Repeating a pattern seen across the West in recent years, Washington State recently discovered the highly transmissible and fatal Mycoplasma ovipneumoniae in its bighorn sheep strongholds of the Cascade Mountains.

On Oct. 20, the Washington Department of Fish & Wildlife announced that a dead lamb in the Cleman Mountain herd tested positive for the fatal bacterium, often abbreviated to “M. ovi” or simply “movi.” A ram tag holder for that area reported the deceased lamb and other young sheep acting lethargic. The state then issued five additional ram tags for the area to applicants who had failed to draw this year in order to test for the disease.

Earlier in October, WDFW discovered a lost domestic ewe in close proximity to several wild bighorn rams in the Quillomene Herd, the largest population of wild sheep in the state at 220 to 250 head. The ewe, which is believed to have been wandering in their territory for weeks, tested positive for movi, so WDFW culled 12 adjacent bighorns. None tested positive, but managers remain gravely concerned for the health of this important population. The nearby Umtanum Herd in the popular Yakima Canyon is already pervaded with movi and is expected to decline sharply.

This is yet another example of an unfortunate pattern in wild sheep herds across North America. Every bighorn population is just one lost domestic ewe or wandering wild ram away from collapse.

Domestic sheep and goats carry and transmit the mycoplasma bacterium, often referred to as “sheep pneumonia,” but are unaffected by it. In wild sheep and mountain goats it causes “a devastating, population-limiting disease of bighorn sheep,” according to the CDC. Some controlled studies have seen as many as 98% of bighorns die after simply comingling with domestic sheep, which happens with relative frequency because the two species are attracted to one another. Domestic sheep will often wander into bighorn country, and young, dispersing bighorn rams will regularly range far and wide in search of mates.

Infected animals are often witnessed coughing, sneezing, and behaving in a sluggish manner. Unable to treat the disease, wildlife managers often choose to exterminate entire bighorn populations infected with movi, then restock the area with healthy sheep captured elsewhere—often at great expense to taxpayers and conservation groups. Many experts and conservation groups believe this disease is the single greatest threat to wild sheep populations in North America.

Within the last ten years, movi outbreaks have been documented in Alaska, Arizona, California, Colorado, Idaho, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming—nearly every state that holds bighorn or Dall sheep. These outbreaks are very frequently associated with domestic sheep and goat grazing.

In 2013, Washington State and federal wildlife managers removed the entire Tieton bighorn sheep herd to prevent the outbreak from spreading to the Cleman Mountain Herd, which now may have it anyway. The department is not currently considering similar actions there because the nearest population is the Umtanum Herd, where the pathogen is already established.

“Over the last decade we’ve seen one herd wiped out. We’re seeing a second in a very popular area for hunting and recreation and fly fishing, Yakima Canyon, on the decline,” Chase Gunnell, communications director for Conservation Northwest, told MeatEater. “And now two more herds, one of which is the biggest herd in the state, are potentially infected by this disease. So, we’re going from what was the stronghold for bighorn sheep to in the coming years, we could see the sheep gone from the central Cascades.”

WDFW said that just one commercial sheep producer grazes on public lands in that region, the Martinez Ranch.

“That producer has permits with the Okanogan-Wenatchee National Forest, as well as with the state department of natural resources. And there’s been quite a bit of concern over those permits,” Gunnell said. “We don’t want to necessarily point the finger at that sheep producer just yet—it’s unclear exactly where the disease transmission from some of these herds has occurred—but they are the only sheep producer on public lands in the area.

“These are good folks. They’ve been pretty responsible in other regards around their grazing and we don’t want to ruin their business, but the status quo has to change.”

Changing that status quo is one of the Wild Sheep Foundation’s primary missions. “WSF acknowledges there is room on Western landscapes for domestic sheep and goats, and for bighorn sheep, but they cannot occur together,” the group states.

The WSF and its Washington chapter are working with other conservation groups to both encourage changes from the Martinez Ranch and the Okanagan-Wenatchee National Forest.

“Simply stated, allowing domestic sheep to graze on high-risk allotments within the OWNF in the face of overwhelming data, published scientific literature, and numerous USFS NEPA analyses in western states is contrary to Forest Service law and policy. Potential solutions such as alternate, suitable allotments away from bighorn sheep range and/or changes in class of livestock such as conversion to cattle should be explored,” said Wild Sheep President and CEO Gray Thornton in a recent letter to the OWNF forest supervisor.

His organization, along with the National Wildlife Federation, have offered to buy out the Martinez Ranch’s problematic grazing allotments. Simultaneously, environmental groups including Wild Earth Guardians and the Western Watersheds Project have threatened to sue if an arrangement that keeps domestic sheep out of wild sheep territory is not reached.

The same battle is playing out in wild sheep country throughout the West. Conservation and environmental groups want to give bighorns the space they need to prosper. Powerful woolgrowers’ associations want to preserve traditional grazing rights and profits. The Forest Service and BLM are constantly caught in the crossfire, rarely able to please either side. Such is the difficulty of managing under a multiple use mandate.

“The main reason bighorn tags are so hard to come by is that wild sheep are only allowed to exist where domestic sheep don’t,” said Ryan Callaghan, MeatEater director of conservation. “I am not against public land grazing to be clear—grazing can be a good thing when done responsibly, but there are a lot of tools in the toolbox when it comes to disease mitigation on public lands. Domestic sheep have the potential to spread disease to wild sheep, even when properly monitored, where wild sheep bands are growing and attempting to spread. In these high conflict areas, retired grazing allotments need to be kept in retirement. When retirement isn’t a possibility, the switch from sheep to cattle needs to be considered.”

Feature image by John Hafner.