Study: higher buck kill could slow CWD spread

Madison — A just-released University of Wisconsin study of chronic wasting disease recommends focusing more hunting pressure on the deer most likely to carry and spread CWD in whitetails: bucks, the males of the species.

The peer-reviewed study, released March 21 and published online in PLOS ONE, was led by Christopher S. Jennelle, Ph.D., and Professor Michael D. Samuel at UW-Madison. Samuel concedes many hunters will dislike shooting more bucks to reduce CWD, but Jennelle’s study predicts the alternative is CWD increasingly infecting more deer while spreading across the landscape.

Samuel said this “acceleration of the infection rate” is already under way in the CWD’s core area in southwestern Wisconsin. Further, the researchers wrote that current deer-management practices in the area will have a predictable result: “Nearly 50 percent of adult males and 25 to 30 percent of adult females are expected to become infected within another decade.”

In fact, those numbers could be conservative.

“If we learn disease transmission is strongly tied to the amount of prions deer shed into their environment, and if that reservoir continues to build and worsen with the disease’s acceleration, our prevalence prediction would be optimistic,” Samuel said in an interview. “In a worst case, it might look like the 80 percent prevalence we saw on the Hall Farm.”

In contrast, if the DNR implemented a buck-focused management strategy and hunters carried it out, deer herds would soon have fewer adult bucks, but lower disease rates. The study projects a buck-focused plan would soon start reducing the CWD rate and drop it into the 5 percent range in 30 to 40 years.

“We’ll probably never get rid of CWD, so the goal becomes how do we get it to a manageable level, one that doesn’t impact our deer herd; and how do we try to contain it so not every area becomes infected?”

Samuel said. “If we don’t, the disease will grow worse and before long have impacts on the deer population.”

The research article put it this way: “The tradeoff between strategies is clear. CWD can eventually be reduced with fewer opportunities to harvest healthy bucks, or more adult bucks may be available for harvest, but with higher rates of CWD infection.”

But “more adult bucks” is no certainty in that second scenario. Samuel said as CWD worsens, fewer bucks will reach older age classes, and hunters who managed properties for bigger bucks will start losing more of them to CWD.

“These are sobering options, but the sooner we act, the less severe the problems and choices we’ll face,” Samuel said. “If we don’t do anything, and just stick with what we’re now doing, we’ll lose more adult males because we’ll get such high infection rates. Most of those deer will probably get killed, one way or another.”

Jennelle, Samuel, and their research team reached those conclusions after studying deer-harvest data from Wisconsin’s 2002 to 2013 hunting seasons, evaluating rates of CWD infection, and analyzing how alternative management plans would affect CWD and the herd.

The 12 years of data showed CWD infection rates are twice as high in males than females, and support the idea that CWD is a “frequency-dependent,” not a “density-dependent,” disease. That means the rate at which deer become infected is driven by CWD prevalence in the herd, not the size of the herd.

Therefore, as prevalence rises, the rate and number of new infections also rise. Unlike many frequency-dependent diseases, however, CWD can probably be reduced because hunters could target CWD’s most likely carriers.

“Frequency-dependent diseases are typically hard to control with generalized removal strategies,” Samuel said. “If male and female deer had the same disease prevalence, the only way to reduce prevalence would be to selectively remove more infected animals than uninfected animals. We’d have to capture deer and test them, which would be very difficult and costly. Because we can easily distinguish bucks (by their antlers) from females, we’re lucky to have a management option.”

Samuel said the research reinforces his skepticism that evolution and natural selection (the genetic resistance theory) is a viable management option.

“The sooner we actually do something about CWD, the better off we’ll be,” he said. “We still don’t know how the disease spreads, but one likely way is by infected yearling bucks dispersing to new areas. The worse the disease prevalence, the higher the infection rate we’ll see in young bucks, and the more likely they’ll spread the disease.”

Samuel said CWD prevalence and its potential impacts were mostly ignored shortly after the disease was discovered in Wisconsin in November 2001. However, this new research also found that focusing hunting pressure on females to reduce deer densities won’t, by itself, reduce CWD prevalence either.

“Shooting more bucks to reduce prevalence is one key to managing this disease, but we still need to work on abundance to some extent,” Samuel said. “These things go hand in hand.”

Further, reducing buck numbers and the overall deer population in CWD areas would make vaccination a more practical possibility, should a vaccine ever become available.

“The lower the disease prevalence, the smaller the affected landscape, and the lower the deer population, the higher percentage of the herd you’d be able to treat,” Samuel said. “The better we can manage this disease now, the better we’d be able to manage it if we ever find that silver bullet.”

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