Fish and Game will lethally remove small bighorn sheep herd in response to pneumonia outbreak in Southwest Idaho

In response to a pneumonia outbreak caused by the bacterium Mycoplasma ovipneumoniae (M.ovi) in a small herd of California bighorn sheep in the Reynolds Creek portion of Unit 40, Fish and Game officials plan to lethally remove the affected group this winter to prevent the spread of the deadly disease to other nearby herds.

"This was a difficult decision because no one, including IDFG staff, wants to see these sheep die," said Shane Roberts, Wildlife Bureau Chief. "Unfortunately, it's the right and necessary decision to protect the larger bighorn population in the Owyhees."

Outbreak poses risks to hundreds of bighorn sheep in nearby herds

M.ovi outbreaks often have population-level effects on bighorns, with about half of infected animals dying during the initial outbreak and longer-term effects on lamb survival. Once present, it also tends to spread widely. Some sheep that survive the initial pneumonia outbreak caused by M.ovi remain carriers and spread it to other sheep that weren't previously exposed, like newborn lambs.

The bighorns in Reynolds Creek are one of several herds of California bighorn sheep in the Owyhees. They are relatively isolated, about 50 miles from Idaho's core populations of California bighorns farther to the south and east, which number around 250 animals. However, some intermingling occurs among them, which poses substantial risks to the broader conservation of California bighorn sheep in Idaho.

"M.ovi doesn't kill quickly," said IDFG staff biologist Hollie Miyasaki. "Once bighorns are sick they can remain infectious for months, some for years, creating a wide window for an individual animal to spread the bacteria to other sheep."

Intensive monitoring led to early detection, opportunity to limit the outbreak

For the past several years, Fish and Game staff have been conducting an intensive research project on bighorn sheep in the Owyhees. Part of that project has included regular disease monitoring. This routine monitoring enabled early disease detection in these sheep, giving Fish and Game a unique opportunity to limit the outbreak. This is the first M.ovi outbreak in wild sheep detected in any of the Owyhee herds.

Herds in the Big Jack's Creek, Little Jack's Creek, and Castle Creek drainages were also tested for M.ovi in December 2025, and all samples came back negative, meaning that for now, they remain disease-free.

"We test for both active infections and past exposures, so we can say with some confidence that we detected this before it had a chance to spread beyond Reynolds Creek," said Regional Supervisor Josh Royse.

A difficult decision, but the right one

All these factors shaped IDFG officials' decision to remove this infected herd.

"As an agency, we've been working in the Owyhees for a long time, and we've invested a ton of resources-including several generations of our staff's blood, sweat and tears-into conserving these California bighorn sheep populations," Royse said. "As gut-wrenching as that makes this decision, we are choosing to be proactive. We have an opportunity to remove a small group of about 30 sheep, a portion of which will likely die from the disease in the coming weeks and months anyway, to protect hundreds in the long term."

A longtime partner in Idaho bighorn sheep conservation, the Wild Sheep Foundation, also recognizes the difficulty of the decision, but shared a statement of support for the removal effort.

"The Wild Sheep Foundation recognizes that our fish and game agency partners have to periodically make difficult decisions in managing wild sheep and other wildlife," said WSF President and CEO Gray N. Thornton. "Of course, WSF is concerned with any loss of bighorn sheep. These losses in southwest Idaho, in particular, hit close to home given the significant success achieved there in wild sheep recovery and conservation efforts. WSF remains unequivocally supportive of the leadership of the Idaho Department of Fish and Game and their agency managers for their swift and continuing efforts to put and keep wild sheep on Idaho's mountains."

To ensure the removal effort's success, sheep will be shot from a helicopter, which offers the greatest likelihood of removing the entire infected herd in the shortest time. Any sheep that can feasibly be salvaged will be processed by a local meat processor, and IDFG will work with local organizations to distribute the meat to Idahoans in need.

"We anticipate questions about why we can't turn to hunters for this removal effort," Royse said. "In normal circumstances, that would be our preference, but time and efficiency are of the essence here. Our objective for this management action is to prevent M.ovi from spreading outside of Reynolds Creek. Right now, all these sheep are tightly congregated, and any sustained hunting pressure is likely to disperse them, increasing the risk of spreading M.ovi to neighboring herds and making it harder to ensure all infected animals have been removed."

Triston Warner, President of the Idaho Wild Sheep Foundation, said that the chapter supported this approach.

"The news of M.ovi in the Reynolds Creek band is of course disappointing to the Idaho Wild Sheep Foundation," Warner said. "While hunter harvesting is usually preferable to professional culling, in this case, we believe that aggressive removal of this small band to prevent the spread of M.ovi to larger herds is the correct course of action."