Identification and characterization of Anopheles spp. breeding habitats in the Korhogo area in northern Côte d'Ivoire: a study prior to a Bti-based larviciding intervention.

backgroundAlthough larviciding may be a valuable tool to supplement long-lasting insecticide nets (LLINs) in West Africa in different ecological settings, its actual impact on malaria burden and transmission has yet to be demonstrated. A randomized controlled trial was therefore undertaken to assess the effectiveness of larviciding using Bacillus thuringiensis israeliensis (Bti) in addition to the use of LLINs. In order to optimally implement such a larviciding intervention, we first aimed to identify and to characterize the breeding habitats of Anopheles spp. in the entire study area located in the vicinity of Korhogo in northern Côte d'Ivoire.

methodsWe conducted two surveys during the rainy and the dry season, respectively, in the thirty villages around Korhogo involved in the study. In each survey, water bodies located within a 2 km radius around each village were identified and assessed for the presence of mosquito larvae. We morphologically identified the larvae to the genus level and we characterized all of the habitats positive for Anopheles spp. larvae based on a predefined set of criteria.

resultsOverall, 620 and 188 water bodies positive for Anopheles spp. larvae were sampled in the rainy and the dry season, respectively. A broad range of habitat types were identified. Rice paddies accounted for 61% and 57% of the habitats encountered in the rainy and the dry season, respectively. In the rainy season, edges of rivers and streams (12%) were the second most abundant habitats for Anopheles spp. larvae. More than 90% of the Anopheles spp. breeding habitats were surrounded by green areas. Dams, ponds and drains produced higher numbers of Anopheles spp. larvae per square meter than rice paddies (RR = 1.51; 95% CI: 1.18-1.94; P = 0.0010). The density of Anopheles spp. larvae was significantly higher in habitats surrounded by low-density housing (RR = 4.81; 95% CI: 1.84-12.60; P = 0.0014) and green areas (RR = 3.96; 95% CI: 1.92-8.16; P = 0.0002] than habitats surrounded by high-density housing. Turbid water [RR = 1.42 (95% CI: 1.15-1.76; P = 0.0012) was associated with higher densities of Anopheles spp. larvae. The likelihood of finding mosquito pupae in Anopheles spp. breeding habitats was higher in the dry season (OR = 5.92; 95% CI: 2.11-16.63; P = 0.0007) than in the rainy season.

conclusionsRice paddies represented the most frequent habitat type for Anopheles spp. larvae in the Korhogo area during both the rainy and the dry seasons. Anopheles spp. breeding habitats covered a very large and dynamic area in the rainy season whereas they were fewer in number in the dry season. In this context, implementing a larviciding strategy from the end of the rainy season to the dry season is presumably the most cost-effective strategy.