Wisconsin biologists expect to soon complete the nation’s first and most-advanced system for estimating the past, current and future impacts of chronic wasting disease on white-tailed deer.
Creating the model wasn’t easy. For starters, the nation’s scientific community didn’t even know of the always-fatal disease until its 1967 discovery in a Colorado mule deer. And 25 years later, science couldn’t know how fast the disease would spread in Wisconsin when it became the first state east of the Mississippi River to verify CWD’s presence. That happened when three whitetails killed in November 2001 by Dane County hunters tested positive in February 2002.
And now, 57 years after discovering CWD, science is still untangling its nuances, like why it behaves differently by region, and even by township.
Wisconsin recorded a record 1,580 CWD cases in 2023, and so far 44 of the state’s 72 counties have documented at least one case in wild deer. But 13 of those CWD counties didn’t find a new case in 2023, while Iowa County added 243 cases to its No. 1 total of 1,714; and Richland County led the state with 381 new cases.
My, how things have changed. When the Department of Natural Resources used 40,116 tests to cover every Wisconsin county in 2002, only five counties found sick deer: Dane, 94; Iowa, 107; Sauk, two; Richland, one; and Walworth, one. In other words, of the 205 CWD cases found statewide, all but four (2%) came from Dane and Iowa counties. Adjacent Green, Rock and Columbia counties found their first cases in 2003, while Lafayette found its first in 2004 and Jefferson County followed in 2005.
Since then, CWD boomed in Dane, Iowa, Sauk and Richland counties, with infection rates ranging from 17% in Dane to 29% in Iowa County the past five years. From 2019 through 2023, Green County’s infection rate averaged 18.5%, with those 167 CWD cases accounting for 81% of Green’s 206 total cases since 2003.
In contrast, infection rates for 2019-2023 in neighboring Jefferson County was only 3%, while it hovered at 11% in Walworth and 12% in Rock counties.
Bryan Richards, the CWD project leader at the USGS’s national wildlife health center in Madison, said society’s land-use practices, more so than soil type/quality, probably affect CWD rates. The disease seems to spread gradually in marginal deer habitats, such as range dominated by small, scattered woodlots, sprawling agricultural lands, and developed areas with many people and relatively few deer. But CWD spreads faster and with higher infection rates in prime deer habitats, where the range features large, contiguous woodlots, smaller farm fields, fewer people and overabundant deer herds.
Iowa County, west of Madison, typifies those differences, with higher infection rates in the quality deer range north of Highways 18-151 than in the more open landscapes south of the four-lane corridor.
To better assess CWD’s myriad infection rates, the DNR launched its southwestern Wisconsin CWD, deer and predator study in 2017. This effort—the largest and longest-running deer study in state history—involved trapping, sedating, collaring and monitoring nearly 1,250 whitetails, bobcats and coyotes in Iowa County the past seven years.
Before releasing each animal, DNR crews also collected tissue samples to learn which deer already had CWD. The deer’s collars included GPS transmitters and mortality sensors so researchers could quickly find its remains, test for CWD, and conduct necropsies to determine if disease, predators, malnutrition or something else caused the deaths.
The study’s chief goal was to create an “integrated population model” for assessing and predicting how CWD affects deer herds. The DNR originally hoped to unveil this model two years ago after collecting and analyzing five years of data on the 810 adult deer, 323 fawns and 116 predators (coyotes and bobcats), including where they lived, ate, bedded, traveled and died.
Dan Storm, the DNR’s chief deer researcher, said Wisconsin has collected more CWD data the past two decades from hunter-killed deer than any state in the country. The challenge, however, is that much of that data came from hunters who wanted to learn if their deer was diseased. Little data came from scientific sampling that collected specific numbers of deer from designated blocks on a grid.
“Melding data from the collared deer with all the data from our hunters is a big technical challenge that’s never been done before,” Storm said. “There’s some ‘noise’ in that data, but we’re getting close to achieving our goals and creating a reliable model. I’m excited about it. I wish we had reached this point two years ago, but I’m confident we’ll be there by June.”
Storm said the study has revealed many interesting details. For instance …
-- Of the 323 fawns collared, the annual percentage reaching their first birthday ranged from 43% to 51%. Storm said that’s “reasonably high,” given that fawn survival rates across North America in recent decades ranged from 10% to 90%.
-- Predation was the No. 1 mortality factor, causing 31% of fawn deaths, with coyotes doing most of it. Other fatal factors were diseases (6%) like pneumonia and enterocolitis (inflamed intestines); hunters, 4%; other human-related causes, 4%, (vehicle collisions, domestic dogs and haying equipment); and starvation, 3%.
Storm was mildly surprised how few Iowa County fawns died from starvation, the biggest fawn killer in the DNR’s eastern farmlands study in Shawano and Waupaca counties a decade ago. In that study, 29% to 35% of fawns died before Sept. 1, primarily from starvation. Predators, mostly coyotes, caused about 8% of the fawn deaths.
“Both study areas were mostly farmland habitats with heavily browsed brush and woodlands, so the only real difference is that southwestern Wisconsin is slightly warmer,” Storm said.
-- In the DNR’s Northern Forest study a decade ago, predation was also the most common cause of mortality, with 35% to 60% of fawns dying before Sept. 1 each year. Most fawn predation was caused by black bears, followed by coyotes and bobcats.
-- Coyotes seldom killed or scavenged adult whitetails in the southwestern Wisconsin study. “I thought we’d see more predation on adult deer once they had CWD and were growing weak, but many ‘droopy droolers’ reached their end stage and died untouched,” Storm said. “I thought predation at that point would be substantial. Coyotes will kill prime-age deer. It happens. Why don’t they do it more often as CWD weakens deer? We don’t know. They don’t scavenge those deer much after they die, either.”
The DNR researchers will continue testing their new model in the weeks ahead as more data come in and more questions arise. “We tracked these deer for years, so we still have lots to learn about their habitat choices, food choices and travel routes,” Storm said. “Those things vary by season, age, gender and CWD’s impacts at different stages.”
So far, only nine CWD cases have been found in the state’s Northern forests, with Oneida County accounting for five. Some people try to attribute CWD’s absence up north to wolves, but Storm remains skeptical. “It’s probably beneficial, but to what extent, we really don’t know,” he said.
To read more about the DNR’s deer and CWD studies, visit https://dnr.wisconsin.gov/topic/research/projects/dpp/publications.
Above, a yearling buck pauses after being trapped, drugged, evaluated and fitted with a GPS collar in February 2018 as part of a long-term DNR research project.
— Patrick Durkin photos
Below, a 2.5-year-old buck walks off after DNR crews attached a GPS monitoring collar in February 2018.
Thank you for the infomative article and the good work you do trying to keep the public informed on these issues.