[Population dynamics and infestation with Bulinus globosus in a Sudan-Sahelien zone of Cameroon]

The population dynamics of Bulinus globosus was followed at seasonal and perennial habitats in three villages in the Extreme North Province, Cameroon, known to have high prevalences of Schistosoma haematobium. The study covered 2 rainy and 1 dry season between June 1987 to November 1988. Of several environmental factors monitored, only rainfall and the resulting changes in water level were strongly correlated with changes in snail density, and even then only when values were at their extremes. In seasonal pools, adult snails emerge from estivation and begin laying eggs shortly after the first rains. A decline in the number of larger snails 3 to 6 weeks later suggests a die off of the founding population. In general, the population density increases modestly during the rainy season and then rapidly declines after torrential rains have ended in September or October. Adults are again dominate just before the habitats dry up. In seasonal streams, population densities fluctuate dramatically in association with rainfall and subsequent flooding, and populations stabilize only after the torrential rains have ceased. Unlike populations in temporary ponds, young snails constituted a large percentage of the population very late in the season. In perennial habitats, seasonal fluctuations in population density were related to manipulation of water levels in a rice irrigation scheme. The lowest densities occurred when the water levels were at their most extreme levels, between November and April. But declines in the population densities often preceded significant changes in the water level, indicating that this factor alone is not responsible for these declines. Of the B. globosus shedding schistosome cercariae, more than 90% were infected with S. haematobium and the remainder with S. bovis. Of 153 infected B. globosus, 87% were found in seasonal habitats. In those sites the first infected snails were found in July but the large numbers were not present until late August and September. Infected snails declined rapidly before the final decline in the general population of B. globosus in November and December. In perennial habitats, infected snails were most common in May and June, just before and early in the rainy season. These findings may prove useful in developing more effective schistosomiasis control measures in sahelian climates.