The aggregated nature of helminth parasite populations has led to the suggestion that selectively treating heavily infected hosts can efficiently reduce parasite abundance and morbidity within a host population. Moreover, it has been postulated that a selective treatment protocol might have long-term effects on parasite transmission by disrupting the stability attributed to aggregated parasite population distributions by theoretical models. Long-term investigation has demonstrated year-to-year consistency in the population dynamics of Halipegus occidualis in green frogs from Charlie's Pond, North Carolina. In 1996, removal of all but 1 worm from each frog with ≥15 worms reduced the estimated component worm population by 45%, thereby decreasing mean intensity and aggregation (variance-to-mean ratio) of H. occidualis in the frogs by 85% and 63%, respectively. The following year, mean intensity, aggregation, and host colonization trends returned to pretreatment levels, indicating no effect of worm removal and demonstrating the stability of this host–parasite system. Although this result might be attributable to inefficient treatment or the presence of infection reservoirs, it is suggested that parasite population stability in this system might be governed by prevalence rather than intensity of adult worms. Therefore, repeated selective treatment might effectively modify intensity-dependent morbidity in similar host–parasite systems but should not affect further parasite transmission.
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