Bighorn sheep were once common in Southern California and Nevada, but by the 1960s their numbers had been sharply reduced by disease, unregulated hunting, and habitat loss. Forty years of cooperative efforts to rebuild the herds were paying off, but now a major disease outbreak at Mojave National Preserve poses a major threat to the majestic animals.

The problem was discovered in mid-May by a National Park Service employee at Mojave National Preserve who was inspecting wildlife guzzlers (a tank that collects rainwater for wild animals to drink). The employee found four desert bighorn dead on Old Dad Mountain, 15 miles southeast of Baker, California, and also observed other sick animals that appeared to be weak and unsteady with labored breathing.

Results of laboratory analysis of blood and tissue samples were sobering: the animals were infected with pneumonia. The mortality rate for animals infected with the disease is 50 to 90 percent, and there is no vaccine or cure. This form of pneumonia is not transmissible to humans.

Bighorn Sheep Have No Resistance to the Disease Transmitted By Domestic Animals

The disease typically enters into a population from domestic sheep or goats. Although no one knows for certain where the most recent outbreak disease originated, an angora goat was found near Marl Springs about 12 miles east of Old Dad Mountain where the outbreak is centered. Biologists caution the public to avoid releasing domestic sheep or goats into the wild as they often are disease carriers even if they have no symptoms. Desert bighorn have no natural defenses to diseases carried by domestic animals.

That lack of natural resistance results in high infection and death rates in bighorn populations. The few animals that survive become carriers, infecting new lambs that often die within a few months of birth. An outbreak such as this one typically causes a long term decline in the population that can last for more than a decade. Scientists believe that pneumonia outbreaks have reduced herds of bighorn sheep in western states by up to 90 percent.

Biologists from the National Park Service and California Department of Fish and Wildlife have conducted field surveys to monitor the scope and spread of this wildlife disease outbreak, but the work is difficult in the steep, rugged, and remote terrain. Using volunteers from the Society for the Conservation of Bighorn Sheep and the Sierra Club to expand their capacity, biologists have visited springs and guzzlers where bighorn congregate on Old Dad Mountain and in nearby areas to determine the extent and seriousness of the problem.

Surveys suggest "a large mortality event has occurred."

To date, about 30 sheep carcasses have been identified. A helicopter survey in mid-July indicated that there were significantly fewer sheep than have previously been surveyed in the vicinity of Old Dad Mountain, suggesting a large mortality event has occurred.

Scientists are considering what, if anything, they might try experimentally as they continue to monitor the outbreak, but there are no good management options. One goal may be to attempt to prevent spread to outlying populations; with the rut beginning in the coming months, biologists believe the disease could spread rapidly as animals from separate herds mix.

An interagency working group has been formed to monitor the outbreak, suggest best practices for limiting effects of the disease, and provide recommendations for management actions. Members of the working group represent the California Department of Fish and Wildlife, National Park Service, Oregon State University, and affiliated researchers. Funding and staff for this work is provided by the Wild Sheep Foundation, Society for the Conservation of Bighorn Sheep, California Department of Fish and Wildlife, and the National Park Service.

Options are being weighed, but there are no easy solutions

Management actions currently under consideration include collaring healthy bighorn sheep in areas peripheral to the outbreak to monitor disease spread and determine sources of mortality, strategically using water sources to discourage connectivity between sub-herds in an attempt to limit disease spread, and removing clinically ill animals from the population to limit disease spread.

There are five separate population groups of bighorn in Mojave National Preserve, although individuals sometimes move from group to group. Before this current disease outbreak began, scientists estimate there was a total of between 425 and 750 animals in the five groups. A potential reduction of those numbers by up to 90 percent would represent a major setback to recovery efforts in the desert Southwest.