Studies show that domestic sheep can pass a bacterial pneumonia (pasteurella) that is fatal to wild sheep.
With the current information that is available, most wildlife professionals wildlife veterinarians and researchers have concluded that bighorn sheep and domestic sheep should not occupy the same range because of the potential adverse effects from disease on bighorn sheep.
Bighorn sheep are highly susceptible to some strains of pasteurella carried by domestic sheep, while domestic sheep are immune to strains carried by bighorn sheep. Bighorn sheep usually die after exposure to specific strains of pasteurella from healthy domestic sheep.
When bighorn sheep experience a pasteurella die-off, all age classes usually suffer. Lambs that are born into these populations generally experience low survival rates for approximately three to five years, or more.

What is the scientific evidence?
In 14 fenced studies, when bighorns and domestic sheep were confined together, there were ten cases where all bighorns die from pasteurella. In the other four studies from 50% to 83% of the bighorns died. Additionally, 18 incidents involving free ranging bighorns in eight states or provinces, linked contact with domestic sheep to bighorn die-offs.

All ungulates carry pasteurella, but all strains are not fatal to wild sheep.
All ungulates carry strains of pasteurella. Experimentally, elk, deer, mountain goats, cattle, horses and llamas that associated with bighorn sheep did not result in pasteurella problems with bighorns. A large number of strains are known to exist, of which only a few are deadly to bighorn sheep. There are some strains of pasteurella carried by domestic sheep that are not lethal to bighorn sheep. Young bighorn rams commonly travel outside their primary range. Domestic sheep can stray from the main flock. Because of these habits, buffers, unless very large, have often failed.

How about a vaccine?
Although attempts have been made, no effective vaccine exists that will prevent bighorn sheep from developing pneumonia after contact with harmful strains of pasteurella. If one were developed, it would be difficult to administer to free ranging bighorns.

If you see bighorn sheep in close proximity of domestic sheep, it is extremely important to contact your nearest Wildlife Department Office as quickly as possible. Wildlife Department personnel may need to remove bighorns from your area to prevent the spread of potentially fatal disease to the herd.

Taken from the Foundation for North American Wild Sheep website with permission.