Four dead bighorn sheep found in the California desert along with reports of other sickly animals in the region have prompted an investigation into a possible outbreak of animal pneumonia.

A National Park Service employee recently found the dead bighorn on Old Dad Mountain, about 15 miles southeast of Baker, Calif., which is near the Nevada border. A news release from the California Department of Fish and Wildlife (CDFW) reported that, in addition to the dead sheep, the employee observed animals walking unsteadily and exhibiting labored breathing.

An analysis of blood and tissue samples taken from one animal indicated it had pneumonia.

Pneumonia is usually fatal to desert bighorn sheep, which may acquire the disease from domestic sheep or goats.

Biologists and state officials are investigating animals in nearby areas to learn the extent of the pneumonia outbreak. As many as 300 bighorn sheep roam in the area of the Mojave Desert area under investigation.

"While we do not yet know the full extent of this disease event, we are taking this situation seriously. Bighorn sheep are highly susceptible to pneumonia," said Ben Gonzales, Senior Wildlife Veterinarian with CDFW's Wildlife Investigations Laboratory.

Pneumonia is an inflammation of the lungs that results in respiratory difficulties. Outbreaks of the disease are not uncommon in herds of domestic ruminants, who may contract the disease due to the stress of transportation, viral infections, lung parasites or overcrowded living conditions.

Signs of a pneumonia outbreak may not be noticed until several animals in a heard of sheep of goats have died, according to a report by Dorothy Ford of Tarleton State University in Texas.

Though rare, pneumonia has been observed in household pets like cats and dogs.