M. ovi reaches Oregon’s largest John Day bighorn herd

Version 2 — Full analysis

Oregon’s largest bighorn sheep herd has lost a key distinction. On September 8, 2026, the Oregon Department of Fish and Wildlife said several California bighorn sheep in the John Day River herd tested positive for Mycoplasma ovipneumoniae, marking the first known detection of the respiratory pathogen in a population estimated at about 1,000 animals. ODFW has begun necropsies, diagnostic testing, and field surveillance to assess the scale of the outbreak and identify the strain involved. (myodfw.com)

That matters because the John Day herd has long been treated as an outlier in the West: large, productive, and, until now, free of M. ovi. Oregon Wild Sheep Foundation recently described the lower John Day herd as having grown to more than 1,000 animals while remaining free of the pathogen, and the herd has been important enough to serve as a source for translocation and augmentation work, including recent restoration efforts in Idaho. ODFW field materials also show animals from John Day were being captured and moved as part of broader bighorn management work before this detection. (oregonfnaws.org)

In its announcement, ODFW said several sheep had tested positive and that the agency is now sending samples to the Washington Animal Disease Diagnostic Laboratory while surveying the canyon for additional sick or dead animals. Assistant Wildlife Biologist Trent Gianella called the news “disappointing,” noting that the herd had stood out for its size and health in Oregon and across the West. ODFW said there’s currently no known vaccine or treatment for M. ovi, and that management options under consideration as more data come in could include measures used elsewhere in the West, such as removing infected individuals or reducing herd density to slow spread. The agency also said it will work with the Confederated Tribes of the Warm Springs and the Confederated Tribes of the Umatilla Indian Reservation on next-step planning. (myodfw.com)

The broader science helps explain why agencies take these detections seriously. Wildlife and veterinary research has consistently linked M. ovi to all-age pneumonia outbreaks in bighorn sheep, often followed by years of low lamb survival because some adults become chronic carriers. A Washington State University veterinary extension summary says even brief contact between bighorn sheep and infected domestic sheep or goats can trigger outbreaks, and notes that some herds can lose a large share of animals quickly, with lamb losses persisting for years. Washington Department of Fish and Wildlife similarly warns that carrier animals can sustain disease in herds long after the initial outbreak. (fs.usda.gov)

There’s also an Oregon-specific backdrop. ODFW said M. ovi has previously caused major problems in places such as Hells Canyon and the Owyhees. Idaho’s current bighorn management plan describes multiple genetically distinct M. ovi strains in the Hells Canyon system, with each first detection associated with an all-age pneumonia outbreak, underscoring why strain identification in John Day will matter. ODFW also flagged a growing regional aoudad population as a newer management concern, alongside the longstanding risk of contact with domestic sheep and goats. (myodfw.com)

Why it matters: For veterinarians, diagnosticians, and animal health professionals, this story sits at the intersection of wildlife disease surveillance, livestock-wildlife interface risk, and long-horizon herd management. The immediate concern is mortality and transmission within the John Day herd, but the bigger issue is what happens afterward: whether the outbreak burns out, whether chronic carriers remain, and whether lamb recruitment drops for multiple seasons. Because the John Day population has been both unusually robust and operationally important as a source herd, any change in its health status could affect future movement decisions, surveillance protocols, and conversations with producers and land managers about separation, testing, and biosecurity. (myodfw.com)

What to watch: The next key signals will be lab characterization of the strain, evidence of lamb impacts during the next recruitment cycle, and whether ODFW shifts from surveillance to active intervention. If the agency identifies a limited cluster and a manageable transmission pattern, the response could stay targeted; if widespread carriage or spillover risk emerges, management choices could become much more consequential for bighorn conservation and for disease-prevention messaging to nearby sheep and goat producers and pet parents with small ruminants. That last point is partly an inference from how other Western agencies have handled M. ovi outbreaks and from the disease’s known persistence dynamics. (myodfw.com)