Transmission dynamics of a trematode parasite: exposure, acquired resistance and parasite aggregation.

This paper examines the relative importance of exposure and susceptibility to the infection of rainbow trout ( Oncorhynchus mykiss) with the trematode parasite Diplostomum spathaceum under natural conditions. A total of 93 individually marked, similarly aged fish were introduced into three cages at regular time intervals and the intensity of infection in individuals recorded by counting parasites in live fish using ophthalmic techniques. Fish introduced into the cages became infected faster than fish that were already in the cages, indicating that fish developed resistance to infection after repeated exposure. Fish kept in the cages experienced similar levels of exposure and the distribution of parasites between these fish was not significantly different from a random distribution. In contrast, parasites from 16 Finnish wild roach populations were highly aggregated. The differences between the caged fish and the wild fish indicate that the aggregated distribution in wild fish might be determined by variations in exposure rather than variations in susceptibility between fish. This is one of the few studies to demonstrate the development of resistance in fish against the parasite under natural conditions, and to attempt to separate exposure and susceptibility as causative agents of parasite aggregation.