Laboratory Resistance Induction of Anopheles Minimus and Anopheles Dirus by the Exposure of Adult Females to Permethrin Treated Mosquito Nettings

Abstract

A laboratory resistance induction study was conducted to determine the effects of permethrin impregnated nettings on the two major vectors of malaria in Thailand, Anopheles minimus and Anopheles dirus. To simulate the field use of bed nets, only females of the two species were exposed. An. minimus and An. dirus were selected using WHO bioassay tubes and cones, respectively, both using permethrin treated netting materials for three minutes. Changes in the phenotypic expression of resistance were monitored through the baseline and subsequent assessments of susceptibilities from the reference strains and within the generations of selected populations. The effects of permethrin selection on adult size were observed between the reference and last selected generation of both vectors. The activity of detoxifying enzymes (mixed function oxidases and non-specific esterases) were also compared between the reference and selected groups of both species for possible resistance mechanisms. Results showed a decrease in susceptibility level of 1.56 folds from the ninth selected generation (S9 ) of An. minimus. An. dirus developed low level resistance of 4.88 folds at S12. No significant effect was observed on adult sizes of the progenies of selected An. minimus and its reference strain. Varying results however were observed from An. dirus, with body weight significantly increased at S12 while no significant difference was observed between the wing lengths of the selected (S12) and reference strains. Significant increase in the levels of mixed function oxidases were detected from both selected generations of the two species. Further studies are needed to directly determine the presence of the resistance genes and the magnitude by which they are expressed in successive generations of vectors.