Washington is home to an estimated 1,690 wild bighorn sheep that range in 17 herds in central and eastern areas of the state. Many of these wild sheep can be easily seen from highways and roadways, such as those in the Yakima River Valley and along Highway 12 near Naches.
Like other wildlife, bighorn sheep are subject to periodic disease outbreaks. A respiratory disease that takes a toll on wild bighorns is pneumonia. Unlike in many wildlife situations, however, the bacteria causing pneumonia in bighorns is not native to North America, and thus bighorns have not had evolutionary time to adapt to it.
Wildlife health researchers across the west have found that pneumonia in bighorns is most often associated with a bacterium named Mycoplasma ovipneumoniae, although other bacteria in the family Pasteurellaceae typically take advantage of its presence and cause death.
Domestic sheep and goats carry both M. ovipneumoniae and the various species within Pasteurellaceae, but are not affected clinically. However, wild bighorn sheep infected by these bacteria often develop acute pneumonia and die.
Unfortunately, there is no effective treatment or preventive vaccination for pneumonia in wild bighorn sheep.
Pneumonia is not transmitted from sheep to humans, nor to domestic livestock.
Pneumonia outbreaks have killed bighorn sheep in other western states and in some Washington herds. Infected bighorns were found in late 2009 and early 2010 in the Umtanum herd in the Yakima River Valley. In early 2013, a separate disease outbreak occurred in the Tieton Herd, near Naches. Because this outbreak was particularly lethal, and to protect the adjacent Cleman Herd, all surviving animals in this herd were euthanized.