Domestic sheep, bighorn sheep, and respiratory disease: a review of the experimental evidence

Introduced infectious diseases pose a significant threat to wildlife populations and are exceptional conservation challenges, in part because they can precipitate much more rapid and devastating population declines than habitat encroachment. Pneumonia epizootics have played a major role in the dynamics and conservation challenges of bighorn sheep (Ovis canadensis) populations. A large proportion of native bighorn sheep populations south of Canada went extinct beginning in the second half of the 19th century. It has long been postulated, based on temporal and spatial correlations, that diseases transferred from domestic sheep (Ovis aries) played a major role in those losses. Although experimental research has repeatedly tested the hypothesis that domestic sheep carry strains of respiratory tract pathogens potentially fatal to bighorn sheep, debate continues over the role of domestic sheep in this disease process. In the context of a hierarchical set of hypotheses we review this experimental research that includes (1) contact trials involving bighorn sheep penned with domestic sheep and a variety of other native and domestic animal species; (2) inoculation experiments with no animal contact; (3) attempts to isolate and identify specific organisms responsible for pneumonia in bighorn sheep; and (4) vaccination experiments. Our review reveals that (1) experiments have repeatedly corroborated the hypothesis that bighorn sheep have a high probability of contracting fatal pneumonia following contact with domestic sheep; (2) low disease and mortality rates in numerous co-pasturing pen studies involving bighorn sheep and animals other than domestic sheep do not support the alternative explanation that the results of the co-pasturing studies involving domestic sheep were an artifact of captivity; (3) the identification of which organism(s) cause pneumonia in bighorn sheep following contact with domestic sheep remains unresolved, possibly because of disease complexity (multiple pathogens) and limitations of Domestic sheep, bighorn sheep, and respiratory disease: a review of the experimental evidence