 0.75% permethrin, 4% DDT, 5% malathion, 0.1% bendiocarb and 4% dieldrin were performed on wild-collected adult anopheline mosquitoes and F-1 progeny of An. arabiensis reared from wild-caught females. Molecular techniques were used for species identification as well as to identify knockdown resistance (kdr) and ace-I mutations in individual mosquitoes. Biochemical assays were used to determine the relative levels of detoxifying enzyme systems including non-specific esterases, monooxygenases and glutathione-S-transferases as well as to detect the presence of an altered acetylcholine esterase (AChE). Results: Anopheles arabiensis was the predominant member of the Anopheles gambiae complex. Of the 436 An. arabiensis females, 0.5% were positive for Plasmodium falciparum infection. WHO diagnostic tests on wild populations showed resistance to the pyrethroid insecticide permethrin at a mean mortality of 47% during February 2006 and a mean mortality of 68.2% in January 2008. DDT resistance (68.4% mean mortality) was present in February 2006; however, two years later the mean mortality was 96%. Insecticide susceptibility tests on F-1 An. arabiensis families reared from material from two separate collections showed an average mean mortality of 87% (n = 758) after exposure to 4% DDT and 65% (n = 587) after exposure to 0.75% permethrin. Eight families were resistant to both DDT and permethrin. Biochemical analysis of F-1 families reared from collections done in 2006 revealed high activity levels of monooxygenase (48.5% of families tested, n = 33, p < 0.05), glutathione S-transferase (25.8% of families tested, n = 31, p < 0.05) and general esterase activity compared to a reference susceptible An. arabiensis colony. Knockdown resistance (kdr) and ace-I-R mutations were not detected. Conclusion: This study confirmed the presence of permethrin resistance in An. arabiensis populations from Gwave and emphasizes the importance of periodic and ongoing insecticide susceptibility testing of malaria vector populations whose responses to insecticide exposure may undergo rapid change over time.