A Survey of Lungworm Infection in Bighorn Sheep of Montana

A survey of lungworm infection (Protostrongylus stilesi and P. rushi) in the Rocky Mountain bighorn sheep (Ovis canadensis) of Montana was conducted from 1958 to 1963. Ninety-one percent of 900 fecal samples collected from 10 bighorn herds in western Montana contained protostrongylid larvae. Ninety-three percent of 143 lungs examined were infected with P. stilesi and 40 percent with both P. stilesi and P. rushi. No lungs contained only P. rushi. Total lesion area of P. stilesi infections averaged 2,332 mm2 and varied from I to 12,462 mm2. Numbers of adult P. rushi averaged 10 per infected lung and varied from 1 to 162. Sheep appeared to become infected during their first year of life. There seemed to be some association between infections of P. stilesi and P. rushi in lungs and the numbers of first-stage protostrongylid larvae in the feces when samples were compared on a herd basis. Numbers of first-stage larvae in individual fecal droppings from the same samples, in samples collected daily from the same animal, and in samples collected monthly from a herd of bighorns varied considerably. Feces collected during the spring contained more larvae than samples collected at other times of the year. Yearly variation in intensity of infection appeared to be directly correlated to the amount of precipitation in the preceding year. There has been a general decline in the number of Rocky Mountain bighorn sheep since the nineteenth century. During this period there have been some dramatic dieoffs which have stimulated a number of studies (Buechner 1960). Various disease conditions have been noted in these studies, and recently a lungworm-pneumonia complex has been implicated as the most important. Hunter and Pillmore (1954: 118) suggest that many of the earlier losses attributed to hemorrhagic septicemia may actually have been due to this disease complex. The parenchymal lungworm (P. stilesi) and the broncheolar lungworm (P. rushi) have been commonly associated with the complex. Protostrongylus stilesi has been reported to be the more virulent of the two species (Honess 1942:23). Basic to an understanding of the impact of lungworm on bighorn sheep is information on the intensity and distribution of fections in natural populations. Reports of lungworms in bighorn sheep have been reviewed by Buechner (1960:94-104). The most extensive and detailed survey was conducted in Colorado (Hunter and Pillmore 1954:125-126, Pillmore 1961:8793). The present survey in Montana was initiated in 1958 and continued to 1963. Its purpose was to determine the frequency and degree of protostrongylid lungworm infection so that they could be used with other data for evaluation of bighorn sheep population dynamics and management. This study was supported in part by a grant from the National Science Foundation (NSF G-19422). The 1958-60 data are from a thesis submitted by the senior author in partial fulfillment of the requirements for the degree of Master of Science in Wildlife Technology at Montana State University, Missoula. The authors wish to thank the personnel of the Montana Fish and Game Department and the hunters who helped in collecting bighorn lungs and fecal samples. R. E. Pillmore, for'Present address: Department of Veterinary Microbiology, University of California, Davis. 2 Present address: Department of Biology, Western Washington State College, Bellingham.