INTRODUCTION
The fact that bighorn-domestic sheep interaction often leads to fatal bighorn disease is well-established, despite some uncertainties about precise transmission components (Brigham, Rominger, and Espinosa T. 2007). The technical staff of the Desert Bighorn Council (DBC) sums up the trend:
“Bighorn die-offs have occurred in every state in the western United States. In broad perspective, when there has been contact between apparently healthy bighorns and domestic sheep, the bighorns die within a few days to a few weeks. While many diseases or stress factors may be involved, bighorns exposed to domestic sheep almost invariably die from pneumonia” (1990, 33).
According to the USFS: “[Bighorn] pneumonia outbreaks frequently result in mortality of many to most individuals within the herd. All age classes of bighorns are typically affected. In addition to high mortality of all age classes during the pneumonia outbreak, lamb survival and thus recruitment typically remains depressed for 2 or more years following the epizootic” (2006, 3). Pneumonia is not the only disease that impacts bighorns, bighorns can experience disease die-offs without contacting domestic sheep, and bighorns may sometimes mingle with domestic sheep without immediately getting sick (BLM 1999; Aune et al. 1998).
DOCUMENTATION
Bighorn-domestic sheep interaction leading to a bighorn epizootic is often not well-documented. Significant bighorn die-offs may occur with no well-documented cause, or domestic sheep interaction may be likely but not proven (Arthur et al. 1999; WAFWA 2010). George et al. note: “Our experiences suggest that contact between bighorn sheep and domestic sheep may go undetected on native ranges” (2008, 398). Interaction is often not observed because of the remoteness, rugged terrain, and lack of human presence common in bighorn ranges (George et al. 2008). George et al. add: “It follows that when pneumonia epidemics occur in free-ranging native North American wild sheep it may not be possible to conclude that contact with domestic sheep did not occur, but only that contact was not detected” (2008, 398).
REFERENCES
Arthur, Steven M., Ian Hatter, Alasdair Veitch, John Nagy, Jean Carey, Jon T. Jorgenson, Raymond Lee, John Ellenberger, John Beecham, John J. McCarthy, Gary Schlichtemeier, Larry T. Gilbertson, Bill Dunn, Don Whittaker, Ted A. Benzon, Jim Karpowitz, George Tsukamato, Kevin Hurley, Steven G. Torres, Craig Mortimore, Mike Oehler, Patrick Cummings, Craig Stevenson, Eric Rominger, and Doug Humphreys. 1999. Appendix A: Wild sheep status questionnaires. In proceedings of 2nd North American Wild Sheep Conference, Reno, NV. April 6-9.
Aune, Keith, Neil Anderson, David Worley, Larry Stackhouse, James Henderson, and Jen’E Daniel. 1998. A comparison of population and health histories among seven Montana bighorn sheep populations. In proceedings of Northern Wild Sheep and Goat Council’s 11th Biennial Symposium, Whitefish, MT. April 16-20.
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.
Bureau of Land Management (BLM). 1999. Attachment 7: 1998 Revised Guidelines for Domestic Sheep and Goat Management in Native Wild Sheep Habitats. In Challis Resource Management Area: Record of Decision and Resource Management Plan. Salmon, ID. http://www.blm.gov/pgdata/etc/medialib/blm/id/plans/challis_rmp.Par.8185.File.dat/ entiredoc_508.pdf (accessed May 12, 2012). [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.
U.S. Forest Service (USFS). 2006. Risk Analysis of Disease Transmission Between Domestic Sheep and Bighorn Sheep on the Payette National Forest. McCall, ID. http://www.mwvcrc.org/bighorn/payette bighornreport.pdf (accessed October 15, 2011). [govt. doc.]
Western Association of Fish and Wildlife Agencies (WAFWA). 2010. Summary on 9 BHS Die-offs in 5 Western States: Winter 2009-10 (June 22, 2010). Cheyenne: WAFWA. http://www.wafwa.org/documents/wswg/Summary9BHSdie-offsin5westernstateswinter 2009-10.pdf (accessed May 17, 2012).