Interactions between a brain-encysting trematode and its intermediate host, the fathead minnow

Abstract

Determining the extent to which parasites influence natural populaions of hosts is a major focus of studies in parasitology. Addressing this issue requires host-parasite systems that can be monitored under natural conditions and can be manipulated in the laboratory. I study a model system involving the larval trematode Ornithodiplostomum ptychocheilus that encysts in the brains of its intermediate host, the fathead minnow (Pimephales promelas). This parasite was the most common and abundant of 13 other parasites found in minnows in four boreal lakes in Alberta, Canada. In two of these lakes, prevalence of infection reached 100% in most years and mean intensity ranged from 4 to 40 parasites/host. Field and laboratory experiments showed that the size, the rate of parasite development, and time to encystment were intensity-dependent. However, parasite intensity had no effect on host or parasite survival after a simulated winter in the laboratory. One effect of infection was that infected fish and significantly greater cranial heights and widths than controls. The expression of this parasite-induced alteration in host phenotype was dependent on the size of the fish at infection and on parasite intensity. The cranial distortion led to significantly higher mortality of fish maintained on poor diets and altered the host's phototactic response.