A rash of bighorn sheep deaths this winter, most likely from pneumonia, has caused concern among wildlife officials for their unusual patterns.
Dozens of bighorn lambs have died in the Upper Yellowstone herd near the North entrance to Yellowstone National Park since December, with one group of 25 to 30 animals now down to 17 sheep, according to reports.
Almost the entire lamb crop from the two herds has been decimated, said Karen Loveless, a wildlife biologist for Fish, Wildlife, and Parks to the Billings Gazette, though she added that the deaths seem to have slowed down.
The death of bighorns is not unusual, and pneumonia is most often the cause, but what’s troubling wildlife experts is the presence of more adults than usual among the fatalities.
The deaths aren’t restricted just to Montana. There are ongoing problems in Hells Canyon between Idaho and Oregon, as well, Nez Perce interim wildlife director Angela Sondenaa told Indian Country Today Media Network.
“That whole population has had ongoing problems,” Sondenaa said, adding that wildlife officials had already captured and removed a small group from Hells Canyon. “The decision was made to capture the remaining animals and send them to South Dakota as part of a research project.”
Pneumonia is the most common way that nature culls bighorn populations, said Mark Drew, wildlife veterinarian for Idaho Fish and Game.
“The primary limiting factor in sheep populations is disease, primarily pneumonia although it can be other things,” Drew told ICTMN.
A number of pathogens can cause pneumonia in sheep, and the illness usually stems from a combination of bacteria, Drew said. Domestic cattle, sheep and goats can carry these bacteria and not show problems, “but bighorn sheep don’t deal with those pathogens well,” Drew said.
“Bighorn sheep in western North America have been exposed to those cattle and sheep pathogens since the advent of white settlement,” he said. “Let’s call it 150 years, 200 years give or take a little bit. When pneumonia hits bighorn sheep, they respond relatively poorly, and they tend to have mortality issues with it.”
That mortality takes “two completely different forms,” Drew said. “We have the chronic adult, ewe primarily, that has pneumonia and she never really gets over it—just constantly coughs and has a snotty nose and tends to give all that stuff to her lambs. The ewe mortality to pneumonia is relatively low, not zero but relatively low. Eventually they’ll die of pneumonia, but it takes a long time.”
The larger issue, he said, is the lambs that get pneumonia and die in the summertime.
“In a lot of the bighorn sheep herds that are affected with pneumonia, lamb survival goes down to the bottom and stays that way for 10, 15, 20 years, where there is no lamb recruitment for a decade or more,” Drew said. “When that happens your populations go down, and that’s the issue.”
Periodic die-offs are not unusual in and of themselves, he emphasized.
“Everybody’s losing animals to some degree,” he said. “In the last three or four years Washington, Montana, Utah and Nevada all had die-offs in particular populations. Montana is having one now.”
Short of attempting to sequester domestic lambs and other livestock from bighorns, there is not much that can be done, Drew said.
“There’s a lot of politics involved in bighorn sheep pneumonia because of this potential contact with domestic sheep and goats,” he said, adding that there are no easy solutions.
Vaccinating the bighorns against the livestock-borne pneumonia pathogens is both pricey and dicey, given the $1,000-per-sheep annual cost and the near impossibility of rounding up every single bighorn for a shot.
On the upside, the disease does not seem to have spread to bighorns within Yellowstone National Park, Loveless said, telling the Billings Gazette, “That’s amazing, because we know that they mix.”