a1 School of Biology and Biochemistry, The Queens's University of Belfast, Belfast BT7 1NN, Northern Ireland, UK
a2 Department of Pathology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73190, USA
a3 Section of Parasitology, Department of Zoology, Aligarh Muslim University, Aligarh 202002, U.P., India
a4 Department of Statistics, University of Oxford, OX1 3TG, UK
Natural populations of the freshwater gastropods Bithynia tentaculata, Gyraulus convexiusculus, Helicorbis coenosus, Indoplanorbis exustus, Lymnaea acuminata, L. luteola and Vivipara bengalensis, were examined from October 1985 to September 1986 at seven transmission sites for paramphistomiasis, fascioliasis and other gastropod-borne infections in Aligarh, northern India. Gastropod diversity and the type and abundance of vegetation were highest at sites containing permanent water bodies. The distributions of H. coenosus and L. acuminata were positively correlated with the type and abundance of vegetation at these sites. Ephemeral water bodies contained B. tentaculata, I. exustus and L. luteola which were resistant to desiccation during the hot season. Aestivation in these gastropods preceded pronounced breeding activity which began with the onset of the monsoon season. Multivariate analysis (by principal components) was used to investigate relationships between each species of gastropod, their site of occurrence and levels of infection with larval helminths. Seasonal variations in numbers and size class structure in these gastropods were found to influence their prevalences of infection with larval helminths. Metacercarial and cysticercoid infections were more prevalent in smaller size classes of B. tentaculata and V. bengalensis, whereas larval paramphistomes were more prevalent in larger size classes of G. convexiusculus, I. exustus and L. luteola. Attempts were made to equate these findings with seasonality in egg shedding by certain species of adult paramphistomes occurring in domestic ruminants.
(Accepted September 28 1994)
c1 Present address and address for all correspondence: Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymounth PL1 2PB, UK.