Potential control measures for pyrethroid-resistant malaria vectors

Over the past decade, the success of long-lasting insecticidal nets (LLINs) in African countries has been countered by the rapid development of pyrethroid resistance in vector mosquitoes. Few studies have investigated the efficacy of LLINs in areas in which vector mosquitoes are resistant to pyrethroids. The use of excito-repellency chemicals may be bio-rational, because this type of repellency does not induce physiological resistance. However, limited information is available on the relationship between the mode of insecticide resistance and excito-repellency in pyrethroid-resistant mosquitoes. The aims of the present study were to investigate (1) differences in reactions to LLINs between individual and populations of vector mosquitos in an area in which multimodal pyrethroid resistance has developed, (2) the effects of LLINs on these malaria vectors, and (3) the development of new control techniques to supplement LLINs. Laboratory contact tests for repellency showed that resistant species governed by kdr (knockdown resistance) (Anopheles gambiae sensu stricto (s.s.)) lost repellency to pyrethroids, whereas those lacking kdr (Anopheles arabiensis and Anopheles funestus s.s.) maintained strong repellency irrespective of whether they possessed cytochrome P450-related metabolic resistance factors. LLINs were effective against these pyrethroid-resistant malaria vectors because they limited feeding on humans overnight. However, notable time shifts in human blood-feeding activity developed in An. arabiensis and An. funestus s.s., but not in An. gambiae s.s. These time shifts may be partially explained by differences in repellency by pyrethroids for these species. LLINs may not be effective because most blood feeding occurs when people are active outside of bed nets. Screening eaves with pyrethroid-impregnated wide-mesh nets was found to be effective in reducing human exposure to malaria vectors. The excito-repellency of pyrethroids, which act as a spatial barrier or reduce the feeding motivation of mosquitoes, may represent another countermeasure.