There's a bighorn sheep killer on the loose, and it's not a natural predator. Instead, it's a bacterial disease —Mycoplasma ovipneumoniae — a respiratory disease that can cause a die-off for sheep of all ages if a herd is infected.

California bighorn sheep don't just live in the Golden State — they also live in Oregon, inhabiting some of the most rugged environments in the state. But even in high altitudes over rocky terrain, they can't outrun this respiratory disease.

The pneumonia has killed wild sheep in Oregon and other Western states over the past few decades, and is considered the largest risk to wild sheep populations, according to the Oregon Department of Fish and Wildlife.

Oregon State University researchers have taken on the study of a range of aspects of the California bighorn sheep herd in the state to gain insight into the animal's risk of contracting this killer bacterium known as M. ovi (pronounced "m-ovee"). The researchers are monitoring sheep movement, how the animals use their habitat and their survival rates. The disease can spread through contact between domestic sheep and bighorn sheep, or from bighorn to bighorn.

While California is part of the bighorn sheep's name, Oregon counts 3,700 of these animals in 32 different herds in the state. They live in central and southeast Oregon. ODFW traditionally captures and relocates California bighorn sheep around the state each year, to improve genetic diversity and restore the species to its historic range in Oregon.

But the relocation is on hold this year while wildlife managers learn more about M. ovi, in part through the OSU work.

A long-term killer
Clint Epps, OSU wildlife biologist, noted that the first contact with a particular strain of pneumonia kills bighorn of all ages. Some adults survive, but then as the infection persists, their lambs die every year. A herd may not recover for decades.

Wildlife managers strive to keep wild and domestic sheep and goats separate to avoid disease transmission. In the OSU report, Epps noted: "There is a high-stakes need to understand where the pathogen is likely to enter a bighorn population, and where it's likely to move after that. In the past few years, wildlife agencies in the West have made decisions to remove certain individual animals, or all individuals in the herd, to prevent the spread of disease."

A die-off of the bighorn sheep herd in the Lower Owyhee River Canyon in 2015-16 raised concerns about how M. ovi is impacting Oregon’s wild sheep populations. Also that year, the Nevada Department of Wildlife made the difficult choice to euthanize an entire herd of sick bighorn sheep just south of Oregon’s border to stop the spread of M. ovi to neighboring populations.

ODFW wildlife biologists and veterinarians have sampled and collared more than 120 bighorn sheep in the past two years. Recent samples from sheep in the 120 — some tested in OSU's Veterinary Diagnostic Laboratory — will provide more information about diseases and animal health. That work will also determine whether the strain found in the eradicated Nevada herd made its way into Oregon.

While disease and hunting were big reasons for a die-off in Oregon in the 1940s, sport hunters today have been instrumental in helping restore the bighorn sheep population in the state. ODFW has an annual auction and raffle of special bighorn sheep tags that has generated money for management and research.

Source: Oregon State University/Chris Branam