Desert bighorn sheep in Texas are on the decline — by as much as 50 percent. But this time, the culprit is different, it’s a bacterium which weakens the immune system and makes the sheep susceptible to pneumonia.
Desert bighorns were eliminated from the state with the last observations of native sheep around 1960. Competition with livestock, disease and predation were the causes of the extirpation.
Through the efforts of West Texas landowners and the funding by hunters, along with the Texas Bighorn Society and the Texas Parks and Wildlife Department, bighorns were restored and reached a 2020 estimate of 1,500 desert sheep in 11 different herds, one of the state’s most recognized conservation achievements.
However, over the past three years, significant declines in desert bighorn numbers have been observed and officials say the reason is pneumonia caused by the bacterium Mycoplasma ovipneumonia, also known as MOVI among sheep biologists. MOVI weakens the immune system and makes them susceptible to other bacteria which cause the pneumonia. The pneumonia primarily affects lambs and adults that survive and can become chronic carriers, infecting subsequent generations of sheep.
MOVI can be found in a number of species including deer and cattle, but is most often associated with domestic sheep and goats. In Texas, MOVI has been documented in exotic aoudad and it’s the aoudad that pose the risk to desert sheep populations. In other states, declines in wild sheep populations due to MOVI have been documented for decades, and sheep biologists have been working on research, policy to provide adequate separation between domestic and wild sheep and management options for years. To date, no effective treatment has been found and, in many cases, removal of infected sheep or entire herds has been necessary.
The Texas declines began to appear in 2021, with the most recent counts indicating a 50-percent decline in overall numbers. Most telling is the decline in lamb survival, slipping from an average of 43 percent pre-2020 to an average of 25 percent since 2021.
“In Texas, it’s an aoudad problem,” said John Silovsky, TPWD’s wildlife division director. TPWD said it has undergone efforts to keep aoudad away from bighorns in critical breeding populations like the Elephant Mountain WMA, but on a large scale, such as the Sierra Diablo Mountains, it’s very difficult. A single aoudad intermingling with a herd of ewes and lambs can infect the herd.
Managing the risk and dealing with outbreaks is a huge task and entire working groups are addressing it across western North America. Research is ongoing and developing selective management methods is being tested in numerous sheep herds across the U.S. and Canada.
In Texas, TPWD is working to protect existing populations and has initiated an effort to establish a new population in Franklin Mountain State Park outside of El Paso — and also outside of the existing range of aoudad.
Eliminating the expanding aoudad population isn’t a practical solution at this point in time, but ways to prevent bighorn deaths due to MOVI are needed. In the interim, sheep researchers and landowners will be fighting to keep the wild sheep conservation story alive in Texas.
