Wild Sheep Disease and Research
Respiratory disease is widely recognized as the primary threat to bighorn sheep populations in Colorado and across the western United States. These diseases, particularly pneumonia, often originate from contact with domestic sheep, which can carry bacteria such as Mycoplasma ovipneumoniae. While domestic sheep are frequently unaffected carriers, the bacteria can prove deadly to bighorn sheep, whose immune systems are not adapted to these pathogens. Even a single interaction between the two species can result in a significant disease outbreak in a wild bighorn herd.
When a bighorn sheep population is exposed to these bacteria, the consequences can be devastating. Entire herds can experience catastrophic die-offs, often involving all age groups. These outbreaks frequently kill 50–90% of infected individuals within a short period, and in extreme cases, local extirpation can occur. Surviving adults often remain chronically infected or serve as disease reservoirs, which complicates recovery efforts and hinders the success of lamb recruitment in subsequent years.
The aftermath of a die-off is not quickly remedied. Bighorn sheep populations recover slowly, often taking decades to return to pre-outbreak numbers—if they recover at all. Low lamb survival, likely due to recurrent pneumonia infections passed from carrier ewes to their offspring, further slows population growth. Additionally, the social structure and strong site fidelity of bighorn sheep make natural recolonization and movement into vacant habitat unlikely without active management.
Efforts to prevent disease transmission often involve spatial separation between domestic and wild sheep, as well as targeted monitoring and removal of chronically infected individuals. However, balancing conservation with livestock grazing remains a complex challenge. Given the long-term impacts and the difficulty of restoring herds once they're infected, proactive prevention of initial contact remains the most effective strategy for protecting bighorn sheep populations across the West.
Please read the publications and research papers below for more information about respiratory disease in wild sheep.
Keeping Wild Sheep Healthy
The Wild Sheep Foundation has produced this excellent reference on wild sheep respiratory disease and what you can do to prevent outbreaks.
Western Association of Fish and Wildlife Agencies Wild Sheep Initiative
Recommendations for Domestic Sheep and Goat Management in Wild Sheep Habitat 2025
The Wild Sheep Initiative (WSI) is comprised of a representative from each WFWA state, province, and territory with wild sheep, plus representatives from the U.S. Forest Service and the Bureau of Land Management, National Park Service, Tribal entities, and the Wild Sheep Foundation. Their mission:
“To identify priority topics and management challenges to wild sheep in western North America; collaboratively develop solutions to those challenges; and foster strong relationships between state/ provincial/territorial jurisdictions, federal land management agencies, indigenous groups, and wild sheep advocates; and engage with domestic livestock owners and users that share the same landscapes inhabited by wild sheep.”
In 2025, the Wild Sheep Initiative updated their unified recommendations intended to guide state, provincial, and territorial wild sheep managers, federal or crown land management agencies, First Nation or tribal entities, wild sheep conservation organizations, private or public land domestic sheep or goat producers, and sheep or goat hobbyists to take actions that likely will reduce transmission of pathogens to wild sheep. Download the recommendations below.
Epidemic pasteurellosis in a bighorn sheep population coinciding with the appearance of a domestic sheep
George JL, Martin DJ, Lukacs PM, Miller MW. 2008. Epidemic pasteurellosis in a bighorn sheep population coinciding with the appearance of a domestic sheep. Journal of Wildlife Diseases, 44(2), 2008, pp. 388–403.
Abstract - A pneumonia epidemic reduced bighorn sheep (Ovis canadensis) survival and
recruitment during 1997–2000 in a population comprised of three interconnected wintering herds (Kenosha Mountains, Sugarloaf Mountain, Twin Eagles) that inhabited the Kenosha and Tarryall Mountain ranges in central Colorado, USA. The onset of this epidemic coincided temporally and spatially with the appearance of a single domestic sheep (Ovis aries) on the Sugarloaf Mountain herd’s winter range in December 1997. Although only bighorns in the Sugarloaf Mountain herd were affected in 1997–98, cases also occurred during 1998–99 in the other two wintering herds, likely after the epidemic spread via established seasonal movements of male bighorns. In all, we located 86 bighorn carcasses during 1997–2000. Three species of Pasteurella were isolated in various combinations from affected lung tissues from 20 bighorn carcasses where tissues were available and suitable for diagnostic evaluation; with one exception, b-hemolytic mannheimia (Pasteurella) haemolytica (primarily reported as biogroup 1G or 1aG) was isolated from lung tissues of cases evaluated during winter 1997–98. The epidemic dramatically lowered adult bighorn monthly survival in all three herds; a model that included an acute epidemic effect, differing between sexes and with vaccination status, that diminished linearly over the next 12 mo best represented field data. In addition to the direct mortality associated with epidemics in these three herds, lamb recruitment in years following the pneumonia epidemic also was depressed as compared to years prior to the epidemic. Based on observations presented here, pasteurellosis epidemics in free-ranging bighorn sheep can arise through incursion of domestic sheep onto native ranges, and thus minimizing contact between domestic and bighorn sheep appears to be a logical principle for bighorn sheep conservation.
Genetic structure of Mycoplasma ovipneumoniae informs pathogen spillover dynamics between domestic and wild Caprinae in the western United States
Kamath, PL, Manlove K, Cassirer EF, Cross PC, Besser TE. 2019. Genetic structure of Mycoplasma ovipneumoniae informs pathogen spillover dynamics between domestic and wild Caprinae in the western United States. Scientific Reports 9:15318.
Abstract - Spillover diseases have significant consequences for human and animal health, as well as wildlife conservation. We examined spillover and transmission of the pneumonia-associated bacterium Mycoplasma ovipneumoniae in domestic sheep, domestic goats, bighorn sheep, and mountain goats across the western United States using 594 isolates, collected from 1984 to 2017. Our results indicate high genetic diversity of M. ovipneumoniae strains within domestic sheep, whereas only one or a few
strains tend to circulate in most populations of bighorn sheep or mountain goats. These data suggest domestic sheep are a reservoir, while the few spillovers to bighorn sheep and mountain goats can persist for extended periods. Domestic goat strains form a distinct clade from those in domestic sheep, and strains from both clades are found in bighorn sheep. The genetic structure of domestic sheep strains could not be explained by geography, whereas some strains are spatially clustered and shared among proximate bighorn sheep populations, supporting pathogen establishment and spread following spillover. These data suggest that the ability to predict M. ovipneumoniae spillover into wildlife populations may remain a challenge given the high strain diversity in domestic sheep and need for more comprehensive pathogen surveillance.
Natural history of a bighorn sheep pneumonia epizootic: Source of infection, course of disease, and pathogen clearance
Besser, TE, Cassirer EF, Lisk A, Nelson D, Manlove KR, Cross PC, Hogg JT. 2021. Natural history of a bighorn sheep pneumonia epizootic: Source of infection, course of disease, and pathogen clearance. Ecology and Evolution 11:14366–14382.
Abstract - A respiratory disease epizootic at the National Bison Range (NBR) in Montana in
2016–2017 caused an 85% decline in the bighorn sheep population, documented by observations of its unmarked but individually identifiable members, the subjects of an ongoing long-term study. The index case was likely one of a small group of young bighorn sheep on a short-term exploratory foray in early summer of 2016. Disease subsequently spread through the population, with peak mortality in September and October and continuing signs of respiratory disease and sporadic mortality of all age classes through early July 2017. Body condition scores and clinical signs suggested that the disease affected ewe groups before rams, although by the end of the epizootic, ram mortality (90% of 71) exceeded ewe mortality (79% of 84). Microbiological sampling 10 years to 3 months prior to the epizootic had documented no evidence of infection or exposure to Mycoplasma ovipneumoniae at NBR, but during the epizootic, a single genetic strain of M. ovipneumoniae was etected in affected animals. Retrospective screening of domestic sheep flocks near the NBR identified the same genetic strain in one flock, presumptively the source of the epizootic infection. Evidence of fatal lamb pneumonia was observed during the first two lambing seasons following the epizootic but was absent during the third season following the death of the last identified M. ovipneumoniae carrier ewe. Monitoring of life-history traits prior to the epizootic provided no evidence that environmentally and/or demographically induced nutritional or other stress contributed to the epizootic. Furthermore, the epizootic occurred despite proactive management actions undertaken to reduce risk of disease and increase resilience in this population. This closely observed bighorn sheep epizootic uniquely illustrates the natural history of the disease including the
(presumptive) source of spillover, course, severity, and eventual pathogen clearance.
Fatal interactions: pneumonia in bighorn lambs following experimental exposure to carriers of Mycoplasma ovipneumoniae
Weyand LK, Felts BL, Cassirer EF, Jenks JA, Walsh DP, Besser TE. 2025. Fatal interactions: pneumonia in bighorn lambs following experimental exposure to carriers of Mycoplasma ovipneumoniae. J Clin Microbiol. 2025 Feb 19;63(2):e0132824
Abstract - We hypothesized that bighorn sheep ewes with chronic nasal Mycoplasma ovipneumoniae carriage are the source of infection that results in fatal lamb pneumonia. We tested this hypothesis in captive bighorn ewes at two study facilities over a 5-year period, by identifying carrier ewes and then comparing lamb fates in groups that did (exposed pens) or did not (non-exposed pens) include one or more carrier ewes. Most (23 of 30) lambs born in exposed pens, but none of 11 lambs born in non-exposed pens, contracted fatal pneumonia. In addition, surviving lambs in exposed pens showed obvious signs of respiratory disease while lambs in non-exposed pens did not. In crossover experiments, individual non-carrier ewes had lambs that experienced fatal pneumonia in years when housed in exposed pens, but not in years when housed in non-exposed pens. The results of these studies clearly associate lamb pneumonia to exposure to M. ovipneumoniae carrier ewes, consistent with a necessary role for this agent in epizootic pneumonia of bighorn sheep. These data specifically highlight the role of chronic M. ovipneumoniae carriage by some bighorn ewes in the epidemiology of this population-limiting wildlife disease.
Host vs. pathogen evolutionary arms race: Effects of exposure history on individual response to a genetically diverse pathogen
Walsh DP, Felts BL, Cassirer EF, Besser TE and Jenks JA (2023) Host vs. pathogen evolutionary arms race: Effects of exposure history on individual response to a genetically diverse pathogen. Front. Ecol. Evol. 10:1039234.