Biology of Disease Connection

DISEASE TRANSMISSION EVIDENCE

Introduction
The DBC’s technical staff emphasizes that: “There is a preponderance of evidence . . . strongly linking the presence of domestic sheep with the subsequent loss of part or all of the affected bighorn population” (1990, 33).

Captivity Cases
Numerous studies involving penned wild and domestic sheep have confirmed a fatal disease connection (Wehausen, Kelley, and Ramey II 2011). One of the most compelling and recent studies was described in a Journal of Wildlife Diseases article entitled: “Transmission of Mannheimia haemolytica from domestic sheep (Ovis aries) to bighorn sheep (Ovis canadensis): Unequivocal demonstration with green fluorescent protein-tagged organisms” (Lawrence et al. 2010). In this study, scientists marked M. haemolytica isolates from four clinically normal domestic sheep with a plasmid containing green fluorescent protein genes. Via intranasal spraying, the tagged genes were then colonized back into the four sheep they came from. The marked domestic sheep were kept 10 m (33 ft) away from four bighorns for one month. During that time, scientists did not observe pneumonia symptoms in the bighorns (Lawrence et al. 2010).

bighorn and domestic sheep contact experiment at Washington State University

The marked domestic sheep were then permitted to have fence-line contact with bighorns for two months. In that time, three bighorns contracted marked bacteria from domestic sheep. After the two months of allowed fence-line contact, scientists let the bighorns and domestic sheep commingle (Lawrence et al. 2010). Lawrence et al. describe what happened next: “All four bighorn sheep died 2 days to 9 days following commingling. The lungs from all four bighorn sheep showed gross and histopathologic lesions characteristic of M. haemolytica pneumonia. Tagged M. haemolytica were isolated from all four bighorn sheep” (2010, 706). The researchers go on to state: “These results unequivocally demonstrate transmission of M. haemolytica from domestic to bighorn sheep, resulting in pneumonia and death of bighorn sheep” (Lawrence et al. 2010, 706). The fluorescent protein-tagging study was funded by the Wyoming Wildlife-Livestock Disease Research Partnership Funds, the Wild Sheep Foundation, and the U.S. Forest Service (Lawrence et al. 2010).

In general, the closer bighorns and domestic sheep get to each other, the closer the bighorns get to death. For example, in 10 planned pen experiments (results published in 1988, 1989, 1990, and 1994) conducted by three independent research groups, all “23 bighorn sheep tested . . . died of respiratory disease following contact with domestic sheep, or were euthanized when close to death” (Wehausen, Kelley, and Ramey II 2011, 10). Domestic sheep in the experiments remained healthy (Wehausen, Kelley, and Ramey II 2011).

Evidence for one-sided, fatal wild-domestic sheep disease transmission also comes from an unplanned 1980 disease outbreak that occurred in a penned bighorn population at Lava Beds National Monument in northern California. In 1971, wildlife managers transplanted bighorns to a 540 hectare (1,334 acre) pen in the Monument. As Wehausen, Kelley, and Ramey II explain:

“In 1980, nose-to-nose contact was observed through the enclosure fence between bighorn sheep and domestic sheep grazed on adjacent National Forest lands. Bighorn sheep began dying of pneumonia 2-3 weeks later and all 43 bighorn subsequently died” (2011, 10).

Anecdotal Cases
Much of the disease transmission evidence associated with cases of free-ranging bighorns is anecdotal. At the DBC’s 2007 meeting, Brigham (retired biologist) discussed wild-domestic sheep disease transmission and referred to “reams of anecdotal cases . . . dating all the way back to the 1800s” (Brigham, Rominger, and Espinosa T. 2007, 1). Wehausen, Kelley, and Ramey II elaborate on the historical context of bighorn disease anecdotally associated with domestic sheep:

“The large region where bighorn sheep extirpations have been so widespread coincides spatially with where domestic sheep have been grazed in North America, and temporally with the beginning of that grazing. While one cannot infer cause and effect from spatial and temporal correlations alone, it has long been hypothesized that diseases transferred from domestic sheep were a key factor in the widespread loss of bighorn sheep populations. For example, the principal cause of the first large-scale population losses in the 19th century was attributed to scabies introduced by domestic sheep, based largely on clinical evidence of scabies in bighorn sheep during die-offs, and the temporal association of these scabies outbreaks with the introduction of domestic sheep” (2011, 9).

According to the DBC’s technical staff: “Documented bighorn die-offs were recorded as early as the mid-1800s” (1990, 33). Rocky Mountain, California, and desert bighorns have suffered documented die-offs, which have continued into the modern era (DBC Technical Staff 1990). For example, Toweill and Geist state that: “Early records tell of massive die-offs of California bighorns [in Idaho]: one near the community of Jordan Valley in the winter of 1884-85, another along the Owyhee River (due to ‘scab’ following the initial introduction of domestic sheep into that country) [in] about 1902” (1999, 134). Toweill and Geist also mention that: “Records indicate that [Owyhee County] supported a minimum of 150,000 [domestic] sheep by 1898” (1999, 134).

Owyhee Plateau domestic sheep

In 1953, one of the biggest documented wild sheep die-offs in American history occurred when bighorns in Colorado’s Tarryall and Kenosha Mountains suffered a pneumonia outbreak “that reduced the population from an estimated 1,000 animals (some observers have said 2,000) to 30 within two years” (CDOW 2009, 1). Regarding this and earlier epizootics, the Colorado Division of Wildlife states: “The causes of these early die-offs are hard to verify retrospectively, but contact with domestic livestock that led to the introduction of exotic diseases and parasites seems the most logical explanation” (CDOW 2009, 1).

Shannon et al. describe the circumstances of a more recent bighorn die-off likely caused by domestic sheep. It occurred in the Beaver Creek bighorn population, which was established in northeastern Utah near the Colorado border in 1983 (Shannon et al. 2008). Shannon et al. state: “In the early 1990s, a collared [bighorn] ram was observed with a herd of domestic sheep 15 km [(9 mi)] away near Cold Springs Mountain, Colorado. Shortly after, a major die-off occurred, reducing the estimated 80-90 bighorn sheep to 10 individuals” (2008, 182). Shannon et al. go on to say: “This reintroduction was considered a failure because of fatal interactions with domestic sheep” (2008, 182-183).

From 1997-2000, a pneumonia epizootic once again reduced the number of bighorns in a population inhabiting central Colorado’s Kenosha and Tarryall Mountains (George et al. 2008). According to George et al.: “The onset of this epidemic coincided temporally and spatially with the appearance of a single domestic sheep. . . .” (2008, 388).

domestic sheep in the Upper Animas River Valley, Colorado

In 1988, a pneumonia-caused die-off completely wiped out an introduced bighorn population in northeastern California’s Warner Mountains (Bleich et al. 1990). Bleich et al. state that the bighorns’ “pneumonia [was] attributed to pathogens transferred to the wild sheep from domestic sheep” (1990, 24).

Open Range Proof
An example of open range wild-domestic sheep disease transmission occurred in Nevada in 1994 (Jeffress 2008). In the Trout Creek Mountains, a bighorn ewe “was documented to have been in contact with 23 domestic rams for a period of less than 24 hours” (Jeffress 2008, 160). Wildlife managers captured the bighorn ewe within 17 hours of her documented domestic sheep interaction. They then translocated the ewe out of the wild, and five days later, she died from pneumonia. Tissue and swab samples taken from the dead bighorn ewe revealed bacteria isolates that were identical to those taken from all 23 domestic rams documented with her earlier. Thus, biological (rather than anecdotal) evidence backs this case of wild-domestic sheep disease transmission between free-ranging animals (Jeffress 2008).

REFERENCES
Bleich, Vernon C., John D. Wehausen, Karen R. Jones, and Richard A. Weaver. 1990. Status of bighorn sheep in California, 1989 and translocations from 1971 through 1989. In transactions of Desert Bighorn Council’s 34th Annual Meeting, Hermosillo, Sonora, Mexico. April 4-6.

Brigham, William R., Eric M. Rominger, and Alejandro Espinosa T. 2007. Desert bighorn sheep management: Reflecting on the past and hoping for the future. In transactions of Desert Bighorn Council’s 49th Annual Meeting, Las Vegas, NV. April 3-6.

Colorado Division of Wildlife (CDOW). 2009. Colorado Bighorn Sheep Management Plan: 2009-2019. Edited by J.L. George, R. Kahn, M.W. Miller, and B. Watkins. Denver. http://wildlife.state.co.us/SiteCollectionDocuments/DOW/WildlifeSpecies/Mammals/ ColoradoBighornSheepManagementPlan2009-2019.pdf (accessed October 15, 2011). [govt. doc.]

Desert Bighorn Council (DBC) Technical Staff. 1990. Guidelines for the management of domestic sheep in the vicinity of desert bighorn habitat. In transactions of DBC’s 34th Annual Meeting, Hermosillo, Sonora, Mexico. April 4-6.

George, Janet L., Daniel J. Martin, Paul M. Lukacs, and Michael W. Miller. 2008. Epidemic Pasteurellosis in a bighorn sheep population coinciding with the appearance of a domestic sheep. Journal of Wildlife Diseases 44, no. 2 (April): 388-403.

Jeffress, Jim. 2008. Transmission of Pasteurella haemolytica between domestic sheep and a free-ranging bighorn ewe. In proceedings of Northern Wild Sheep and Goat Council’s 16th Biennial Symposium, Midway, UT. April 27-May 1.

Lawrence, Paulraj K., Sudarvili Shanthalingam, Rohana P. Dassanayake, Renuka Subramanium, Caroline N. Herndon, Donald P. Knowles, Red R. Rurangirwa, William J. Foreyt, Gary Wayman, Ann Marie Marciel, Sarah K. Highlander, and Subramanium Srikumaran. 2010. Transmission of Mannheimia haemolytica from domestic sheep (Ovis aries) to bighorn sheep (Ovis canadensis): Unequivocal demonstration with green fluorescent protein-tagged organisms. Journal of Wildlife Diseases 46, no. 3 (July): 706-717.

Shannon, Justin M., Daniel D. Olson, Jericho C. Whiting, Jerran T. Flinders, and Tom S. Smith. 2008. Status, distribution, and history of Rocky Mountain bighorn sheep in Utah. In proceedings of Northern Wild Sheep and Goat Council’s 16th Biennial Symposium, Midway, UT. April 27-May 1.

Toweill, Dale E., and Valerius Geist. 1999. Return of royalty: Wild sheep of North America. Missoula, MT: Boone and Crockett Club and Foundation for North American Wild Sheep.

Wehausen, John D., Scott T. Kelley, and Rob R. Ramey II. 2011. Domestic sheep, bighorn sheep, and respiratory disease: A review of the experimental evidence. California Fish and Game 97, no. 1 (Winter): 7-24. http://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=4651 1(accessed May 23, 2012).