An Inconvenient Truth of Pyrethroid - Does it have a promising future? -

Abstract

Pyrethroid is the general term for a group of chemicals that includes natural pyrethrins derived from Chrysanthemum flowers and their structurally related synthetic chemicals. Most of pyrethroids are highly nontoxic to mammals and possess high knockdown activity. Pyrethroid resistance is envisioned to be a major problem for the vector control program since, at present, there are no suitable chemicals substitutes for pyrethroids. Cross-resistance to knockdown agents, which are mainly used in mosquito coils and related products as spatial repellents, is the most serious concern. Since this is a global phenomenon, we have started to monitor the distribution of mosquito resistance to pyrethroids. The first pilot study was carried out in Vietnam. We periodically drove along the national road from the north end to the Mekong Delta in Vietnam and collected mosquito larvae from used tires. Simplified susceptibility tests were performed using the fourth instar larvae of Aedes aegypti (L.), Aedes albopictus (Skuse), and Culex quinqufasciatus (Say). The susceptibility of the abovementioned species against d-allethrin was lower in the southern part as compared to in northern part of Vietnam. Compared with the other species, Ae. aegypti demonstrated the most prominent reduction in susceptibility. For Ae. aegypti, significant increases in the susceptibility indices with decrease in the latitude of collection points were observed, indicating that the susceptibility of Ae. aegypti against d-allethrin was lower in the southern part including mountainous areas as compared to in the northern part of Vietnam. The significant correlation was observed between the susceptibility indices and the annual pyrethroid use for malaria control in 4e. aegypti. This might explain that the use of DDT and pyrethroids as residual treatment inside houses and pyrethroid-impregnated bed nets for malaria control are attributable to low pyrethroid susceptibility in Ae. aegypti. Such insecticide treatment appeared to have been intensively administered in the interior and along the periphery of human habitation areas where incidentally, the breeding and resting sites of Ae. aegypti are located. This might account for the strong selection pressure toward Ae. aegypti and not Ae. albopictus.