Division of Geographic Medicine, Johns Hopkins University, The Baltimore City Hospitals, International Center for Diarrheal Disease Research of Bangladesh, Dacca University, Department of Zoology, British Volunteer Service Overseas, Baltimore, Maryland 21224, Bangladesh
Stool surveys were conducted on children 2–10 years of age in 27 villages within Dacca District and around this index area (1,668 children were sampled), revealing an endemic focus of Fasciolopsis buski infection to the south and the east of Dacca District. In order to determine the seasonal variation in the total snail populations and the natural rate of F. buski infection in the snails, two species of planorbid snails, Segmentina (Trochorbis) trochoideus and Hippeutis (Helicorbis) umbilicalis, were periodically sampled for 12 months from a village endemic for F. buski infection. Gymnocephalous cercariae were found in S. (T.) trochoideus snails during August, September and October. The size of the snail population (n = 1,275) was significantly correlated with inches of rainfall (r = +0.62; P < 0.05) and water temperature (r = +0.59; P < 0.05). The natural infection rate of F. buski in the snails ranged from 0.5–2%. Snails from non-endemic areas were exposed to 3–10 miracidia. A total of 13 of 49 (27%) of H. (H.) umbilicalis and 6 of 14 (43%) of S. (T.) trochoideus had gymnocephalous cercariae present 4 to 6 weeks after exposure to miracidia. Thus, snail strain variation is unlikely to be a barrier to F. buski transmission.