An invasive species found across West Texas may pose a greater threat to native bighorn sheep than previously understood.
In a new study published in Scientific Reports, Texas A&M University researchers in collaboration with veterinarians and biologists from the Texas Parks and Wildlife Department (TPWD) found that 80% of bighorn sheep experimentally exposed to respiratory pathogens carried by aoudad died from pneumonia, providing new evidence that helps explain how disease-causing pathogens may persist in bighorn sheep habitat. The findings also provide wildlife managers with new information to support conservation efforts.
"The experiment showed us the lethality of aoudad-bighorn contacts, that some aoudad may pose a greater risk than others, and that aoudad populations across the Trans-Pecos carry different strains at different rates," said Dr. Logan Thomas, an assistant professor at Kansas State University and the study's lead author.
Tracking disease across the landscape
Respiratory disease remains one of the greatest challenges facing bighorn sheep conservation across North America. Pneumonia outbreaks can cause widespread illness and mortality within herds, making it critical for researchers to understand how disease spreads and where pathogens persist between outbreaks.
Aoudad, also known as Barbary sheep, were introduced to Texas decades ago and have since established large populations throughout portions of the state. In some regions, they share habitat with bighorn sheep, raising concerns that they may contribute to disease transmission.
To investigate those risks, researchers evaluated whether aoudad could maintain and transmit Mycoplasma ovipneumoniae and leukotoxigenic Pasteurellaceae — two groups of pathogens associated with pneumonia in bighorn sheep.
"Wildlife managers have been concerned about the potential aoudad disease threat for our native species since the late '70s, but relatively little work had been done to characterize these risks," Thomas said.
The study found that while exposed bighorn sheep frequently developed severe disease, aoudad continued shedding pathogens for extended periods while typically showing relatively few signs of illness. This combination may allow the invasive species to act as a source of infection for nearby bighorn sheep populations.
The findings highlight an important difference in how the two species respond to infection. While bighorn sheep can experience severe illness and death following exposure, aoudad may be capable of carrying and spreading pathogens without exhibiting the same level of disease.
"Because aoudad show little, if any, signs of infection, every aoudad on the landscape should be conservatively considered a risk to bighorn sheep," Thomas said.
Because hosts can continue spreading pathogens without appearing sick, they may allow disease to persist on the landscape and increase the risk of transmission to more vulnerable species.
"Some other work we did on this suggests aoudad handle these pathogens far better than bighorn sheep, possibly because aoudad evolved with some level of exposure to them whereas bighorn did not," Thomas said.
What researchers found across Texas
The team also surveyed 351 free-ranging aoudad from populations across Texas to better understand how widespread the pathogens may be in the wild, finding that approximately 9.4% of sampled animals throughout the state carried M. ovipneumoniae DNA in nasal swabs, while 55.8% had antibodies indicating previous exposure. The study also found differences among populations and that juvenile aoudad shed more pathogens than adults.
"There's a lot of work ongoing to learn what makes aoudad populations tick," Thomas said. "We have good reason to believe that, just like the disease risk posed by domestic sheep to bighorn, that juveniles may be the major players, as they shed the pathogen more frequently," Thomas said.
Together, these findings provide a more complete picture of how disease may persist across the landscape.
The statewide sampling effort also demonstrates that exposure is not isolated to a single region, emphasizing the potential importance of aoudad in the spread and persistence of disease throughout portions of Texas, as that exposure to these pathogens is already occurring in wild aoudad populations across the state.
"Texas Parks and Wildlife Department initiated surveillance around 2014 to assess any pathogen-related risks to bighorn conservation," Thomas said. "While this disease system is always complex, the strains and distribution seem to be unique in Texas, warranting continued surveillance and vigilance."
Implications for conservation
The findings provide wildlife managers with new information about factors that may influence the long-term health and recovery of bighorn sheep populations.
"This research has changed the way TPWD manages bighorn in Texas," said Dr. Walter Cook , a clinical professor in VMBS' Department of Veterinary Pathobiology . "For example, they recently relocated over 70 uninfected bighorn to Franklin Mountains State Park in El Paso, where there is very little chance of aoudad reaching the area."
Because aoudad populations continue to expand across portions of Texas, understanding how pathogens move among wildlife species has become increasingly important for conservation planning. The researchers say strategies such as reducing opportunities for disease transmission and improving long-term surveillance may help protect vulnerable bighorn sheep herds.
For wildlife managers working to protect and restore bighorn sheep populations, understanding where pathogens persist and how they spread between species is critical to reducing disease risks and supporting long-term herd health.
"Important questions remain," Cook said. "For example, how virulent are the strains carried by aoudad in different parts of Texas? Also, can shared water sources post a threat? Finally, what is the best technique to control aoudad numbers and spread?"
While additional research is needed, as aoudad populations continue to expand across portions of Texas, the researchers hope their findings will help inform future surveillance and management efforts aimed at protecting native bighorn sheep and reducing disease risks across the American Southwest.
By Camryn Haines, Texas A&M University College of Veterinary Medicine and Biomedical Sciences
