How sheep ranching spreads disease

How sheep ranching spreads disease

We have a major problem with domestic sheep diseases causing the demise of native bighorn sheep on our western public lands. Domestic sheep are carriers of the diseases Mannheimia haemolycta, which causes a deadly form of pneumonia that can wipe out an entire herd of wild bighorns from a single contact, and Mycoplasma ovipneumoniae, a sort of HIV for bighorn sheep that attacks their immune systems and causes pneumonia in its own right. These diseases do not affect domestic sheep, which evolved with them. Yet, a single nose-to-nose contact between the species can wipe out an entire herd of bighorn sheep.

Bighorns are naturally curious about their domestic cousins, and these diseases are the main problem preventing bighorn sheep recovery across the mountains of the American West. Contact and transmission can decimate existing bighorn herds and prevent their recovery in formerly-occupied mountain ranges. 

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Even the possibility of bighorn sheep contracting domestic sheep diseases sometimes compels state agencies to kill off entire herds of bighorns, to prevent further spread of disease. Sometimes they do this through special trophy hunts, to gain the support of sport hunters who look upon bighorn sheep as that rare hunt-of-a-lifetime. This is a rather brilliant way to distract hunters from the fact that without domestic sheep roaming the public lands, bighorns might well be as common as elk or mule deer in our Western mountains, with hunting opportunities to match.

The disease problems posed by domestic sheep to bighorns are only the beginning of the story. It turns out that chronic wasting disease, a prion-based brain disease currently spreading through deer and elk populations across America, most likely originated with the domestic sheep disease called “scrapie.” This disease, also caused by prions which are a twisted form of protein that infects brain tissue much like “mad cow disease,” is relatively prevalent among domestic sheep herds.

Domestic sheep were housed in an agricultural experiment station in Fort Collins, Colorado, at a time when mule deer and elk penned up in the same facility contracted the first known cases of chronic wasting disease. It soon spread throughout elk and mule deer herds in Colorado, and now is making its way to the doorstep of Yellowstone National Park, one of the nation’s premiere elk-viewing locations.

Researchers testing chronic wasting disease have documented its transmission from elk to monkeys through the consumption of infected meat. This has caused grave concern among hunters that the disease may be transmitted from infected elk and deer to humans, if it hasn’t happened already. Chronic wasting disease can take years of incubation before the first outward symptoms arise, making it difficult for hunters to be sure their quarry is disease-free, and also making it difficult to tell if CWD is already present in humans or not.

The sheep industry has become adept at rhetorical gymnastics to cast doubt on their role as disease vectors on western public lands. Domestic sheep don’t spread diseases, they argue, they transmit pathogens (bacteria, viruses, and prions) and it’s the pathogens that cause the diseases, not the sheep. To untangle this rhetorical knot, we offer this logical proof: Sheep ranchers spread their livestock across the land, domestic sheep spread pathogens, pathogens cause disease and therefore domestic sheep (and the ranchers themselves) are vectors of disease.

Perhaps it is time the ranchers took responsibility for hamstringing the recovery of bighorn on western public lands, and started taking preventative measures to protect our natural heritage.  Such measures should include maintaining adequate separation — at least 10 miles, according to the “foray distance” of young bighorn rams seeking herds of females they might breed — between domestic sheep grazing leases on public lands and bighorn sheep herds.  

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That wouldn’t account for risks posed by sheep and goats on private land, which suggests a mandatory test-and-slaughter program to purge diseases from domestic herds of sheep. But this goal may be unattainable: Some 85 percent of domestic sheep operations are infected by Mycoplasma alone, and more than half the sheep in infected herds were found to be carriers of the pathogen. If these measures don’t stop sheep diseases from threatening the survival of wildlife on western public lands, we could always stop leasing public lands to domestic sheep producers. This last solution might be the most humane and sensible option of all.

Erik Molvar is a wildlife biologist and is executive director of Western Watersheds Project, a nonprofit environmental conservation group dedicated to protecting and restoring watersheds and wildlife throughout the American West.