Effects of Ivermectin on Lungworm Infection and Lamb Survival of Bighorn Sheep in Custer State Park, South Dakota

I examined the effects of the anthelmintic drug, ivermectin, on lungworm (Muellerius capillaris) densities, reproductive rates, and survival of lambs of free-ranging bighorn sheep in Custer State Park (CSP), South Dakota. Three distinct ewe herds in CSP allowed for comparisons among ewes given o, l, and 2 treatments of ivermectin. Treatments were given in February and March, 1987, and January and February, 1988. Ivermectin was sprayed on alfalfa hay and ingested by sheep. Within 2 weeks of treatment, densities of lungworm larvae were lower in feces of bighorn sheep that received ivermectin than in control animals. This effect was short-term, however, and larval densities of treated sheep began to increase within 4-6 weeks of treatment. Mean densities of lungworm larvae in feces were not different among treatment groups by the summer following treatment; nor were they different from larval densities calculated between 1980-85, the years prior to this study. There were no differences among treatment groups in reproductive rates of ewes or survival rates of lambs. Reproductive rates and lamb survival during 1987 and 1988 were similar to rates calculated for the bighorn sheep in CSP during 1983-85. Although oral administration of ivermectin did decrease densities of lungworm larvae, it appeared that ivermectin was not 100% effective against all stages of M. capillaris. Surviving lungworm eventually matured and reproduced, perpetuating infection levels. Multiple treatments per year with ivermectin may extend the short-term effects of drug treatment. Management activities which may eventually increase bighorn sheep populations in CSP include increased drug treatments to control lungworm for longer time periods and programs designed to increase the quality and quantity of habitat available to the bighorn sheep