A Long-Term Study on Various Aspects of the Population Biology of Ornithodiplostomum ptychocheilus in a South Carolina Cooling Reservoir

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Abstract
The population biology of Ornithodiplostomum ptychocheilus was examined in mosquitofish, Gambusia affinis, within ambient temperature and thermally altered locations of a South Carolina cooling reservoir. During the 53-mo study period, both prevalence and mean density of metacercariae fluctuated erratically in mosquitofish from ambient areas. In heated parts of the reservoir, seasonal changes were also unpredictable although there was a tendency to be highest in spring and decline in late summer and early fall. For 31 of the 53 mo, mean infrapopulation densities were significantly different at the two collecting sites; for 20 of the 31 mo, densities were significantly higher at the thermally altered location. During one 12-mo period, prevalence and density of O. ptychocheilus metacercariae were followed within four, arbitrarily established, size classes of mosquitofish. At the ambient temperature site, prevalence remained consistently high (> 85%) in the largest fish (⩾36 mm), but declined from high levels in smaller size classes beginning in November and continuing into March. Seasonal changes in mean densities closely approximated prevalence. Thermal effluent significantly affected recruitment of metacercariae and cercarial shedding from infected snails (Physa sp.) during summer and winter months. For a period of several weeks in late summer and early fall, both cercarial shedding and metacercarial recruitment were interrupted, resulting in a precipitous decline in prevalence and mean infrapopulation densities within mosquitofish of all size classes. Cercarial shedding persisted throughout the winter months while prevalence and density of metacercariae were elevated above the low levels during summer. These changes in prevalence and density within various size classes at the ambient temperature site were a consequence of variable growth rates of hosts, host reproduction and host mortality, in combination with seasonal changes in cercarial shedding and metacercarial recruitment. All of these variables were affected greatly by artificially elevated temperature, to the extent that prevalence and density dropped sharply during periods when summer temperatures were highest. While remaining somewhat regular at the heated location, both prevalence and mean density declined significantly at the ambient temperature site over the last 24 mo of the study. This drop may have been caused by a change in foraging or nesting behavior by the definitive host which, in turn, depressed the numbers of infective agents being dispersed into the study area.