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Background   Anopheles funestus  is a major malaria vector in southern Africa. Vector control relies on the use of insecticide chemicals to significantly reduce the number of malaria vectors by targeting that portion of the female population that takes blood meals and subsequently rests indoors. It has been suggested that the intake of a blood meal may assist female mosquitoes to tolerate higher doses of insecticide through vigour tolerance. It is hypothesized that during the process of blood digestion, detoxification mechanisms required for the neutralizing of harmful components in the blood meal may also confer an increased ability to tolerate insecticide intoxication through increased enzyme regulation.    Methods  Bottle bioassays using a range of concentrations of the pyrethroid insecticide permethrin were performed on pyrethroid susceptible and resistant laboratory strains of  An. funestus  in order to detect differences in insecticide susceptibility following a single blood meal. Based on these results, a discriminating dosage was identified (double the lowest dosage that resulted in 100% mortality of the susceptible strain). Blood-fed and unfed females drawn from the resistant strain of  An. funestus  were then assayed against this discriminating dose, and the percentage mortality for each sample was scored and compared.    Results  In the insecticide dose response assays neither the fully susceptible nor the resistant strain of  An. funestus  showed any significant difference in insecticide susceptibility following a blood meal, regardless of the stage of blood meal digestion. A significant increase in the level of resistance was however detected in the resistant  An. funestus  strain following a single blood meal, based on exposure to a discriminating dose of permethrin.    Conclusion  The fully susceptible  An. funestus  strain did not show any significant alteration in susceptibility to insecticide following a blood meal suggesting that vigour tolerance through increased body mass (and increased dilution of internalized insecticide) does not play a significant role in tolerance to insecticide intoxication. The increase in insecticide tolerance in the pyrethroid resistant strain of  An. funestus  following a blood meal suggests that insecticide detoxification mechanisms involved in insecticide resistance are stimulated by the presence of a blood meal prior to insecticide exposure, leading to enhanced expression of the resistance phenotype. This finding may be significant in terms of the methods used to control indoor resting populations of  An. funestus  if the mass killing effect of insecticide application proves increasingly inadequate against blood-feeding females already carrying the insecticide resistance phenotype.

The effect of a single blood meal on the phenotypic expression of insecticide resistance in the major malaria vector Anopheles funestus