The trematode infection was detected by examining
the tissue microscopically after crushing the snails or
by inducing the cercarial emergence of parasites by
placing the animals singly in de-chlorinated tap water
in 0.5 l plastic beakers at a temperature rising to 20 ±
1C. The first method was used for the populations
sampled in June (1999) and the emergence method
for those sampled in August (2005 and 2006). In
June, when Diplostomum spp. infections in snails are
recent and only a few fully developed cercariae are