[Malaria transmission in lagoon areas in Côte d'Ivoire: the villages of N'gatty and Allaba].

The ecological and climatic variations in tropical areas of Africa make the transmission of malaria very heterogeneous. Several species of mosquitoes of the Anopheles genus transmit the disease. In Côte d'Ivoire, studies in the southern and northern forests have shown the involvement of Anopheles gambiae, An. funestus and An. nili in this transmission. This study was conducted in N'gatty and Allaba, two villages in a lagoon area of southern Côte d'Ivoire, in the district of Dabou. The district is located between 05 degrees 18 N and 04 degrees 27 W, about 40 kilometres from the economic capital of Côte d'Ivoire, and has a surface of 2257.8 km2. Two lagoon villages, N'gatty and Allaba, about a kilometre apart, were selected as sites for this research. The topography of N'gatty is rugged, with two important sides: a banana plantation on the west and the lagoon on the east, covered by aquatic plants (Pistia stratiotes, Acrosticum aureum, nympheacae and Eichhornia crassipes). This aquatic vegetation separates the last plots of the village from the lagoon. Mosquito populations were sampled from December 2007 through September 2008 by nighttime captures on volunteers three consecutive nights each month, inside six living rooms (4 in N'gatty and 2 in Allaba, a smaller village). The mosquitoes were captured when they landed on the volunteer's legs. There were two teams of catchers for each home; the first worked from 18:00 h to 24:00 and the second from 24:00 to 06:00. The ovaries and glands of An. gambiae were dissected. The physiological age of the females was determined by the appearance of the trachea on ovarian dissection. The sporozoites were sought by optic microscopy in cooled glands in a drop of physiological salt solution. During the 10 months of this study, 21 137 mosquitoes were caught in N'gatty and 7163 in Allaba. An. gambiae species accounted for less than 2% of the mosquito fauna in the two villages. Overall, the average bite rate was 203.24 bites per man per night (b/m/n) in N'gatty and 137.75 in Allaba. The average number of An. gambiae bites was 4.04 b/m/n in N'gatty and 2 b/m/n in Allaba. Annually, the average An. gambiae bites totalled 1474.6 bites per man per year (b/m/y) in N'gatty and 730 in Allaba. No An. funestus mosquito was caught in Allaba, and its average number of bites per man per year in N'gatty was only 3.65. No infected glands (n = 420 dissected) were found in N'gatty throughout the investigation. The mean sporozoite rate in Allaba was only 0.285% (n = 104). The mean entomological inoculation rate (EIR) was estimated at 0.017 infecting bites per man and per night (ib/m/n) in N'Gatty and 6.2 in Allaba. This weak entomological inoculation rate leads us to consider continuing the research for infection of An. gambiae by ELISA techniques. In conclusion, malaria transmission by An. gambiae in this area is very low.