intended to be a disease project, the more we worked on restoring the sheep, the more we realized that disease was the biggest limiting factor.” In early experiments, when domestic sheep and bighorn sheep were put in the same pen, the bighorns died. “We’ve known for a while that there’s something domestic sheep carry to which bighorns have little resistance,” she said. “But it took a long time to understand exactly what that was.” In recent years, biologists have identified bighorn sheep interaction with domestic sheep and goats as the source of the pathogen’s transmission. Both wild and domestic sheep and goats are gregarious, and don’t shy away from interacting with each other. But while domestic herds have immune resistance to the effects of M.ovi, wild herds do not. All it takes is a sneeze or nose-to-nose contact to transmit M.ovi.
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HERE IS NO effective treatment or vaccine for M.ovi that will stop transmission, but there are two, equally important solutions, both fraught with challenges. One is to eliminate M.ovi from a herd, a strategy the DFW has adopted with “test and remove.” That’s logistically challenging given the number of bighorn sheep and the complexities of catching and testing each one in terrain and weather conditions that are often inhospitable.
“Initially, recommendations were to test 95 percent of the herd and to capture and test them twice,” Moore said. “Today we’re trying to optimize our test and remove strategy by using data to determine the herds’ interaction. We can track bighorn sheep interactions with their GPS collars, and by knowing how the populations are interacting, we can test more effectively.” Test and remove is underway in bighorn sheep herds in the Hells Canyon herd, the Umtanum herd in Yakima County and the Cleman Mountain herd in Kittitas County. Moore’s team is planning new test and remove projects in the Mount Hall herd in Okanagan County, near Omak, in two years’ time. The strategy has been effective in the past in Hells Canyon, where 850 bighorns reside in sixteen herds that traverse 5.6 million acres of land in Washington, Idaho and Oregon. “Hells Canyon was one of the places where test and remove was first tried, starting in 2012, after over a decade of research into infection and low lamb survival,” Cassirer said. “Five carriers were removed in 2015 and placed in captive research facilities. After that, those Washington herds were M.ovi-free until 2024, when a new strain of M.ovi occurred in Hells Canyon. We hope to do the same on this round to gain a better understanding of why some sheep are chronic carriers and how to better identify them to minimize the amount of capture and testing.”
Bighorn sheep lambs are more at risk to M.ovi than their elders. (photo: Washington Department of Fish & Wildlife)
58 1889 WASHINGTON’S MAGAZINE FEBRUARY | MARCH 2025
