Sierra Nevada Bighorn Sheep - Disease

Historical photo showing domestic sheep grazing in Tuolumne Meadows
Diseases from domestic sheep can be fatal to native bighorns.

The stance of a bighorn ram with its wide sweep of magnificent horns is well known in the American vernacular as ram tough. Indeed, their phenomenal strength and balance propel them along vertical cliffs and up to the highest of rocky precipices. That such a strong and resilient creature could be so profoundly affected by disease, almost to the extent of extinction, is a critical reality faced by the Sierra Nevada Bighorn Sheep Recovery Program.

Diseases from domestic sheep pose the single largest threat to the existence of Sierra Nevada bighorn sheep. Native bighorns lack resistance to these diseases and can become fatally infected through such limited exposure as touching noses. The infected bighorns can then spread the diseases back to the herd, resulting in a “wildfire” of death that sweeps through the herds and can kill hundreds of bighorns within several months. Devastation by disease helps to explain much of the extinction of large swaths of bighorns that once roamed along the entire Sierra crest and the Great Western Divide resulting in the dangerously low numbers of bighorns alive during the past decades.


The reintroduction of Sierra Nevada bighorn sheep to Yosemite National Park’s Cathedral Range in March 2015 was preceded by intense scrutiny of the location’s possible vulnerability to contact with domestic sheep as a necessary precaution to the transfer of disease to the bighorns. Not only was the Cathedral Range surrounded by thousands of acres of federally protected wilderness as a secure buffer against domestic sheep contact, but its high altitude rockiness served as a natural fortress.

The secure locale of the Cathedral herd within the heart of the Yosemite wilderness is not paralleled for the Mt. Warren herd in their habitat near Tioga Pass, north of Lee Vining Canyon. The Mt. Warren herd (one of two herds formally known as the Yosemite Herd) was one of the earliest herds to be reintroduced to the Sierra in 1986, and their range includes small portions of Yosemite National Park and larger areas within the adjoining Inyo National Forest. Recent GPS satellite tracking of the bighorns confirmed that some Mt. Warren bighorns had descended in elevation to within a half mile of sheep grazing pastures owned by Mono County as a conservation easement.

For all the many people worldwide who celebrated the return of the Sierra Nevada bighorns to Yosemite’s Cathedral Range, the potential for severe consequences if there is contact between the Mt. Warren bighorns and domestic sheep cause deep apprehension. The successful establishment of 16 herds of Sierra Nevada bighorns, now numbering over 600 animals, has been dependent on relationships forged with partnering agencies, on scientifically based practices and knowledge and on the best intentions of those who believe that bighorns will only survive with acts of men and women reversing the devastation of the past.

The Sierra Nevada Bighorn Sheep Recovery Program has identified measures that will prevent contact between domestic sheep and the Mt. Warren herd, which include alternative uses of Mono County pasturelands that would exclude domestic sheep. In keeping with the spirit of the bighorn recovery effort, there is every hope that partnerships can begin now to ensure the survival of Sierra Nevada bighorn sheep within Mono County.

Map showing distribution of bighorn sheep and domestic sheep in and near Yosemite
Bighorn sheep live in close proximity to pastures where domestic sheep graze in Mono County.

Thanks to volunteer Bonnie Cassel for writing these bighorn sheep pages.

Last updated: December 3, 2018

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