'Worst-Case Scenario' Disease in Mojave Bighorn Sheep Crosses State Lines

Bighorns in the Old Dad Mountains, Mojave National Preserve. | Photo: Le Hayes

In what wildlife agency staff are calling a "worst case scenario," two different killer strains of a pathogen that causes infectious pneumonia have been found in Southern Nevada's desert bighorn sheep, and one of those strains is the same one that's been killing bighorns in the Mojave National Preserve in California.

After testing bighorn in mountain ranges in Southern Nevada to see whether the sheep had been exposed to a strain of the pathogen Mycoplasma ovipneumoniae that's been killing sheep in the River Mountains near Hoover Dam since August, biologists with the Nevada Department of Wildlife (NDOW) confirmed that strain's presence in the Eldorado, Spring, and McCullough mountains.

And to their horror, the biologists also found that a different strain of the pathogen responsible for killing bighorn in the Mojave National Preserve in California was also present, raising the possibility that the disease travels through the desert's wide-ranging bighorn populations more readily than suspected.

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The River Mountains strain of the pathogen was detected in sheep in the Spring, McCullough, and Eldorado mountains, which run north from the Mojave Preserve area, connecting bighorn sheep habitat between the Preserve and the desert mountains north of Las Vegas. The strain found in the Mojave Preserve was also found in the Spring Mountains.

"This is a worst case scenario," said NDOW wildlife biologist Pat Cummings in a press statement. "Given the geography between the Spring Mountains and the outbreak area in California, we were concerned this might be possible, especially with the ability of bighorn rams to cover vast amounts of territory in their wanderings. There is no way to limit these animals' movements."

Bighorn sheep have limited immunity to pneumonia, and the mortality rate generally runs between 50 and 90 percent. Sheep that recover can remain infected, spreading the disease to their companions for as long as a decade. Lambs are especially vulnerable to the disease.

The Preserve's bighorn sheep are thought to have contracted the disease through illegal dumping of domestic sheep carcasses or manure by livestock haulers. Outbreaks in the Old Dad mountains in the southwest corner of the Preserve and the Marble Mountains just south of the Preserve were detected in May of 2013. Between those two ranges and the Spring Mountains, where the same strain of pathogen was detected, the Providence, New York, Ivanpah and Clark mountains make up a nearly unbroken corridor of bighorn habitat stretching for a hundred miles.

As no vaccinations or other palliative treatments exist for bighorn sheep suffering from pneumonia, NDOW biologists are limited to monitoring the situation to try to determine how big the outbreaks are, and what effect the two different strains actually have on the sheep in seasons to come.

As part of that monitoring, NDOW is asking hikers and others who encounter sick or dead bighorn sheep in Nevada to document the animal's location with photos and GPS coordinates, and then share them with NDOW biologists.

About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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