Game managers charged with expanding bighorn sheep (Ovis canadensis) populations must perform a delicate balancing act (1). All they have to do is find suitable habitat that is not too close to domestic sheep which may transmit diseases to bighorn, but not so remote that hunters can’t get at them. Those have been vexing problems, but it turns out that efforts to establish new herds or bolstering numbers of existing populations by transplanting in new sheep has another complication. The transplanted animals themselves might be reservoirs of diseases that could threaten the health and survival of the herd (1).
Bighorn sheep populations are believed to have once numbered in the millions and extend over wide regions of North America. Incompatible with human settlement, the species had declined almost to the point of extinction by the start of the twentieth century (2). Establishment of national parks, wildlife refuges and active conservation efforts have enabled some recovery, although only to levels of probably less than 1% of the pre-settlement era (2).
Because small herds are at greater risk of inbreeding and dying out from predation or other factors, managers would like to supplement their numbers by transplantation. Now they have recognized that activity brings with it some risk of transplanting potentially deadly disease agents as well (1). Damned if they do and damned if they don’t, managers are caught in a bind trying to figure out how to keep small, isolated bighorn herds viable over the long term.
The bighorn sheep transplant story reminds me of reports of a human disease condition known informally as “stranger’s cough” or “boat cough” (3). Geographic isolation may have set up a situation where persons inhabiting the remote Scots archipelago of St. Kilda seem to have become more susceptible to respiratory diseases that were comparatively mild in persons from the mainland. Living in physical isolation inadvertently kept them away from a lot of respiratory bugs that were commonly passed from person-to-person in heavily populated locations. That lack of routine exposure set the islanders up to have nasty epidemics when travelers from afar brought in forms of disease agents they had not encountered. A retrospective analysis suggests that the most probable etiologic agent for the severe condition termed stranger’s cough was rhinovirus, the cause of the usually rather benign common cold (3).
Drawing from the stranger’s cough story we can see some parallels to the problems facing bighorn managers in Montana and other locations today. Some of the descriptions in a recent news article (1) fit the basic St. Kilda model in which new arrivals carry a risk to transmit disease to the rest of the herd. In addition, the observations that wild sheep now harbor quite a few pathogens and could be “infecting themselves” coupled with the fact that large herds near Yellowstone carry pathogens, but do not have mass die-offs (1) fit in as well. Taken as a whole, the information suggests the bighorn problem is directly analogous to the plight of the isolated islanders (3) with epidemics touched off by the arrival of a novel strain of “M. ovi” (1).
The article (1) does not explicitly state that “M. ovi” is Mycoplasma ovipneumoniae, but this pathogen is known to cause disease in domestic and wild sheep. It certainly is a good place for veterinarians seeking the bighorn disease outbreaks culprit to start. However, they may have a hard time figuring out which agent(s) to blame. What is actually killing bighorn after transplant actions may not necessarily be the same microbes that typically create problems in wild and domestic animals. M. ovipneumoniae is known to promote secondary infection by viruses or bacteria that finally kill their weakened hosts. Worse, deaths may not necessarily be due to the same agent every time in the different herds. Each bighorn subpopulation existing in fragmented pockets of suitable habitat may succumb to something different due to sheer happenstance. This is a complicated situation, one that may be hard to control with antibiotics or vaccines.
Looking at the numbers, it is clear the vestigial bighorn sheep population has been on the verge of complete extinction for over a century. This difficult – and dangerous – situation makes careful and systematic examinations of animal transplant practices and outcomes essential. At this stage it is hard to decide when reintroduction efforts have succeeded (4) or whether they are now even advisable. Bighorn sheep once roamed the west in numbers thousands of times greater than today. They were important enough to the peoples of ancient times that quite a number of petroglyphs were created that seem to depict them (2). May the game managers of our time find a way to preserve this species.
Special thank you to Cyndi Tuell, Arizona wildlife expert and advocate for pointing out the article by Laura Lundquist.
(1) Laura Lundquist. Montana FWP Backs Away From Plans to Establish New Bighorn Sheep Herds. Missoula Current, 15 November 2018. https://www.missoulacurrent.com/outdoors/2018/11/montana-bighorn-sheep/
(2) The National Wildlife Federation. Bighorn Sheep. https://www.nwf.org/Educational-Resources/Wildlife-Guide/Mammals/Bighorn-Sheep
(3) P. Stride. 2008. The St. Kilda Boat Cough Under the Microscope. Journal of the Royal College of Physicians of Edinburgh 38:272-279. https://www.rcpe.ac.uk/college/journal/st-kilda-boat-cough-under-microscope
(4) Doug Kreutz. Bighorn Reintroduction Near Tucson Could Soon be Declared a Success. Arizona Daily Star, 1 September 2016. https://tucson.com/entertainment/outdoors/hiking/bighorn-reintroduction-near-tucson-could-soon-be-declared-a-success/article_aba02910-5e8f-5d35-bb0e-fef79a25e872.html