Bighorn sheep (Ovis canadensis) diseases: a brief literature review and risk assessment for translocation

Prior to European settlement in western North America, bighorn sheep (Ovis canadensis) were more widespread and abundant than they are today (Buechner 1960). The species arrived via the Bering land bridge approximately 70-100,000 years before the present (YBP) (Kurten and Anderson 1980) and slowly spread to occupy most mountainous regions of western North America from southern British Columbia and Alberta, Canada to the Cape of Baja California and northern Sierra Madre in Mexico (Brown 1989). Based on fossil records, it is likely that bighorn sheep arrived in the southwestern United States at the end of the Pleistocene era approximately 9-12,000 YBP (Findley et al. 1975). It is clear that bighorn sheep underwent dramatic declines in both occupied area and numbers throughout their range in North America in the 3 decades prior to 1900. The most probable cause of declines in this era was the introduction of domestic sheep with a suite of diseases to which bighorn sheep were naïve (DeForge et al. 1981, Brown 1989, deVos 1989). Subsequent to 1900, bighorn sheep population declines continued due to several causes including habitat fragmentation and degradation, unregulated harvest for trophies and subsistence, and competition with domestic livestock. One strategy to repatriate bighorn sheep populations is translocation of groups from healthy source populations to repopulate vacant historic habitat. Translocation is also used as a management tool to bolster populations that are below demographic objectives. Managers overseeing translocations need to be cognizant of the potential to introduce diseases when moving animals, and their potential impacts on indigenous wildlife or domestic livestock. To facilitate translocations and minimize disease risk, managers need to develop an understanding of diseases that play roles in bighorn sheep demographics, and develop methods to minimize any risk to bighorn sheep, other wildlife, and livestock. This is particularly important when managers move bighorn sheep between jurisdictions and across international boundaries (typically Canada to the U. S., and bidirectional from U. S. – Mexico). In this paper, we review several diseases of livestock and bighorn sheep and propose recommendations for health screening of bighorns to minimize disease risks to animals in the recipient area and to aid in reestablishing healthy bighorn sheep populations.