(November 14) Biologists surveying the Mojave Desert’s mountains for the National Park Service and the California Department of Fish and Wildlife have noted a significant decline in the bighorn sheep population in a substantial portion of the region.
Scouts for Fish and Wildlife, utilizing helicopters, since late October visually scoured over 75,000 acres on and around Old Dad Mountain, the Marble Mountains, and the Kelso, Bristol, Clipper, Soda, Providence, Granite, Hackberry and Woods ranges in a concentrated effort to spot bighorn, which have been decimated by pneumonia since earlier this year.
Scouts were able to spot fewer than half the number of bighorn than they typically see during similar flyovers.
Wildlife officials are concerned that the pneumonia outbreak among what was considered one of the hardiest of the state’s herds, which killed as many as 45 bighorn sheep in the Mojave Desert between May and August, is persisting and could result in a catastrophic die-off.
Pneumonia is a pernicious condition with respect to bighorn. It can incubate in the animals for months with little or no outward indication for weeks and then strike the animals dead in a short time after it manifests. Sick animals are highly contagious and the disease can spread quickly to other members of the heard.
The disease has infested an unknown number of a herd of more than 300 of the sheep near Old Dad Mountain after first showing up in some of the desert ruminants around nearby Kelso Peak.
Wildlife officials were initially put on alert that there was a pneumonia outbreak among the bighorn in May, when three dead rams turned up at guzzlers – man-made watering founts – near Old Dad Mountain. Tests performed on the dead animals by UC Davis’s extension Animal Health Laboratory in San Bernardino confirmed they had succumbed to pneumonia. Biologists have speculated that pneumonia was introduced into the herd when a hunting guide felled a feral angora goat near Marl Springs, a dozen miles east of Old Dad Mountain.
Bighorn are particularly vulnerable to pneumonia and can contract it from domestic goats and sheep or cattle, which have a greater resistance to the disease. Once hit with a full blown case, the sheep become extremely lethargic and their air passageways coated with mucous, making respiration difficult. Upwards of 80 percent of an infected herd can die as a result of the contagion.
In August, wildlife officials were contemplating wholesale killing and removal of animals confirmed to be infected as a means of bettering the odds of survival for those yet uninfected. At least ten suspected bighorn sheep were shot. All but one was indeed infected.
The helicopter survey was augmented with efforts to fit a large number of the bighorn with radio beacon collars so their movements can be closely monitored. More than 70 such beacons were successfully affixed to the sheep. The upshot of the helicopter survey and the direct contact with the sheep, which in several instances included taking blood samples and nasal swabs, was the observation that the pneumonia outbreak is most serious in the environs of Old Dad Mountain.
Bighorn sheep can be inoculated against pneumonia but the resulting immunity lasts only about four weeks.
Herds of bighorn sheep south of San Bernardino County in Riverside, Imperial and San Diego counties were significantly depleted from a pneumonia outbreak that ranged all the way down to the Mexican border in the 1980s. A strategy that was somewhat successful in that episode consisted of isolating the healthy sheep and removing feral animals, including cattle, from the area in which they roamed.
The situation in the Mojave Desert comes less than two years after biologists had reported an uptick in the number of bighorn in San Bernardino County. According to the Society for the Conservation of Bighorn Sheep, surveys that group had completed in recent years in conjunction with the California Department of Fish and Game and the Forest Service showed steady increases in the bighorn sheep population locally. Those entities, which have conducted surveys for bighorn sheep in the San Gabriel range annually since 1979, said the San Gabriel Mountains, which lie south of the westernmost portion of the Mojave Desert, once held an estimated 740 sheep, making the San Gabriel population the largest population of desert bighorn sheep in California.  The bighorn population declined over 80 percent through the 1980s but appeared in 2011 to be on the increase. Estimates at that time postulated approximately 400 animals.
There is no indication that the pneumonia outbreak has extended into the mountain population of bighorn at this time.