Resistance management strategies in malaria vector mosquito control. A large-scale field trial in Southern Mexico

Abstract

A resistance management programme comparing rotations, mosaics and single use of insecticides for residual house-spraying against the insect vectors of malaria is being carried out in Southern Mexico. The area was chosen because of its prior history of insecticide use, relatively sedentary vector, and physical features of the area which limit inward migration of insects to the study area. A high level of resistance to DDT and low levels of organophosphorus (OP), carbamate and pyrethroid resistance were detected by WHO discriminating-dose assays in field populations of Anopheles albimanus in the pre-spray period in the region where this resistance management project is being undertaken. After the first year of spraying, resistance, as measured by a discriminating-dose assay, was still at a high level for DDT and had risen for all the other insecticides. Biochemical assays showed that DDT resistance was primarily caused by elevated levels of glutathione S-transferase (GST) activity leading to increased rates of metabolism of DDT to DDE. The numbers of individuals with elevated GST and DDT resistance were well correlated, suggesting that this is the only major DDT resistance mechanism in this population. The carbamate resistance in this population was conferred by an altered acetylcholinesterase (AChE) mechanism. The level of resistance in bioassays correlated well with the frequency of individuals homozygous for the altered AChE allele. This suggests that the level of resistance conferred by this mechanism in its heterozygous state is below the level of detection of the bioassay. The low levels of OP and pyrethroid resistance could be conferred by either the elevated esterase or monooxygenase enzymes. The esterases, however, are elevated only with p-nitrophenyl acetate (PNPA), and are unlikely to be causing broad-spectrum OP resistance. The altered AChE mechanism may also be contributing to the OP but not the pyrethroid resistance. There were significant differences in some resistance gene frequencies for insects obtained by different indoor and outdoor trapping methods. To determine whether the different sampling methods were effectively sampling the same interbreeding population, RAPD analysis of insects obtained by different collection methods in different villages was undertaken. There was no observed variability in the RAPD patterns for the different mosquito samples with a number of primers. ©1997 SCI