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December 7, 2021

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Not in any kind of control:’ Pneumonia killing Nevada bighorns

Desert Bighorn Sheep at Hemenway Park

Steve Marcus

Desert big horn sheep rest on the grass at Hemenway Park in Boulder City, Nev. Saturday, July 13, 2019.

Desert Bighorn Sheep at Hemenway Park

A desert bighorn sheep is shown at Hemenway Park in Boulder City, Nev. Saturday, July 13, 2019. Launch slideshow »

From climate change to habitat loss to invasive species, desert bighorn sheep face a host of threats. But the No. 1 concern for the official state animal, says Nevada Department of Wildlife veterinarian Peregrine Wolff, is pneumonia.

First identified in 2012 as a problem afflicting endangered desert bighorn sheep across the country, bacterial pneumonia continues to kill off entire bighorn herds statewide, said Pat Cummings, a game biologist for the department of wildlife. While wildlife officials have a better understanding of the disease’s origins and how it spreads, the problem persists among Nevada’s approximately 10,400 desert bighorn sheep.

“It’s not in any kind of control,” Cummings said.

Desert bighorn sheep likely first contracted the pneumonia-causing bacteria generations ago from domestic sheep brought to the American West from Europe. While domestic sheep had already developed a degree of tolerance or immunity to the bacteria, wild bighorn sheep were never previously exposed to the disease, making them more likely to succumb to it.

“These animals have no immunity to the actual bacteria itself,” Wolff said.

The wildlife department is trying to get a grip on the problem by surveying all of the approximately 100 bighorn herds statewide, an effort that is expected to be completed by the end of February, Wolff said. During the surveys, Wolff and others have collected tissues from sheep belonging to each herd to identify “pre-disease conditions” that could contribute to their survival rate after being exposed.

It hasn’t been cheap or easy, given that staff need to locate the notoriously shy animals and capture them via helicopter to collect the samples, she said.

“It costs a lot of money, and it’s almost impossible to get all of the animals,” Wolff said. “But even eliminating a few can help make a big difference.”

Through the surveys, biologists and wildlife managers hope to determine how many bighorns in Nevada have died from the disease and what percentage are infected. They also want to understand why some herds seem to recover while others can be virtually wiped out after being exposed to the disease, Wolff said.

“One of the big questions and major disease research focuses is to look at factors within the sheep, within the bug itself, or within other bacteria involved,” Wolff said. “Also, what’s happening in the environment? What’s the impact of the environment on the ability of these animals to either not get sick, or once they get sick, to maybe rebound?”

Researchers have reached at least one conclusion: Fatalities are higher in lambs than in rams and ewes. Adult bighorns that survive the disease seem to develop immunity to it, which they don’t pass on to their offspring, Wolff said. Young seem to spread it easily amongst one another as well, while healthy survivors will often spread it to other members of their herd or other herds with whom they come into contact.

“Bighorn sheep, by their very nature, are highly social animals,” Cummings said. “So they’re routinely interacting with one another.”

They can also travel great distances, from the Idaho border to the southern tip of the state, increasing the likelihood that they sicken other herds, Cummings added. That’s part of why the disease isn’t concentrated in particular parts of the state; it has been observed in herds in the Spring Mountains, Lake Mead, Laughlin and parts of Northern Nevada.

Hallmarks of the disease include deep chest coughs and nasal discharge. In some cases, entire herds affected by pneumonia will die off, Cummings said. Other times, especially in southern parts of the state, only the lambs will succumb to the disease.

“We have infected populations that, as years go by, we have very little lamb recruitment or negligible recruitment,” he said.

In the long run, that can actually be more devastating than the herd-wide die-offs, because it allows the disease to continuing spreading, according to Cummings. Part of the reason why the disease has persisted could have to do with the fact that it isn’t guaranteed to kill every infected animal.

“You have some animals that appear clinically healthy, but they’re shedding the microbes that cause disease,” Cummings said.

Even though the Department of Wildlife isn’t sure if the disease is less widespread than it was five or 10 years ago, Cummings says there is reason to be hopeful. During an aerial survey this fall, he saw greater lamb representation than expected in some of the infected herds, indicating that disease recovery could be on the horizon.

“But it’s still too early to know,” he said.