Abstract
BackgroundPyrethroid insecticides are widely used for insect pest control in Cameroon. In certain insect species, particularly the malaria vector Anopheles gambiae, resistance to this class of insecticides is a source of great concern and needs to be monitored in order to sustain the efficacy of vector control operations in the fields. This study highlights trends in DDT and pyrethroid resistance in wild An. gambiae populations from South Cameroon.MethodsMosquitoes were collected between 2001 and 2007 in four sites in South Cameroon, where insecticides are used for agricultural or personal protection purposes. Insecticide use was documented in each site by interviewing residents. Batches of 2-4 days old adult female mosquitoes reared from larval collections were tested for susceptibility to DDT, permethrin and deltamethrin using standard WHO procedures. Control, dead and survivors mosquitoes from bioassays were identified by PCR-RFLP and characterized for the kdr mutations using either the AS-PCR or the HOLA method.ResultsFour chemical insecticide groups were cited in the study sites: organochlorines, organophosphates, carbamates and pyrethroids. These chemicals were used for personal, crop or wood protection. In the four An. gambiae populations tested, significant variation in resistance levels, molecular forms composition and kdr frequencies were recorded in the time span of the study. Increases in DDT and pyrethroid resistance, as observed in most areas, were generally associated with an increase in the relative frequency of the S molecular form carrying the kdr mutations at higher frequencies. In Mangoum, however, where only the S form was present, a significant increase in the frequency of kdr alleles between 2003 to 2007 diverged with a decrease of the level of resistance to DDT and pyrethroids. Analyses of the kdr frequencies in dead and surviving mosquitoes showed partial correlation between the kdr genotypes and resistance phenotypes, suggesting that the kdr mechanism may act with certain co-factors to be identified.ConclusionThese results demonstrate the ongoing spread of kdr alleles in An. gambiae in Central Africa. The rapid evolution of insecticide resistance in this highly dynamic and genetically polymorphic species remains a challenge for its control.
Highlights
Pyrethroid insecticides are widely used for insect pest control in Cameroon
Study sites and insecticide usage The study was conducted in 4 localities (Figure 1) of Southern Cameroon that are characterized by high insecticide usage for agro-industry or personal protection: (1) Ipono district (2°22'N, 9°50'E), an area of intensive forest exploitation and timber storage, with agricultural activities limited to the households' supplies, (2) Bonanloka district (4°03'N, 9°43'E), a highly urbanized area in the centre of Douala, the major economic city in Cameroon, where inhabitants use insecticides against arthropod nuisance, (3) Nkolondom district (3°51'N, 11°30'E), a market gardening area located in the outskirts of Yaoundé, the capital city of Cameroon, and (4) Mangoum district (5°31'N, 10°37'E), a locality with extensive manual and mechanized agricultural settings producing spices, vegetables and cereals
Females An. gambiae s.l. were used for insecticide resistance monitoring
Summary
Pyrethroid insecticides are widely used for insect pest control in Cameroon. In certain insect species, the malaria vector Anopheles gambiae, resistance to this class of insecticides is a source of great concern and needs to be monitored in order to sustain the efficacy of vector control operations in the fields. Agriculture and timber production are the main components of the economical activities in most equatorial African countries These economic activities require intensive use of pesticides including insecticides belonging to the four main chemical groups used in public health: organochlorines, carbamates, organophosphates and pyrethroids. In Cameroon, organochlorines such as DDT have been used extensively for both vector control and agricultural purposes in the southern areas of the country, especially during the 1950s malaria eradication campaign [13]. These compounds have progressively been replaced by alternative more specific and less toxic chemicals, in part because of the emergence of insecticide resistance in the target species [1,4]. 56% are crop pests, 39% are arthropods of medical or veterinary importance and 5% are beneficial species [14]
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