An invasive sheep called aoudad (Ammotragus lervia) is causing problems throughout Texas, but the biggest problem has probably gone unexamined for decades. A study recently published in the journal Scientific Reports suggests that apparently asymptomatic aoudads are spreading respiratory pathogens that cause pneumonia to the state’s native bighorn sheep. According to recent experiments by a team of wildlife biologists and veterinarians, sick bighorn sheep have only a one in five chance of surviving.

“Wildlife managers have been concerned about the potential threat of aoudad diseases to our native species since the late 1970s, but relatively little work has been done to characterize these risks,” study co-author Logan Thomas, a biologist at Kansas State University, said in a statement.

Aoudads first arrived in West Texas shortly after World War II, when veterans returning from the Barbary Coast of North Africa brought home the large, horned sheep with the intention of raising them for hunting big game. Since then, the number of aoudad has soared by more than 1,800 percent, and an estimated 30,000 of them now live in the region. Their loss of local food sources is driving out the state’s 1,500 remaining bighorn sheep, while bacteria like Mycoplasma ovipneumoniae spread by aoudads that threaten commercial livestock, goats and sheep. The animals are such a nuisance that Texas even recently introduced a bill allowing hunters to shoot them from helicopters.

To better understand the true impact of Aoudad-borne diseases, researchers recently exposed native Maroons to the pathogens that cause pneumonia, M. ovipneumoniae and Pasteurellaceae. They found that aoudads often remain infectious for long periods of time despite showing comparatively few symptoms, while sick bighorn sheep present with much more serious problems. What’s more, the mortality rate of sheep can be as high as 80 percent.

“Other work we did on this suggests that aoudad handle these pathogens much better than bighorn sheep, possibly because aoudad evolved with some level of exposure to them, while bighorn sheep did not,” Thomas explained.

A subsequent survey of 351 aoudad at large throughout Texas showed that almost 10 percent were carriers of M. ovipneumoniae DNA, while more than 55 percent had antibodies from previous exposure. Juvenile aoudads also appear to shed more pathogens than adults.

More research is needed, particularly when it comes to understanding the virulence of various strains. Conservationists also continue to adapt their strategies to protect the state’s remaining bighorn sheep.

“This research has changed the way TPWD manages bighorn in Texas,” explained veterinary pathologist Walter Cook. “For example, they recently moved more than 70 uninfected bighorns to Franklin Mountains State Park in El Paso, where there is very little chance of aoudad reaching the area.”

Meanwhile, Thomas cautioned that any aoudad detected in Texas should be considered a potential problem.

“Because aoudad show little to no signs of infection, every aoudad on the landscape should conservatively be considered a risk to bighorn sheep,” he added.

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