Dynamics of Lymnaea natalensis populations in the Zaria area (Nigeria) and the relation to Fasciola gigantica infections.

Observations on the seasonal changes in the Lymnaea natalensis population and the infection rate of these snails with Fasciola gigantica during 1972-1976 are reported. The data were obtained from snails in three different locations near Zaria in Northern Nigeria. One of the survey sites dried out during the drought of 1972-1974 and had to be abandoned. The snail population showed seasonal variations with the lowest number of snails at the very end of the dry season. Many juvenile snails developed at the end of the wet season and beginning of the dry season, and the population reached its climax at the middle of the dry season. Fasciola infections in the snails occurred throughout the year, but the majority of the infections appeared to be acquired during the end of the wet season resulting in a high infection rate (up to 25%) in the beginning of the dry season. There was evidence of accelerated growth and gigantism in the infested snails. The occurrence of Fasciola infections in snails appears to depend on the presence (or absence) of livestock in snail infected areas during the rainy season. The infection developing in the rainy-season generation of snails is of major importance in the dissemination of fascioliasis in livestock as metacercarial shedding more or less coincides with the river-valley grazing of cattle.