Estimates based on aerial counts in 2009 place the bighorn population in the park at about 250 animals. Cassie Waters, a wildlife program manager at Zion, said she has noticed the sheep dispersing into new areas of the park, but most sightings are along State Road 9 from the east entrance to the tunnel.
"When some wildlife populations reach high density levels," Waters said, "it can stress them and make them more susceptible to things like this."
She said there have been no reports of dead bighorn, "but we have been given lots of reports on sick lambs. There have been no reports of sick adults, and I have seen some healthy lambs."
Lambs are the most impacted because, in addition to developing a sore mouth that makes them not want to suckle milk, their mothers may stop allowing them to feed because their teats also become infected and sore.
Park visitors may notice sick bighorn sheep for months as the disease works its way through the population. National Park Service officials have asked people to report dead or dying bighorn sheep to park dispatch at 435-772-3256.
Anis Aoude, big-game coordinator with the Utah Division of Wildlife Resources, said sore mouth disease has been documented in bighorn herds around the state. It was most recently observed in some animals captured south of Zion National Park during a transplant effort in January.
"We released them so they wouldn't infect the population we were moving," Aoude said. "It usually works its way through the system. Some succumb and remain. Others don't make it."
There is no cure for sore mouth disease in bighorn, and treatment is usually not a viable option because of the number of wild animals involved that could be carrying the virus.
The bighorn population in the area when pioneers arrived eventually disappeared by the 1950s, due likely to a combination of human impacts.
An attempt to reintroduce a dozen desert bighorn sheep from the area around Nevada's Lake Mead in 1973 appeared to fail when the animals simply vanished into the canyon country. But by the mid-1990s visitors in the national park started to spot bighorn again, and the population quickly grew within and just outside of the park.
Zion's sick sheep
I Visitors to Zion National Park are being asked to report dead or dying bighorn sheep to authorities at 435-772-3256.