[Efficacy of permethrin-impregnated Olyset Net mosquito nets in a zone with pyrethroid resistant vectors. I--Entomologic evaluation]

Abstract

The efficacy of permethrin-treated Olyset Net mosquito nets on malaria transmission and morbidity was studied in Kafine, a village located in the savanna region of the Cote d'Ivoire in Africa. After collecting sociodemographic, entomological, and parasitological data, bednets were distributed first in the southern half of the village and then in the whole village. Throughout the study period, mosquito specimens were captured on the skin of inhabitants at four points in the village between 6 PM and 6 AM both inside (but outside bednets) and outside houses. Prior to distribution of bednets, the mean biting rate (MBR) by Anopheles gambiae was 77.4 bites per man per night (b/m/n). The mean parturity rate (MPR) was 40.6 p. 100, the sporozootic index (SI) was 0.99 p. 100, and the mean entomological inoculation rate (MEIR) was 0.7 infectious bites per man per night (b+/m/n). Six months after distribution of bednets in the southern half of the village, MBR was 80.2 b/m/n, MPR was 32 p. 100, SI was 1.8 p. 100, and MEIR was 0.83 b+/m/n. After extending distribution to the whole village, data from November 1996 to July 1997 were as follows: MBR, 67.8 b/m/n; MPR, 20.1 p. 100; SI, 0.65 p. 100; and MEIR, 0.66 p. 100. From August 1977 to July 1998, data were as follows: MBR, 102.6 b/m/n; MPR, 26.2 p. 100; SI, 1.15 p. 100; and MEIR, 0.74 b+/m/n. Comparative analysis of these data showed that use of bednets had no effect on the bite or entomological inoculation rate. This is in agreement with the documented resistance of vectors in the study zone to permethrin. Despite the known stimulation/repulsion effect of permethrin, use of treated bednets had no real impact on transmission. This inefficacy could be related to the high prevalence (80 p. 100) of the Kdr gene (responsible for resistance) in the savanna form of Anopheles gambiae.