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Sunday, February 9th, 2025

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Sportsmen Since 1968

Patrick Durkin: CWD is sinking deer herds in disease areas

According to Wisconsin DNR research, a healthy buck’s annual survival chances are 69% in southwestern Wisconsin, while a CWD-infected buck has only a 17% chance of being alive a year later. The 3-year-old Richland County buck, left, tested positive for CWD in 2020. The yearling buck skeleton couldn’t be tested for CWD, but was found in 2021 on the same infected farm. (Photos by Patrick Durkin)

If you’re seeing too few deer in southwestern Wisconsin for your hunting or viewing pleasure, it’s time to accept the obvious reason: The culprit is chronic wasting disease (CWD), the always fatal prion disease that now kills more female deer in highly contaminated areas than hunters kill with bullets and arrows. Roughly speaking, that’s much of Iowa, Sauk, Richland, and western Dane counties.

The DNR confirmed that fact for the first time Jan. 22 when releasing the latest findings of its long-running $5 million study into how CWD affects deer populations. The study found that once CWD infects 29% or more of an area’s female deer, the herd starts declining as more deer die each year than can be replaced by newborn fawns.

As Jasmine Batten, DNR wildlife health section supervisor, emailed her staff, “CWD mortality has largely replaced antlerless harvest as the primary driver of the deer population’s trajectory in the CWD endemic area (west of Madison).”

Neither Batten nor the agency rushed to that conclusion. The DNR launched its Southwest Wisconsin CWD, Deer and Predator Study in autumn 2016, and then caught, tested, and fitted GPS collars to 1,249 animals over the next four years across northeastern Grant County, northern Iowa County and northwestern Dane County.

The agency then monitored those 810 adult deer, 323 fawns, 69 coyotes and 47 bobcats to learn where they lived, when and where they moved, and when/how they died. When a collar signaled the animal’s death, researchers hurried in, hoping to learn what killed it.

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The DNR says this ongoing study is the “largest and most comprehensive deer research project ever undertaken in Wisconsin.” Although the data will provide more findings, this fact won’t change under current hunting regulations: Other than two-legged hunters targeting bucks, CWD has no deer-killing equal once it’s widespread.

“We can now say it’s not EHD (epizootic hemorrhagic disease), it’s not coyotes, it’s not bobcats, and it’s not earn-a-buck regulations from 15 years ago that are causing the herd declines we’re seeing,” said Dan Storm, the study’s chief researcher. “CWD is the cause. We have solid evidence to back it up. This is what’s going on, and so let’s proceed with what to do about it.”

The study found that a healthy, uninfected doe age 1 or older is twice as likely to be alive a year later than a CWD-infected doe. A healthy doe’s annual survival chances are 83%, while an infected doe’s chances are 41%. CWD-infected bucks age 1 and older fare four times worse than healthy bucks. A healthy buck’s annual survival chances are 69%; an infected buck’s chances are 17%.

CWD-infected deer more often get hit by vehicles, shot by hunters, and killed by starvation and pneumonia. In fact, 51% of dead deer necropsied in the study had pneumonia. Further, preliminary summaries show end-stage wasting – which includes infections and starvation – is the No. 1 cause of death (57%) for CWD-positive adult does. Sick does more often reach that stage than sick bucks, given hunters’ focus on antlers after lawmakers eliminated earn-a-buck regulations in 2011.

So yes, contrary to endemic social-media nonsense, CWD kills deer. In fact, as the cause of death for 57% of infected does, it outpaces the next three causes: hunting, coyotes, and unknown causes. For healthy, CWD-free does, hunting and vehicle collisions caused 75% of deaths. Bacterial infections, coyotes, and unknown causes killed the other 25%.

Fawn survival in the study was high enough to sustain deer herds. Of the study’s 323 fawns, the annual percentage reaching age 1 ranged from 43% to 51%. Fawn survival rates across North America in recent decades range from 10% to 90%.

The study found that predators (mainly coyotes, but also bobcats) kill about 31% of the annual fawn “crop,” while diseases like pneumonia, EHD and enterocolitis (inflamed intestines) kill 6%; hunters, 4%; human-related causes (vehicles, pet dogs and haying/mowing/brush-hogging), 4%; and starvation, 3%.

Skeptics, of course, ignore CWD while blaming predators and EHD for declining herds. Though they clamor for other hunters to quit shooting antlerless deer, no legitimate biologist supports passivity.

“We already did that and look how it went,” Storm said. “Before we lost earn-a-buck (in 2011), hunters dropped Iowa County’s deer herd below 20,000. After earn-a-buck, the herd took 7%, 10%, and 12% annual increases until 2020. That herd should have kept growing, but it didn’t. CWD is pulling it down.”

The DNR’s annual post-hunt population estimates show Iowa County’s herd rose 51.3% from 16,900 in 2011 to 25,566, the 2018-20 three-year average. The herd has since fallen 15.25% to 21,666, the 2021-23 three-year average.

Bryan Richards, CWD project leader at the U.S. Geological Survey’s (USGS) National Wildlife Center in Madison, said backing off would backfire.

“You won’t recover a population by letting CWD run its course,” Richards said. “When you try to stockpile deer by not shooting, you protect sick deer, too. Contamination worsens and the healthy proportion of the herd declines. Shooting removes sick deer from the herd sooner than CWD will. They’ll spread fewer prions over time, and you’ll probably shoot them before CWD reaches its worst stages for shedding prions.”

Storm put it this way: “The more CWD you have in your area, the more the herd will decline.”

This map shows the CWD infection rates by township (6 miles by 6 miles) for all deer tested during the 2024 hunting seasons. (Map courtesy of the Wisconsin DNR)

Which areas already exceed 29% infection rates for adult does? The latest DNR data from a year ago shows southeastern Richland County on the edge at 27%, northwestern Iowa County at 35%, and the Devil’s Lake area in eastern Sauk County at 34%.

Further, CWD testing of hunter-killed deer in autumn 2024 shows overall (bucks and does) detection rates at or above 29% in six townships (6-mile by 6-mile areas) in Columbia County, three townships in Dane County, eight townships in Iowa County, 11 townships in Richland County, and 15 townships in Sauk County.

How low will deer populations drop where CWD is endemic? Storm said CWD won’t exterminate deer, but no one can predict how it will affect specific valleys, woodlands or watersheds. CWD has spread at varying rates in different Wisconsin habitats, and appears to have leveled off at high infection rates in some areas while still rising and spreading in others.

The disease has so far been verified in wild deer in 48 of Wisconsin’s 72 counties, even though testing has been totally voluntary for years. During the 2024 hunting season, 1,755 more deer tested positive for CWD across the state, a record 10.4% detection rate despite the least amount of samples (16,939) volunteered since 2017. Richland County hunters provided the most samples, 1,335, in 2024, and 444 (33.4%) had CWD.

Contact Patrick Durkin at patrickdurkin56@gmail.com.

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