PREVALENCE OF HELMINTHS IN A CYCLIC SNOWSHOE HARE POPULATION

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Abstract
Five species of helminths were monitored in a population of snowshoe hares (L. americanus) near Rochester, Alberta [Canada] during 1961-1977. Prevalence of both Obeliscoides cuniculi and Protostrongylus boughtoni among young hares averaged .apprx. 50% by age 2 mo. then tended to level off. Prevalence of Taenia pisiformis (cysticerci) and Dirofilaria scapiceps rose more slowly, but continued to increase steadily beyond their mean levels of 8% and 1% at age 2 mo. There were well defined seasonal (within-year) cycles in prevalence of O. cuniculi and P. boughtoni that were generated evidently to a major degree by arrested devolopment of larvae in fall and renewed development in late winter. It was hypothesized that renewed larval development was triggered (in Feb.) in O. cuniculi by the seasonal rise of circulating pituitary gonadotropins, and (in April) in P. boughtoni by the seasonal rise of gonadal androgens and estrogens. Indices to gonadal hormone levels in hares indicated that these increased most rapidly among males, and may have accounted for the higher prevalences of P. boughtoni in males during April-May. Neither T. pisiformis nor D. scapiceps exhibited conspicuous seasonal changes in prevalence. Maximum prevalence of T. pisiformis was attained at .apprx. 1 yr of age, whereas D. scapiceps increased among adult snowshoes through age 2 yr before stabilizing. Long-term (between-year) changes in prevalence of O. cuniculi, T. pisiformis, and D. scapiceps were correlated significantly with the cyclic hare population which declined from a peak in fall 1961 to a low in 1965-1966, rose to another peak by fall 1970, and declined again to a low in 1975. There were no detectable time lag between this 10-yr cycle in hare density and the cycles of parasite prevalence among juveniles (< 1 yr of age). Among adult hares, the cycle of O. cuniculi prevalence was likewise synchronous with that of the hare population, but the cycles of D. scapiceps and T. pisiformis lagged by .apprx. 1 and 2 yr, respectively. This lag in T. pisiformis prevalence was largely inexplicable. These data on P. boughtoni were not suitable for analyses of between-year trends; nor were those for the 5th helminth, Taenia serialis (coenuri), because mean prevalence was < 1% among both juveniles and adults. An apparent decline in T. serialis after the early 1950''s, and its continued scarcity thereafter, paralleled a major change in numbers of one important definitive host, the red fox (Vulpes vulpes). Lighter weights of young hares at age 37-96 days, and of adults and fully grown juveniles, were associated with P. boughtoni infections. There was no demonstrable relationship between snowshoe hare reproductive parameters and helminth parasitism.