12 minute read
The Fate of Bighorn Sheep
The Fate of Bighorn Sheep
The race against the pathogen M.ovi, which threatens bighorn herds of the Northwest
written by Lauren Kramer
It’s a crisp, cold December morning in Hells Canyon, and from the helicopter, Taylor Chism, an animal capture specialist, scans the steep, rugged terrain, watching for bighorn sheep. When she spots the herd, pilot Jim Pope, owner of Leading Edge Aviation, carefully maneuvers the chopper to corral them and begins his descent. At 15 feet above ground, the gunner readies a modified shotgun containing a weighted net. Aiming at a large bighorn, they shoot, and the net entangles the animal on the spot. As the helicopter hovers a couple of feet from the ground, the 26-year-old jumps to the ground to begin her work.
In this environment, where captured wildlife can quickly get stressed and overheated, every minute counts, and Chism must act fast. Her first task is to untangle the sheep from the net, hobble it by binding its back and front legs together, and blindfold it. Positioning herself near the sheep’s rear end, where she’s least likely to get kicked and injured, she straps a GPS collar around its neck so biologists can track its location, draws blood and takes nasal swabs that will be sent to a lab. The procedure takes six to eight minutes, and during that time she continually monitors the sheep’s temperature to ensure it doesn’t overheat. After collecting the final swab, she removes the blindfold and leg constraints, and the animal beats a hasty retreat to rejoin the herd.
It’s a good day for the crew of animal capture specialists in Hells Canyon, and by dusk, as they’re headed home, they have collared and swabbed seven bighorns, returning with a few scrapes and bruises, their clothes pungent with the smell of wildlife.
What the bighorns don’t know is that their herds’ future is in the balance. Nose swabs will be tested for Mycoplasma ovipneumoniae, or M.ovi for short, a pathogen associated with fatal pneumonia in bighorn sheep. Those that test positive twice over a period of two years will be tracked and euthanized or moved to captivity for research, to protect the rest of the herd from a virulent bacterium that is decimating their population.
Bighorn sheep are an iconic species in the Pacific Northwest. Drive along a highway in the Yakima Canyon or float down the Columbia River, and you can often see them feeding quietly on steep, rocky terrain, butting heads or rutting in mating season. In Washington, today’s herds number just fewer than 1,700 sheep, but in centuries past, there were tens of thousands of them.
“The populations we have now are just vestiges of what they once were and what our land can support,” said Frances Cassirer, wildlife research biologist for Idaho Fish and Game.
“Mount Mazama, in the Cascade Range, erupted 7,700 years ago, and there are bighorn sheep bones below that ash. Archeological digs were done in Hells Canyon, and the most common bone found was bighorn sheep, even more than deer. Early settlers in the canyon also reported bighorn sheep as the most common ungulate. We know that for thousands of years bighorn sheep lived here and were an important part of tribal culture. But now, we consider them uncommon. In the last couple hundred years, they’ve almost disappeared.”
Highly contagious, M.ovi is one of the biggest threats facing the seventeen herds of bighorn sheep in Washington today. Bighorns are highly susceptible to respiratory disease, but they’re especially vulnerable to M.ovi, a pathogen that is not native to North America.
“We have four herds of bighorn sheep in Yakima County, and all of them have M.ovi carriers,” noted William Moore, an ungulate specialist for the Washington Department of Fish & Wildlife. “We found it in one herd in 2009, another in 2013 and a third in 2020.” In 2019, DFW identified M.ovi in a herd in Okanagan County, just outside of Omak, and in December 2023, it was detected in herds in Hells Canyon, too.
When members of a herd are first infected with M.ovi, a disease outbreak spreads across all ages. Fatalities can range from killing only 5 to 10 percent to up to 80 percent of the herd. Of the animals that remain, some will have been exposed to the pathogen but are resistant to it. Others—between 5 and 15 percent—become chronic carriers, retaining the pathogen in their respiratory system and spreading it to others in the herd that were not initially exposed.
This is particularly threatening to the annual lamb cohort. "When newborn lambs are exposed to M.ovi from a chronic carrier, most of them don’t survive their first year,” Moore said. In wildlife terms, “lamb recruitment” refers to survival rates. A 50 percent survival rate is considered good lamb recruitment, while bad hovers around just 10 percent. “As long as there are chronic carriers in the population, this cycle can continue for years and years.”
Bighorns are very social animals, and as a result, M.ovi is able to spread quickly. The herds raise their lambs in a nursery environment akin to human daycare, so when the lambs get sick, the bacteria infects the entire nursery in no time at all. “Typically lamb recruitment is poor, meaning, producing fewer lambs than you’re losing in adults,” Moore said. “That’s the crushing part of this disease—the lambs born each year are succumbing to pneumonia, so the population never stabilizes. Without intervention, it’s a slow death to the population.”
Biologists and naturalists have known for a century that bighorn sheep are susceptible to pneumonia. What they didn’t know until 2006, when Washington State University microbiologist Dr. Tom Besser joined the research team, was exactly what was causing it.
“Over the last hundred years, there have been many reports of people seeing dead sheep on the hill,” said Cassirer, a top researcher on M.ovi and its effects on the bighorn. She was hired back in 1995 to try and accelerate the restoration of the Hells Canyon herd populations. The effort fell under the Hells Canyon Initiative, a collaborative effort between wildlife agencies and organizations in Washington, Oregon and Idaho.
“State wildlife agencies wanted to bring back the bighorn herds that were extirpated by unregulated hunting and disease in the late 1800s to early 1900s,” she said. “While this wasn’t intended to be a disease project, the more we worked on restoring the sheep, the more we realized that disease was the biggest limiting factor.”
In early experiments, when domestic sheep and bighorn sheep were put in the same pen, the bighorns died. “We’ve known for a while that there’s something domestic sheep carry to which bighorns have little resistance,” she said. “But it took a long time to understand exactly what that was.”
In recent years, biologists have identified bighorn sheep interaction with domestic sheep and goats as the source of the pathogen’s transmission. Both wild and domestic sheep and goats are gregarious, and don’t shy away from interacting with each other. But while domestic herds have immune resistance to the effects of M.ovi, wild herds do not. All it takes is a sneeze or nose-to-nose contact to transmit M.ovi.
There is no effective treatment or vaccine for M.ovi that will stop transmission, but there are two, equally important solutions, both fraught with challenges.
One is to eliminate M.ovi from a herd, a strategy the DFW has adopted with “test and remove.” That’s logistically challenging given the number of bighorn sheep and the complexities of catching and testing each one in terrain and weather conditions that are often inhospitable.
“Initially, recommendations were to test 95 percent of the herd and to capture and test them twice,” Moore said. “Today we’re trying to optimize our test and remove strategy by using data to determine the herds’ interaction. We can track bighorn sheep interactions with their GPS collars, and by knowing how the populations are interacting, we can test more effectively.”
Test and remove is underway in bighorn sheep herds in the Hells Canyon herd, the Umtanum herd in Yakima County and the Cleman Mountain herd in Kittitas County. Moore’s team is planning new test and remove projects in the Mount Hall herd in Okanagan County, near Omak, in two years’ time.
The strategy has been effective in the past in Hells Canyon, where 850 bighorns reside in sixteen herds that traverse 5.6 million acres of land in Washington, Idaho and Oregon. “Hells Canyon was one of the places where test and remove was first tried, starting in 2012, after over a decade of research into infection and low lamb survival,” Cassirer said.
“Five carriers were removed in 2015 and placed in captive research facilities. After that, those Washington herds were M.ovi-free until 2024, when a new strain of M.ovi occurred in Hells Canyon. We hope to do the same on this round to gain a better understanding of why some sheep are chronic carriers and how to better identify them to minimize the amount of capture and testing.”
Equally important in stopping M.ovi transmission is preventing interactions between wild sheep and domestic herds of sheep and goats. That’s difficult, given that they’re close relatives and are socially attracted to each other. Many domestic herds in the state are held on unfenced private land in close proximity to wild sheep habitat, and bighorn sheep, naturally forayers, like to roam. How do you ensure complete separation of herds in a natural environment such that they never interact?
“It’s especially difficult because in Washington state we have rural and agricultural areas abutted right next to our bighorn sheep habitat,” Moore said. “Most of our bighorn sheep populations reside next to Omak, Naches, Selah, Ellensburg, Wenatchee, Chelan and Asotin. Separating wild sheep and domestic sheep and goats is crucial.”
There were 45,000 sheep and lambs in Washington in 2024 and 25,000 goats, according to data from the U.S. Department of Agriculture. When domestic sheep and goats contract M.ovi, the bacteria causes mild weight loss and coughing, but frequently goes unnoticed.
To maintain separation between domestic sheep grazing allotments and wild sheep habitat, the U.S. Forest Service has moved or closed grazing allotments in some places to reduce the possibility of interaction. Other steps relate to private land, where owners of small flocks of domestic sheep and goats live in proximity to wild sheep.
Here, it’s all about education and outreach, an effort Carrie Kyle is heading up for the DFW. She is involved in education and outreach programs with the conservation district in the hells canyon area, as well as in the North-Central populations. “I’m doing presentations and working in different areas with the Grange, the National FFA Organization and other youth groups,” Kyle said, “but first we’re just trying to figure out where the small flock owners are, that are close to our bighorn areas.”
“Our goal is to build connections with kids and their parents who are raising sheep and goats, so that we can get the word out that we have M.ovi in our bighorn sheep populations. A lot of people don’t know that,” she added. “As people become educated about this issue, the situation will become manageable, because the fact is, we can have wild and domestic herds—we just have to be cognizant about this pathogen and diligent about keeping them separated.”
These efforts are running parallel with those of biologists, who are racing against the clock to learn more about M.ovi transmission. “Why are some bighorn sheep carriers of M.ovi, and others not?” asked Cassirer. “If we had more information on who is likely to be a chronic carrier, we could focus our efforts on testing just those individuals. Right now, there’s often just random testing, or testing of the whole population, which is not feasible in a lot of places.”
The importance of separating wild sheep from domestic sheep and goats can’t be underestimated, she said. “The safest thing is not to have domestic herds of sheep or goats, if you’re in bighorn habitat. But if you do have them, put your sheep inside a pen at night. Build good fences. Participate in a testing program like the one Washington offers and be aware if your animals are carriers of M.ovi.”
The future for bighorn sheep hangs in the balance, and Cassirer said it could go either way—the bighorn populations could be completely depleted, or they could recover. “We have the potential to fix this problem if we put our minds to it, but for bighorns to stay healthy, it’s going to take help from the communities that reside in areas that have bighorn sheep. If we give them a chance and put some hard work into this, we could see rebounding populations,” she said.
We have the potential to fix this problem if we put our minds to it, but for bighorns to stay healthy, it’s going to take help from the communities that reside in areas that have bighorn sheep. If we give them a chance and put some hard work into this, we could see rebounding populations.
Moore and his team are devoting their energies and budget precisely to this solution. “Bighorns provide a really excellent, accessible wildlife viewing opportunity in our state, and we are working hard to maintain these species for now and for future generations,” he said. “If we work together to try and keep these animals separated from domestic herds, we have a chance at keeping our bighorn sheep populations safe from possible extinction.”
