Bighorn Sheep “Disease Event” in Montana Mountains

Nevada Department Wildlife makes difficult decision in effort to save remaining healthy bighorns

The Nevada Department of Wildlife (NDOW) has taken necessary actions in the Montana Mountains in Northern Nevada in an effort to save healthy bighorns in the adjacent Nevada and Oregon mountain ranges. The decision was made to depopulate a herd of California bighorn sheep due to an ongoing and aggressive outbreak of polymicrobial pneumonia that has caused an all age die-off in a once healthy herd in the Montana Mountains of Humboldt County.

"We had to make the difficult but necessary decision to completely depopulate the Montana Mountains of the remaining small number of bighorn sheep to prevent the spread of pneumonia to sheep in other Nevada mountain ranges and to prevent the possibility of sick sheep dispersing into the neighboring state of Oregon," said Nevada Department of Wildlife Director Tony Wasley. "This is truly an unfortunate measure, but as the extent of this disease event has become clearer to us, we have determined that it is the proper and responsible course of action. " An important factor in the Department’s decision is that currently, there is no known cure or treatment for the illness.

Indications of disease in this herd were first revealed in early December 2015 during routine capture and radio-marking efforts. Subsequent close monitoring and disease testing of additional bighorn sheep from this area indicated a high occurrence of pneumonia.

Since discovering the outbreak in December, NDOW personnel have been monitoring the herd to determine the breadth of concern. As many as 70 percent of the bighorn sheep in the Montana Mountains may have succumbed to the disease outbreak. Helicopter surveys of the remaining sheep in the Montana Mountains on Feb. 4 further heightened concerns. Not only were very few sheep observed, but many of those that were observed, showed symptoms consistent with the advanced stages of pneumonia.

The Montana Mountains are located north of Winnemucca along the Oregon/Nevada border near the town of Orovada in wildlife management unit 031. That management unit is bordered by the Trout Creek Mountains, to the north, and includes the Double "H" Mountain Range, to the south.

"We have an obligation to take action and try to keep this unfortunate disease event isolated to the Montana Mountains. We have what appears to be a healthy herd of over 100 sheep just south of the Montana Mountains in the Double "H" Mountains," said Wasley. "Quick action now can prevent sick sheep from contacting healthy sheep from the Double "H" Mountains and possibly infecting that herd."

NDOW chose to initiate the operation now because the recent warm weather has caused the snow cover in the area to begin melting, which would allow diseased sheep from the Montana Mountains to disperse. Less confined by snow, the diseased sheep, could wander and spread the pneumonia to the apparently healthy sheep in the Double "H" Mountains or other adjacent mountain ranges.

The operation to remove the sheep began Sunday morning (Feb, 14) with a crew from the U.S. Department of Agriculture Wildlife Services using a helicopter to fly over the area and remove the remaining affected bighorn sheep in the Montana Mountains. Through the end of the day on Monday (Feb, 15), 24 sheep in total had been removed from the targeted area.

"Ten sheep were recovered and brought to the lab for testing. What we have seen so far in the animals examined is a severe pneumonia. I’m surprised that some of these animals were still alive," said NDOW veterinarian Peregrine Wolff. She describes the strain of bacteria contracted by the bighorns as "novel" to Nevada. The source of the pneumonia has not been determined at this time. Pneumonia in wild bighorn sheep is not transmissible to humans.

Although this is the first time Nevada has used this tactic to eliminate an entire herd to protect others, depopulating has been used by other western states and a Canadian province to try and prevent the spread of disease.

"For those of us who have spent our entire careers restoring sheep to historic ranges this is an incredibly painful course of action but one we simply could not avoid," said Mike Cox, NDOW staff biologist. In 2013 the state of Washington used the same tactic to remove 63 diseased animals from the infected Tieton herd to save the healthy herd of sheep on the adjacent Cleman Mountain.

In 2010, the Utah Division of Wildlife Resources euthanized the remaining 25 animals from the Goslin Mountain herd, which had numbered up to 60 bighorn sheep before the die-off, to prevent the spread of pneumonia to adjacent healthy herds. In 2000, in British Columbia, bighorn sheep were removed in the Vaseux herd to protect a healthy herd located in the neighboring Penticton Creek herd.

"Hopefully this will be the first step in what will be a long and thoughtful process that may allow us to restore sheep to the Montana Mountains," said Cox. "But, before we can even consider restoring the herd we have to take this necessary step." Cox says continued surveillance of the area is planned.

Bighorn sheep were reintroduced into the Montana Mountains in 1991 from Hart Mountain, Oregon. The Montana Mountains sheep herd has successfully been used many times as a source stock for translocations and has provided over 60 sheep to areas of Utah and Nevada and until now, had not experienced a disease event.

The Nevada Department of Wildlife (NDOW) protects, restores and manages fish and wildlife, and promotes fishing, hunting, and boating safety. NDOW’s wildlife and habitat conservation efforts are primarily funded by sportsmen’s license and conservation fees and a federal surcharge on hunting and fishing gear. Support wildlife and habitat conservation in Nevada by purchasing a hunting, fishing, or combination license. Find us on Facebook, Twitter or visit us at