State believes disease responsible for big horn die-off

The Nevada Department of Wildlife has been investigating an apparent bighorn sheep die off in the Hay's Canyon Range, a chain of mountains in northwestern Nevada. The news of a possible disease event in this area came from a 2007 bighorn sheep tag-holder after the hunter observed what appeared to be a sick ewe that was found dead a few hours later.

Game wardens retrieved the carcass, which was submitted for veterinary diagnostic work-up and a thorough necropsy examination. The results of the examination, backed up by various laboratory results, confirmed that the ewe died from severe bacterial pneumonia.

With funding from Nevada Bighorns Unlimited, the state performed a follow-up aerial survey of the Hay's Canyon area immediately following the discovery of the first dead ewe and only seven live sheep were observed. Increasingly intensive ground surveys in October and November followed. State biologists and club members were able to locate several decomposed carcasses as well as several sick bighorn sheep. A number of samples were obtained from both sick and dead animals. As expected, bacterial pneumonia was identified in all animals.

All evidence gathered in the fall of 2007 pointed to a die-off occurring in the area and a second helicopter survey was conducted by the state in mid-November covering the entire ridge system and western slope of the Hay's Canyon Range. The survey turned up more carcasses and only two bighorn were seen alive. Several bighorn observed alive during the initial helicopter survey in October were later found dead near or adjacent to water sources. Additional ground surveys failed to locate any live bighorn.

Based on veterinary findings and survey results, state scientists are convinced that a major disease event has seriously impacted the Hay's Canyon bighorn population. Biologists are hopeful that a small number of bighorn survived this catastrophic disease event " some sheep may have a genetic resistance to certain diseases " and are still occupying the range. It is also possible that some of the population migrated to a more hospitable area in response to the severe drought conditions of the past year. Reports made to the state and the local office of the Bureau of Land Management indicate the presence of live bighorn north of the Hays Canyon Range along the Vya Rim and while the numbers of bighorn there remain low, the animals that have been seen appear to be healthy, apparently unaffected by the disease event that occurred further south.

"We're working hand-in-hand with the Nevada Department of Agriculture in diagnostic work," said Dr. Mark Atkinson, NDOW's wildlife veterinarian. "Landowners and grazing permit holders in the area are also participating in the investigation; we're all concerned about the death of these bighorn sheep and are working cooperatively to solve this puzzle."

Unfortunately it may be some time before NDOW biologists have a good understanding of the factors that initiated this disease event or how many resident bighorn survived it. Respiratory disease in bighorn sheep is especially complex, usually involves multiple factors and specific causes can be very difficult to determine. NDOW will continue to investigate this bighorn sheep die-off and through collaboration with the Department of Agriculture, University of Nevada Reno, NBU, BLM and local ranchers hopes to be able to shed light on what may have contributed to this disease event.