POPULATION-DYNAMICS OF FRESH-WATER GASTROPODS AND EPIDEMIOLOGY OF THEIR HELMINTH INFECTIONS WITH EMPHASIS ON LARVAL PARAMPHISTOMES IN NORTHERN INDIA

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 pa ramphistomiasis, fascioliasis and other gastropod-borne infections in Aligarh, northern India. Gastropod diversity and the type and abundanc e of vegetation were highest at sites containing permanent water bodie s. The distributions of H. coenosus and L. acuminata were positively c orrelated with the type and abundance of vegetation at these sites. Ep hemeral water bodies contained B. tentaculata, I. exustus and L. luteo la which were resistant to desiccation during the hot season. Aestivat ion in these gastropods preceded pronounced breeding activity which be gan with the onset of the monsoon season. Multivariate analysis (by pr incipal components) was used to investigate relationships between each species of gastropod, their site of occurrence and levels of infectio n with larval helminths. Seasonal variations in numbers and size class structure in these gastropods were found to influence their prevalenc es of infection with larval helminths. Metacercarial and cysticercoid infections were more prevalent in smaller size classes of B. tentacula ta and V. bengalensis, whereas larval paramphistomes were more prevale nt in larger size classes of G. convexiusculus, I. exustus and L. lute ola. Attempts were made to equate these findings with seasonality in e gg shedding by certain species of adult paramphistomes occurring in do mestic ruminants.