OSU bighorn sheep research sheds light on killer disease

With their ability to climb steep rocky mountain areas, California bighorn sheep live in some of the most rugged environments Oregon has to offer.

No matter how high they go, the wild sheep can’t elude Mycoplasma ovipneumoniae – the bacteria now widely thought to be primarily responsible for fatal infectious pneumonia in bighorns.

Respiratory disease has killed numerous 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.

Once a herd is infected, an all-age die-off can occur, and the disease remains chronic in the population.

Now, Oregon State University researchers are studying several aspects of the California bighorn sheep herd in the state – including movement, habitat use and survival – to gain insight into the animal’s risk for contracting the killer strain known as M. ovi (pronounced m-ovee). The disease spreads through contact between domestic sheep flocks and bighorn sheep, or from bighorn to bighorn.

Oregon is home to about 3,700 California bighorn sheep in 32 different herds in Central and Southeast Oregon.

ODFW traditionally captures and relocates California bighorn sheep around the state each year to improve genetic diversity and restore this rare species to its historic range in Oregon. But these relocation efforts are on hold this year while wildlife managers learn more about M. ovi, partly through the work being done at OSU.

Often, the first contact with a particular strain of pneumonia kills bighorn of all ages, according to OSU wildlife biologist Clint Epps . Some adults survive, but then as the infection persists their lambs die every year. A bighorn herd might not recover for decades.

Wildlife managers strive to keep wild and domestic sheep and goats separate to avoid transmission of the disease.

“There is a high-stakes need to understand where the pathogen is likely to enter a bighorn population and where it’s likely move after that,” Epps said. “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 Owhyee 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.

In 2011, ODFW had to kill five of the 20 bighorn sheep reintroduced to the John Day Fossil Beds after they wandered into an area where they could have been exposed to a domestic sheep farm.

“There’s been a tremendous amount of effort to increase these bighorn populations, and our goal is to provide better information when they make those decisions,” Epps said.

ODFW wildlife biologists and veterinarians have sampled and collared more than 120 California bighorn sheep in the past two years. Recent samples from those captured bighorn sheep, some of which were tested at OSU’s Veterinary Diagnostic Laboratory, will provide extensive information on diseases and animal health, including determining whether the strain that eliminated the Nevada herd has spread to Oregon’s bighorn sheep.

Robert Spaan, an OSU doctoral student, travels to southeastern Oregon from April to August to study the California bighorn sheep herds, which each typically number between 30 and 150 individuals. He tracks the bighorn sheep that have been fitted with GPS collars by ODFW and records birth and death data.

“We’re able to respond to mortalities, and we are able to determine cause of death in most cases,” Spaan said. “We managed to detect a die-off of lambs in one population last year, the only one where we saw active M. ovi infection.”

Disease was one of the factors when bighorn sheep died off in Oregon in 1940s, along with unregulated hunting. But sport hunters have since been instrumental in restoring bighorn sheep in Oregon. The Oregon Department of Fish and Wildlife’s annual auction and raffle of special bighorn sheep tags have generated thousands of dollars for their management and for research.

Among the funders of the study are the Oregon Department of Fish and Wildlife and the Oregon Foundation for North American Wild Sheep.