Our understanding of the causes of respiratory disease in bighorn sheep have evolved over time (Besser et al. 2013, Cassirer et al. 2018). In more recent years, there is significant scientific evidence that the respiratory bacterium Mycoplasma ovipneumoniae (M. ovi) is a necessary agent involved in epizootic and chronic pneumonia in bighorn sheep (Besser et al. 2012, Besser et al. 2013, Cassirer et al. 2017, Butler et al. 2018, Garwood et al. 2020). Among 14 M. ovi-positive herds in Montana (n=7) and in Wyoming (n=7), at least half of the herds have exhibited adequate lamb recruitment and adult survival and have recently been stable or increasing despite some history of pneumonia epizootics (Butler et al. 2018). Conversely, we are unaware of populations that have a recent or more distant history of pneumonia epizootics in which M. ovi has not been detected since diagnostic tools for M. ovi have become available. Based on the evidence accumulated to date, M. ovi is likely a necessary agent involved in pneumonia epizootics but may not be a sufficient indicator of a history, severity, or likelihood of pneumonia epizootics. Understanding the variation in response to infection with M. ovi continues to be the subject of ongoing study across the western US (Besser et al. 2021, Spaan et al. 2021, Martin et al. 2021).

Bacteria in the Pasteurellaceae family, particularly hemolytic strains or those that contain and express the Leukotoxin A gene, are thought to play important roles in respiratory disease, influencing patterns of morbidity and mortality (Dassanayake et al. 2010, Besser et al. 2012, Shanthalingam et al. 2014, Wood et al. 2016).  Past work has indicated that detection probabilities are extremely low (0.12-0.36) for the majority of Pasteurellaceae using culture-based diagnostic methods (Butler 2017, Butler et al. 2017, Walsh et al. 2012). The detection of the leukotoxin gene does not always correspond with the detection of hemolytic activity of the bacteria (the ability of the bacteria to lyse red blood cells), which is assumed to be the sign of toxin expression (Fisher et al. 1999).  This indicates that even when leukotoxin A gene is present in the Pasteurellaceae bacteria, it is not always being expressed. How this relates to future risk of gene expression and associated hemolytic activity is unknown (Almberg et al., 2022).

Respiratory disease continues to have pronounced effects on bighorn sheep populations throughout the western US and Canada (Aune et al. 1998, Gross et al. 2000, Singer et al. 2000, Cassirer et al. 2007, Cassirer et al. 2013), with less certainty about the impacts on mountain goats. In Montana, bighorn sheep populations spread over a variety of landscapes have experienced approximately 25 known respiratory disease epizootic events since 1979 (Sells et al. 2015). During the single winter of 2009-10, 4 bighorn sheep populations covering a large portion of western Montana experienced simultaneous all-age die off events due to respiratory disease (Edwards et al. 2010).

Mycoplasma ovipneumoniae (M. ovi) is primarily a pathogen of wild and domestic sheep and goats, however, impacts vary among wild and domestic species.  In domestic sheep, M. ovi infection may be subclinical and generally causes low mortality with variable morbidity (Highland et al., 2018). In wild sheep and goats, M. ovi has often been associated with all-age die-offs followed by periods of poor lamb recruitment. Due to the potential for pathogen transfer, significant effort is made to maintain separation between domestic and wild sheep and goats as well as, within reason, between known infected and non-infected wild sheep and goats.  While the role of other wild and domestic species in bighorn sheep and mountain goat pneumonia is not clear, M. ovi has been detected in smaller numbers of other species including muskoxen, Biera antelope (Qatar), cattle, white-tailed deer, mule deer, caribou, and moose (Highland et al., 2018).  To date, M. ovi has not been detected among cervids in Montana.

Mycoplasma ovipneumoniae, the bacterium believed to be a necessary pathogen in development of bighorn sheep/mountain goat pneumonia, is shed in respiratory droplets and is transmitted by direct contact or via aerosols at distances up to 12-15 meters (Besser et al. 2014, Felts 2020). When not associated with a host species, the bacteria do not survive long in the environment.

Pneumonia epizootics occur frequently among free-ranging bighorn sheep (Ovis canadensis), resulting in mortality due to both all-age pneumonia die-offs and enzootic (constantly present) pneumonia characterized by sporadic or persistent high rates of pneumonia, particularly among lambs (Cassirer et al., 2007; Besser et al., 2007). Affected bighorn sheep may display various symptoms, including coughing, nasal discharge, ear paresis, headshaking, fever, lethargy, and sudden death (Highland et al., 2018).

There is no evidence that bighorn sheep pneumonia can be transmitted to or cause disease in humans.

Is it safe to eat the meat?

Meat from animals that have mild pneumonia can be consumed by humans; however, if the animal has active disease with abscesses in the lungs or is in poor condition, the meat should not be consumed by humans.  If you harvest an animal and are unsure whether the meat can be consumed, contact your local FWP office as soon as possible after harvest.

Almberg, E.A., Ramsey, J., Becker, M., Carson, K., Freund, G., Gude, J., Bighorn Sheep and Mountain Goat Herd Health Assessments. Federal Aid in Wildlife Restoration Grant W-166-SI. Final Report, 2016-2022, January 6, 2022.

Besser TE, Cassirer EF, Potter KA, VanderSchalie J, Fischer A, Knowles DP, Herndon DR, Rurangirwa FR, Weiser GC, Srikumaran S. Association of Mycoplasma ovipneumoniae infection with population-limiting respiratory disease in free-ranging Rocky Mountain bighorn sheep (Ovis canadensis canadensis). J Clin Microbiol. 2008 Feb;46(2):423-30. doi: 10.1128/JCM.01931-07. Epub 2007 Dec 5. PMID: 18057131; PMCID: PMC2238132.

Besser, T.E., Cassirer, E.F., Lisk, A., Nelson, D., Manlove, K.R., Cross, P.C. and Hogg, J.T., 2021. Natural history of a bighorn sheep pneumonia epizootic: Source of infection, course of disease, and pathogen clearance. Ecology and evolution, 11(21), pp.14366-14382.

Butler, C.J., Edwards, W.H., Paterson, J.T., Proffitt, K.M., Jennings-Gaines, J.E., Killion, H.J., Wood, M.E., Ramsey, J.M., Almberg, E.S., Dewey, S.R. and McWhirter, D.E., 2018. Respiratory pathogens and their association with population performance in Montana and Wyoming bighorn sheep populations. PloS one, 13(11), p.e0207780.

Cassirer, E. F., and A. R. E. Sinclair.2007. Dynamics of pneumonia in a bighorn sheep metapopulation. J. Wildl. Manag.71:1080-1088.

Garwood, T.J., Lehman, C.P., Walsh, D.P., Cassirer, E.F., Besser, T.E. and Jenks, J.A., 2020. Removal of chronic Mycoplasma ovipneumoniae carrier ewes eliminates pneumonia in a bighorn sheep population. Ecology and evolution, 10(7), pp.3491-3502.

Highland MA, Herndon DR, Bender SC, Hansen L, Gerlach RF, Beckmen KB. Mycoplasma ovipneumoniae in Wildlife Species beyond Subfamily Caprinae. Emerg Infect Dis. 2018;24(12):2384-2386. doi:10.3201/eid2412.180632

Martin, A.M., Cassirer, E.F., Waits, L.P., Plowright, R.K., Cross, P.C. and Andrews, K.R., 2021. Genomic association with pathogen carriage in bighorn sheep (Ovis canadensis). Ecology and evolution, 11(6), pp.2488-2502.

Spaan, R.S., Epps, C.W., Crowhurst, R., Whittaker, D., Cox, M. and Duarte, A., 2021. Impact of Mycoplasma ovipneumoniae on juvenile bighorn sheep (Ovis canadensis) survival in the northern Basin and Range ecosystem. PeerJ, 9, p.e10710.