Academic journal article Bulletin of the World Health Organization

Status of Pyrethroid Resistance in Anopheles Gambiae Sensu Lato

Academic journal article Bulletin of the World Health Organization

Status of Pyrethroid Resistance in Anopheles Gambiae Sensu Lato

Article excerpt

Voir page 233 le resume en francais. En la pagina 234 figura un resumen en espanol.

Introduction

Vector control is an important component of the WHO Global Strategy for Malaria Control, whose objective is to break the transmission of malaria parasites using indoor residual spraying or pyrethroid-impregnated materials (bednets and/or curtains). Pyrethroids are preferred for impregnation because they are highly effective and fast-acting insecticides with a strong excitorepellent effect on mosquitos.

During the Global Malaria Eradication Programme of the 1950s and 1960s, dieldrin resistance, involving a specific resistance mechanism ([Gamma]-aminobutyric acid (GABA) receptors), was recorded among most Anopheles gambiae s.l. populations in Africa. In contrast, only a few cases of DDT resistance have so far been recorded in Africa. The first, involving A. gambiae s.s., was observed in 1967 in Bobo Dioulasso (Burkina Faso) and attributed to the use of DDT against cotton pests (1-3, J. Hamon et al., unpublished data, 1968). Soon after, it was also observed among A. arabiensis from Senegal (1). DDT resistance can be due either to a specific detoxification mechanism (glutathione-S-transferase) or to a nerve insensitivity resulting from a modification of the target site (sodium channel). The latter, governed by the kdr gene, reduces both the knockdown and lethal effects of DDT. In West Africa, it induces a cross-resistance to pyrethroids, which also depends on kdr mutation (4). In Zanzibar, however, DDT resistance induced by glutathione-X-transferase did not cross with pyrethroids (5).

Since the 1970s, pyrethroids have been extensively used in urban areas (as domestic coils and aerosols) as well as for agricultural purposes in rural areas. In both cases, the selection pressure exerted on A. gambiae s.l. populations was not negligible: the first case of pyrethroid resistance in A. gambiae s.l. was recorded in Bouake (Cote d'Ivoire) and was attributed to the domestic use of aerosols (6). Later, reduced susceptibility was observed in an area of Kenya where permethrin-impregnated bednets were used (7, 8). In the Gambia, however, no change in the pyrethroid susceptibility of A. gambiae s.s. occurred as a result of an impregnated bednet project (9).

In view of the pyrethroid resistance observed at Bouake and the current prospects for extending the use of impregnated materials, surveys were carried out in Africa through an informal collaboration network. The objectives were first to confirm the resistance recorded in Bouake and evaluate its spread outside that city, and to obtain more information on the susceptibility of A. gambiae s.l. to pyrethroids in other countries.

Materials and methods

Mosquitos were harvested in six countries (see Fig. 1): Benin (Cotonou), Botswana (Sebina), Burkina Faso (Kou valley near Bobo Dioulasso), Cameroon (Yaounde), Cote d'Ivoire (Abidjan, Bouake, Daloa, Kafine, Katiola, Korhogo, Odienne, Tai, and Yao Koffikro) and Senegal (Dakar).

[Figure 1 ILLUSTRATION OMITTED]

Most of the tests were carried out on nonblood-fed, 2-4-day-old females that emerged from field-collected larvae or with the [F.sub.1] progeny of females collected in the field. Only in Cotonou were engorged wild-caught females tested directly. All the specimens were A. gambiae s.s. except in Dakar and Sebina where they were A. arabiensis (determined where possible by polymerase chain reaction (PCR)). A susceptible reference strain of A. gambiae s.s from Kisumu was used as the control.

Resistance tests

WHO test kits for adult mosquitos were used (10). Impregnated papers were prepared using silicone oil (Dow Corning 556) with technical deltamethrin and 25/75 permethrin active ingredient samples. Papers were impregnated using 3.6 mg of 0.25% (w/v) permethrin/oil solution per [cm.sup.2] and 0.025% (w/v) deltamethrin/oil solution per [cm.sup.2] (WHO diagnostic concentration (11)). …

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