Ecology of Biomphalaria (Gastropoda: Planorbidae) in Lake Albert, Western Uganda: snail distributions, infection with schistosomes and temporal associations with environmental dynamics

Abstract

In Lake Albert, an ecological study was conducted, between June 2000 and May 2003, which assessed snail population dynamics, parasite infection patterns and interplay of environmental factors upon Biomphalaria. Monthly sampling surveys were conducted at 29 sites monitoring populations of Biomphalaria stanleyi and Biomphalaria sudanica. Altogether, a total of 21,715 B. stanleyi and 8452 B. sudanica were collected during the period. Both species could be found infected with Schistosoma mansoni although infection prevalence was significantly higher (p < 0.01**) in B. stanleyi (4.4%) than in B. sudanica (3.5%). Each species occupied slightly different aquatic niches with B. stanleyi preferring deeper water habitats whilst B. sudanica was found along the shoreline in shallower water. B. stanleyi was more widely distributed among the sampling locations (19 sites) than B. sudanica (10 sites). Of the four villages included in the study area, snails from sites near Piida and Bugoigo villages had the highest schistosome infection rates, presumably attributable to the closer proximity of people with intestinal schistosomiasis. After inspection of cross-correlation plots which identified most suitable time lags, snail density dynamics could be associated with seasonal variations inclusive of: air temperature, rainfall, lake level, water temperature, water conductivity and water pH. These temporal observations better reveal the relationship between snail populations and environmental factors, providing important information concerning the relative roles of B. stanleyi and B. sudanica in transmission of S. mansoni and development of integrated strategies for disease control around Lake Albert.