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Title: Bighorn sheep pneumonia: Sorting out the cause of a polymicrobial disease

item BESSER, THOMAS - Washington State University
item CASSIRER, FRANCES - Idaho Department Of Fish & Game
item Highland, Margaret
item WOLLF, PEREGRINE - Nevada Department Of Wildlife
item JUSTICE-ALLEN, ANNE - Arizona Game And Fish Department
item MANSFIELD, KRISTIN - Washington Department Of Fish & Wildlife
item DAVIS, MARGARET - Washington State University
item FOREYT, WILLIAM - Washington State University

Submitted to: Preventive Veterinary Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2012
Publication Date: 2/1/2013
Citation: Besser, T.E., Cassirer, F.E., Highland, M.A., Wollf, P., Justice-Allen, A., Mansfield, K., Davis, M.A., Foreyt, W. 2013. Bighorn sheep pneumonia: Sorting out the cause of a polymicrobial disease. Preventive Veterinary Medicine. 108(2-3):85-93.

Interpretive Summary: The epidemiology and cause(s) of pneumonia in bighorn sheep populations has long been the subject of controversial debate. The bighorn sheep pneumonia phenomenon is important both for bighorn sheep and for the domestic sheep industry in the Western United States, because published research has implied at least some domestic sheep may be carriers of the pathogens that cause pneumonia in bighorn sheep. This review provides an overview of the hypothesized causes of bighorn sheep pneumonia and evidence for and against each. These include lungworms, bacteria belonging to the Pasteurellaceae family, and the bacterium Mycoplasma ovipneumoniae. In summary, the authors describe the strong evidence that supports the importance of Mycoplasma ovipneumoniae in bighorn sheep pneumonia.

Technical Abstract: Pneumonia of bighorn sheep (Ovis canadensis) is a dramatic disease of high morbidity and mortality first described more than 80 years ago. The etiology of the disease has been debated since its initial discovery, and at various times lungworms, Mannheimia haemolytica and other Pasteurellaceae, and Mycoplasma ovipneumoniae have been proposed as primary causal agents. A multi-factorial “respiratory disease complex” has also been proposed as confirmation of causation has eluded investigators. In this paper we review the evidence for each of the candidate primary agents with regard to causal criteria including strength of association, temporality, plausibility, experimental evidence, and analogy. While we find some degree of biological plausibility for all agents and strong experimental evidence for M. haemolytica, we demonstrate that of the alternatives considered, M. ovipneumoniae is the best supported by all criteria and is therefore the most parsimonious explanation for the disease. The strong but somewhat controversial experimental evidence implicating disease transmission from domestic sheep is consistent with this finding. Based on epidemiologic and microbiologic data, we propose that healthy bighorn sheep populations are naïve to M. ovipneumoniae, and that its introduction to susceptible bighorn sheep populations results in epizootic polymicrobial bacterial pneumonia often followed by chronic infection in recovered adults. If this hypothesized model is correct, efforts to control this disease by development or application of vectored vaccines to Pasteurellaceae are unlikely to provide significant benefits, whereas efforts to ensure segregation of healthy bighorn sheep populations from M. ovipneumoniae-infected reservoir hosts are crucial to prevention of new disease epizootics. It may also be possible to develop M. ovipneumoniae vaccines or other management strategies that could reduce the impact of this devastating disease in bighorn sheep.