Importance of mosquito population genetic studies in medical entomology.

Some pathogens are known to be transmitted to humans by hematophagous insects. Control of diseases caused by arthropod-borne pathogens has usually been carried out by vector control using insecticides. The response of many vectors to insecticidal pressures often results in selection of resistant populations. Under field conditions, resistance is usually controlled by genes. Susceptibility or resistance to pathogens are usually determined by relatively simple genetic mechanisms. The frequency of such genes within a population will be influenced by evolutionary forces and interpopulation differences in levels of susceptibility to pathogens can be estimated. In view to understand the evolution of selected traits (insecticide resistance or vector capacity), analyses of gene flow provide information on population local dispersion patterns which is critical in vector control, indicating the distance of dispersal of vectors. Given that mosquitoes colonize various restricted and differentiated habitats, studies of mosquito populations appear especially promising for the understanding of vector genetic structure, vector transmission, disease epidemiology and disease control. Knowledge of patterns of gene flow and dispersal rates are useful to define geographic structure of mosquito populations from French Polynesia in relation with the transmission of a parasite (Bancroftian filariasis worm) or a virus (dengue viruses) or with their capacity to resist to insecticides. (1) Experimental infections of various geographic strains of Aedes polynesiensis for Wuchereria bancrofti show differences in the vectorial efficiency related to the geographic origin of the mosquito strain. Mosquitoes were more efficient intermediate hosts when infected with sympatric W. bancrofti. Mosquito strains from the Society archipelago developed the highest proportion of infective stage larvae and exhibited the lowest mortality rate. Isoenzyme frequency differences among Ae. polynesiensis populations confirms the genetic divergence of geographic isolated populations from crab holes. (2) A study on the genetic structure of Aedes aegypti, a vector of dengue viruses, shows a low level of genetic exchanges between mosquito populations. Genetic differentiation is probably due to insecticidal pressure which tends to structure mosquitoes for their oral susceptibility to dengue type 2 virus. The occurrence of dengue haemorrhagic fryer in French Polynesia during the last few years was likely due to the dispersal of dengue viruses via viremic people rather than via infected vectors. (3) The analysis of the distribution of insecticide resistance genes in relation to genetic structure and gene now in Culex quinquefasciatus show that the frequency of resistance genes to organophosphorous insecticides is correlated to human displacements. The resistance genes have first been imported in French Polynesia through the international airport and dispersal of these genes through other islands was proceeded lia the local commercial traffic.