Domestic Sheep Bad Company for Bighorn Sheep? - Rocky Mountain Research Station - RMRS

Researchers have long suspected that domestic sheep might be bad neighbors for bighorn sheep because contact between these species could cause various diseases—including respiratory disease and fatal pneumonia—in bighorn sheep.

A new review by U.S. Forest Service biologists of studies that investigated disease-related conflicts between these species concludes that although the reasons for the negative outcomes aren’t completely understood, bighorn sheep would be better off not sharing space with domestic sheep. Domestic sheep pass along various diseases to bighorn sheep, the most harmful being respiratory disease and fatal pneumonia. This review is another piece of the complicated puzzle of managing bighorn sheep, some of which share space with domestic sheep and goats on National Forest System (NFS) lands.

At one time, bighorn sheep could be found as far north as the Brooks Range in Alaska and as far south as Baja California and northern mainland Mexico, and from western Texas to just east of the Rocky Mountains in North Dakota, South Dakota, and Nebraska. Although the bighorn populations in Alaska and Canada have held steady, many of the populations to the south have become extinct. Populations in the western states of the United States and in Mexico declined since the nineteenth century to less than 5,000 sheep before bighorn sheep recovery efforts began in the 1960s. These efforts—which include transplanting sheep into unoccupied habitat, augmenting existing herds, and manipulating habitat—have helped increase herds. But 1999 estimates set the number of bighorn sheep in the contiguous United States at 47,900—far fewer than historical estimates.

Researchers are amassing evidence that bighorn sheep die-offs follow contact with domestic sheep, and have long suspected that the transfer of diseases from domestic sheep is an important factor. The first large-scale bighorn sheep population losses in the nineteenth century were attributed to scabies introduced by domestic sheep. Other diseases that might be communicable from domestic sheep to bighorn sheep include anaplasma, babesia, and pinkeye.

Researchers took a comprehensive look at scientific literature and expert panels to determine whether existing studies supported the hypothesis that bighorn sheep have a high likelihood of contracting fatal respiratory disease following contact with domestic sheep. The scientists reviewed various studies that used various experimental methods, research to identify bacteria strains causing fatal pneumonia, and results of vaccination trials. The scientists also surveyed legal findings and recommendations from scientific panels.

Although the specific ways in which domestic sheep pass diseases to bighorn sheep are not yet completely understood, the evidence supports the conclusion that if maintaining health bighorn sheep populations is a goal, then bighorn sheep should not cohabitate with domestic sheep. The challenge for managers of bighorn sheep habitat is to address the difficult task of keeping these species separate.