EFFICACY OF IVERMECTIN AGAINST NEMATODES INFECTING FIELD POPULATIONS OF SNOWSHOE HARES (LEPUS-AMERICANUS) IN YUKON, CANADA

From July 1990 to February 1991, nematode numbers in free-ranging snow shoe hares (Lepus americanus) at Kluane Lake, southwestern Yukon, Cana da, were manipulated by subcutaneous injection (0.4 mg/kg) of ivermect in. Three field experiments were conducted to determine the degree of helminth loss associated with a single administration of ivermectin; t he length of time that ivermectin was effective in reducing worm numbe rs; and the effect of repeated ivermectin administration in reducing w orm numbers. Numbers of the nematodes, Protostrongylus boughtoni and N ematodirus triangularis were reduced by approximately 80% 2 wk after t reatment with a single dose of ivermectin, and were still significantl y lower than controls at 4 wk. However, beyond 2 wk, ivermectin did no t affect the rate of acquisition of new worms of either species. All t reated groups contained one or more hares in which numbers of P. bough toni and N. triangularis were not reduced. In addition, ivermectin had no effect on numbers of Trichuris leporis or Passalurus sp. Overall, ivermectin was not as effective against the nematodes of free-ranging hares as has been reported for nematodes of domestic and laboratory an imals.