Nationwide Investigation of the Pyrethroid Susceptibility of Mosquito Larvae Collected from Used Tires in Vietnam

Hitoshi Kawada,Masahiro Takagi,Yukiko Higa,Son H Tran,Hoa T Nguyen,Yen T Nguyen

doi:10.1371/journal.pntd.0000391

Abstract

Pyrethroid resistance is envisioned to be a major problem for the vector control program since, at present, there are no suitable chemical 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 cross-resistance 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, Aedes albopictus, and Culex quinquefasciatus. Compared with the other species, Ae. aegypti demonstrated the most prominent reduction in susceptibility. For Ae. aegypti, significant increases in the susceptibility indices with a 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 that in the northern part of Vietnam. There was a significant correlation between the susceptibility indices in Ae. aegypti and the sum of annual pyrethroid use for malaria control (1998–2002). This might explain that the use of pyrethroids as residual treatment inside houses and pyrethroid-impregnated bed nets for malaria control is 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.

Highlights

One of the most successful events in the development of pesticide chemicals was the discovery of pyrethrum and the successful synthesis of pyrethroids
Cross-resistance to knockdown agents is the most serious concern. Since this is a global phenomenon, we have started to monitor the distribution of mosquito resistance to pyrethroids
Resistance to pyrethroids belonging to the knockdown agent groups that have been used as the spatial repellents, such as mosquito coils and related products, is the most serious concern

Summary

Introduction

One of the most successful events in the development of pesticide chemicals was the discovery of pyrethrum and the successful synthesis of pyrethroids. There are two main groups of pyrethroids: one possessing high knockdown activity but low killing activity and the other possessing high killing activity. The pyrethroids in the former group, such as d-allethrin, are aptly labeled as knockdown agents, and those in the latter group, as killing agents. The pyrethroids belonging to the latter group exhibit high photostability that enables their outdoor use, for example, as agricultural pesticides. Photostable pyrethroids comprise 40% of the insecticides used annually on a global level for indoor residual spraying against malaria vectors and 100% of the WHO-recommended insecticides for the treatment of mosquito nets. The exception is the use of dichlorodiphenyltrichloroethane (DDT) in African countries [2]

Methods

Results

Discussion

Conclusion