It is said that bighorn sheep used to be more common than deer in Hells Canyon. Tragically, by the early 1900s they were completely extirpated from the area, partially because of disease. Reintroduction of bighorn sheep back into Hells Canyon has restored some populations, but they have been affected by repeated pneumonia outbreaks and are only a fraction of what they used to be. Over a decade ago biologists discovered the main perpetrator of these outbreaks.
Mycoplasma ovipneumonia (M.ovi) is a bacterium that can cause fatal pneumonia outbreaks in bighorn sheep. Approximately 50% of domestic sheep and goat flocks in the U.S. have at least one M.ovi positive animal. A study conducted by the USDA in 2011 reports an estimate in commercial sheep flocks is as high as 88.5%. M.ovi usually causes no obvious disease in domestic sheep and goats, although it can stunt lamb growth and increase susceptibility to other diseases. M.ovi is not harmful to humans.
This is not the case for bighorn sheep. Once a bighorn is infected with M.ovi, the infection spreads throughout the herd and 80% can die within months. Some sheep that survive the outbreak become carriers who infect newborn lambs raised in communal nursery groups, often resulting in high rates of lamb death, sometimes up to 100%, for years afterwards. This becomes a fatal annual cycle and without lamb recruitment, the herd will stay small or decline.
Unfortunately, now and in the foreseeable future there is no effective vaccine for either bighorn sheep or domestic sheep or goats. This all may seem bleak, but this story is not without hope.
Content from Asotin County Conservation District, 2023