Date Published: March 10, 2009
Publisher: Public Library of Science
Author(s): Hitoshi Kawada, Yukiko Higa, Yen T. Nguyen, Son H. Tran, Hoa T. Nguyen, Masahiro Takagi, Jeffrey M. Bethony
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.
Partial Text: One of the most successful events in the development of pesticide chemicals was the discovery of pyrethrum and the successful synthesis of pyrethroids. For example, allethrin , a classical synthetic pyrethroid, continues to be used for preventing mosquito bites without any toxicological and operational problems. 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. Generally, the pyrethroids belonging to the latter group exhibit high photostability that enables their outdoor use, for example, as agricultural pesticides. Nowadays, photo-stable pyrethroids are emerging as the predominant insecticides for vector control. In fact, photo-stable 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 .
The most plausible procedure for investigating the knockdown susceptibility of mosquitoes to pyrethroids is to acquire an adequate number of field-collected female adults or laboratory-reared colonies by rearing field-collected larvae and to carry out a knockdown bioassay with the adults and the actual pyrethroid formulation, such as a mosquito coil, in a laboratory. However, it was impossible for us to follow this procedure since 190 field-collected larval samples were acquired in the present study. Susceptibility tests were, therefore, performed on the day of collection according to a simplified protocol by using the fourth instar larvae. In many cases, the adult susceptibility scores correlated with the larval susceptibility scores ,. This is, however, not always true for all cases since mosquitoes might develop different resistant mechanisms with different metabolic pathways in the larval and adult stages. In the present study, the authors focused on knockdown that might not be chiefly dependent on the enhancement of metabolic activity but may be dependent on the nervous insensitivity controlled by the kdr gene. The simple bioassay with mosquito larvae that is presented in this paper might be a convenient and cost-effective method for evaluating mosquito knockdown resistance in the field.