Bighorns From Two Herds are Sick and Dying Due to Disease Spread by Domestic Sheep

Photo by Erik Molvar/WWP.

Last week, wild bighorn sheep herds in Washington and Oregon have been detected with deadly pneumonia, likely caused by a deadly pathogen spread by domestic sheep grazing on public lands. The bighorn are dying slow and painful deaths because they have no natural immunity to the livestock-borne disease.

In Washington, the Department of Fish and Wildlife announced that deadly pneumonia had been detected in the Cleman Mountain herd of bighorn sheep, located primarily in the Okanogan-Wenatchee National Forest northwest of Yakima. A day later, the Oregon Department of Fish and Wildlife announced the detection of pneumonia in the Burnt River herd, southeast of Baker City. Both herds of bighorn likely contracted the illness from domestic sheep or goats.

The Forest Service has been failing to address the threat that domestic sheep allotments pose to bighorn sheep, making infections all but inevitable.  Bighorn sheep pneumonia is caused by pathogens carried by domestic sheep and goats, who typically remain asymptomatic themselves. The pathogens, which include the primary infectious agent Mycoplasma ovipneumoniae as well as a variety of bacteria that act as secondary agents to make bighorn sheep ill, originally traveled with domestic sheep from the old world to the new. The bacteria reached the American West in the Mid-1800s as enormous bands of domestic sheep were brought to graze lush mountain meadows throughout the region, decimating populations of bighorn sheep, who hadn’t evolved with the bacteria and thus had no natural immunity. Several states saw their bighorn populations extirpated entirely, while others saw them reduced to a fraction of historic numbers. The bighorn deaths have continued for more than a century, reducing the remaining native herds as well as reestablished herds through sudden all-age die-offs and through subsequent annual losses of lambs, who can contract the bacteria from their mothers long after the initial disease event appears to have passed.

Science tells us that domestic sheep need to stay far away from bighorn in order to keep our wildlife safe – i.e. social distancing for the Bovidae. But the Forest Service is going to need to start making firm decisions about grazing permits, taking the political heat, and giving priority to native wildlife.

Melissa Cain is Bighorn Conservation Director for Western Watersheds Project.