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To Fight CWD, Wildlife Managers Look To Lower Deer Density

White-tailed deer.
White-tailed deer.

It’s been two years since Chronic Wasting Disease (CWD) was detected in Montana’s deer herds, and in May the disease popped up in the northwest corner of the state in Libby. Montana Fish, Wildlife and Parks (FWP) has a plan to manage the fatal disease based on its prevalence, a strategy born from more than 20 years of trial and error across the country.

FWP has been holding a series of public meetings to educate the public about its management response in Libby after six deer tested positive.

"And so our main objective is not to eradicate it, we’re going to try to manage this disease to the best of our ability, try to keep that prevalence low, typically below 5 percent," says FWP's Neil Anderson.

At the Libby meeting, Anderson said the agency doesn’t yet know how prevalent CWD is in the area. So this fall, the agency plans to sample 400 deer to find out. To get that sample, FWP has issued 600 special whitetail licenses for a 10-mile radius around Libby, and will also trap animals in town.

Once FWP understands CWD’s scope around Libby, it will choose a management strategy to keep prevalence below 5 percent. That could mean increasing hunter harvests, or specifically targeting bucks, which are three times more likely to carry the disease. All the options FWP is considering are focused on limiting spread by lowering deer density.

But some questioned why the agency isn’t attempting to eradicate the fatal disease from the area.

"Wisconsin, when they first found this disease, they tried to drive that prevalence down as close to zero [percent], then hope that because of very low deer densities it would kind of wink out on its own," Anderson says.

Tami Ryan with Wisconsin’s Bureau of Wildlife Management says this scorched-Earth approach began in 2002 with a litany of methods, even hired sharpshooters. Ryan says those efforts showed signs of working, but ended in 2007 due to public backlash.

"After that many years of working outside of, maybe the framework in which hunters are accustomed to, public support and patience kind of waned," Ryan says.

Wisconsin has been reluctant to reduce deer densities since, and now has herds with prevalence rates over 50 percent, something most wildlife managers agree there’s no coming back from. That’s a fate Montana hopes to avoid.

Another tale of caution is Wyoming, where CWD was discovered in the wild in 1985. Wyoming has never reduced deer numbers in infected herds. Like Wisconsin, it has herds with prevalence rates up to 50 percent.

However, director of the nationally-based Chronic Wasting Disease Alliance, Matt Dunfee, says both of these approaches were completely valid when CWD was first being widely detected in wild herds.

"As that was happening, this sense of urgency rose really rapid in the early 2000s because we managers were in a really tough situation. We were finding the disease, but we had no idea what to do about it."

Unlike viruses which persist in herds for a limited amount of time and then disappear, CWD is caused by a misfolded protein called a prion. It spreads through animal-to-animal contact, and prions can persist for years in soil and vegetation contaminated by urine, feces and carcasses. That leaves wildlife managers with few options.

27 states have now detected CWD in wild deer and elk populations. After years of experimental management strategies, Dunfee says game managers have learned that eradication may be a pipedream.

Only New York has accomplished this, but it’s thought the state caught it extremely early. According to state DNR, only a handful of deer tested positive in 2005.

Scientists know that CWD’s spread takes off exponentially when prevalence hits about 20 percent. Dunfee says they are also beginning to understand the long-term impacts of high prevalence rates.

"Things like the herd becomes younger. Older age class of animals just start to vanish, particularly older-age male classes."

That’s been a turning point for U.S. wildlife agencies, which are now attempting to keep CWD levels under 5 or 10 percent, a number most see as manageable. This is based on recommendations from the Western Association of Fish and Wildlife Agencies.

Wyoming and Colorado are both reworking their management plans based on these ideas. FWP Disease Ecologist Emily Almberg says that’s also what Montana’s plan is based on.

"These are tools that we think we’ve got anecdotal or small amount of evidence that they actually work," Almberg says. "And we’re trying to implement those on a much larger scale and figure out whether they’re in fact making the differences we hope they are."

Almberg and others say it could be a decade or more before anyone knowns how effective this new style of management may be.

Management in Libby may be a good test once FWP understands prevalence in a year or so. Work being done near Billings and the northeast corner of the state could also prove valuable. As that work progresses, FWP is also trying to set up controlled studies with neighboring states and Canadian provinces to really assess its management practices.

Aaron graduated from the University of Minnesota School of Journalism in 2015 after interning at Minnesota Public Radio. He landed his first reporting gig in Wrangell, Alaska where he enjoyed the remote Alaskan lifestyle and eventually moved back to the road system as the KBBI News Director in Homer, Alaska. He joined the MTPR team in 2019. Aaron now reports on all things in northwest Montana and statewide health care.
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