Environmental Epidemiology of Intestinal Schistosomiasis in Uganda: Population Dynamics ofBiomphalaria(Gastropoda: Planorbidae) in Lake Albert and Lake Victoria with Observations on Natural Infections with Digenetic Trematodes

Abstract

This study documented the population dynamics ofBiomphalariaand associated natural infections with digenetic trematodes, along the shores of Lake Albert and Lake Victoria, recording local physicochemical factors. Over a two-and-a-half-year study period with monthly sampling, physicochemical factors were measured at 12 survey sites and all freshwater snails were collected. RetainedBiomphalariawere subsequently monitored in laboratory aquaria for shedding trematode cercariae, which were classified as either human infective (Schistosoma mansoni) or nonhuman infective. The population dynamics ofBiomphalariadiffered by location and by lake and had positive relationship with pH (P<0.001) in both lakes and negative relationship with conductivity (P=0.04) in Lake Albert. Of theBiomphalariacollected in Lake AlbertN=6,183, 8.9% were infected with digenetic trematodes of which 15.8% were sheddingS. mansonicercariae and 84.2% with nonhuman infective cercariae. In Lake Victoria, 2.1% of collectedBiomphalaria  N=13,172were infected with digenetic trematodes with 13.9% sheddingS. mansonicercariae, 85.7% shedding nonhuman infective cercariae, and 0.4% of infected snails shedding both types of cercariae. Upon morphological identification, species ofBiomphalariainfected includedB. sudanica,B. pfeifferi, andB. stanleyiin Lake Albert andB. sudanica,B. pfeifferi, andB. choanomphalain Lake Victoria. The study found the physicochemical factors that influencedBiomphalariapopulation and infections. The number and extent of snails sheddingS. mansonicercariae illustrate the high risk of transmission within these lake settings. For better control of this disease, greater effort should be placed on reducing environmental contamination by improvement of local water sanitation and hygiene.