Study finds deadly disease reduces deer survival, population growth in Wisconsin

Wisconsin’s largest study on chronic wasting disease shows the deadly disease substantially reduces deer survival and population growth, especially where it’s most widespread.

The Wisconsin Department of Natural Resources launched the study in 2017 by capturing more than 1,200 animals and collaring 1,089 white-tailed deer in northern Iowa, Dane and Grant counties. The state first discovered chronic wasting disease, or CWD, in that area in 2002. It has since become widespread. 

The study involved collecting rectal samples from live deer to test for CWD. Scientists analyzed data from both infected and healthy deer that were collared to compare annual survival rates.

DNR Research Scientist Dan Storm told the Natural Resources Board on Wednesday that results showed adult females or does that were infected with CWD had half the annual survival rate of healthy deer at 41 percent compared to 83 percent. For adult males or bucks, the survival rate was far lower among infected deer at 17 percent compared to 69 percent.

“CWD does substantially reduce deer survival and suppresses deer population growth (rates). Where CWD prevalence is high, your populations are likely declining,” Storm told the board.

CWD attacks the brains of deer and other animals, leading to drastic weight loss and eventually death. The disease spreads through contact with saliva, urine or feces of an infected deer. It can also be spread by deer who shed the prions, or misfolded protein, as they move from one location to another, or by hunters who unknowingly move infected deer.

Wisconsin is estimated to have more than 1.6 million deer, but they’re not evenly distributed across the state. Deer hunting contributes around $2.2 billion to the state’s economy.

Scientists sampled 766 adult deer and 323 fawns to arrive at their findings. Fawn survival rates ranged from 43 to 51 percent. The study mostly focused on does, which drive population growth. 

“What we see is that chronic wasting disease, through its mortality effect on adult females, immediately chips away at the population growth potential of a deer herd,” Storm said.

The results showed that deer populations are likely to decline in areas where the disease is found at prevalence rates of about 29 percent or higher. In some parts of southern Wisconsin, the prevalence of the disease is as high as 44 percent in does and as high as 55 percent in bucks.

Storm said rates as high as 30 percent are confined to southwestern Wisconsin counties, but he cautioned that CWD will eventually lead to declines in deer populations if the disease continues to spread statewide.

“They’re not going to go extinct, I don’t believe,” Storm said. “There are going to be fewer of them.”

The research involved more than 400 private landowners and thousands of volunteers. The DNR also collaborated with multiple state and federal partners. They include the University of Wisconsin-Madison, USGS National Wildlife Health Center, the Wisconsin Wildlife Cooperative Research Unit and Montana Wildlife Cooperative Research Unit.

Wisconsin now has at least 64 counties the DNR considers CWD-affected. Storm said the disease has been detected within the wild deer herd in at least 48 counties. DNR Wildlife Health Section Supervisor Jasmine Batten said CWD sampling yielded a lot of positive tests in the northern part of the state last year.

“We did have a few areas with new detection,” Batten said.

Chippewa, Manitowoc and Menominee counties all detected CWD for the first time within the wild deer herd. Batten said there’s no easy answer for managing the disease.

“In order to affect prevalence over time or spread over time, it takes a multi-faceted solution,” Batten said. “There’s not that one silver bullet.”

Board member Doug Cox shared concerns that regulation of deer farms is lacking. He said hundreds of positive cases have been detected at a deer farm near Menominee County. Data from the Wisconsin Department of Agriculture, Trade and Consumer Protection shows 842 farm-raised deer have tested positive for CWD in neighboring Oconto County, the most of any county host to deer farms.

“This probably isn’t new to the public when this happens in our counties,” Cox said. “But I think from our standpoint on the board, it warrants us to start talking about it more actively.”

In 2018, Republican former Gov. Scott Walker proposed emergency rules that would’ve included a requirement for double-fencing of deer farms. The proposals drew pushback from the deer farm industry and some GOP lawmakers. Rules that would have required double-fencing or limited movement of deer killed in CWD-affected areas were either scrapped or allowed to expire.

Board member Marcy West also encouraged more research and testing for CWD.

Democratic Gov. Tony Evers proposed six positions for the state’s Veterinary Diagnostic Lab to improve capacity for CWD testing under the 2023-25 state budget. He also proposed setting aside $1 million for one-time grants to purchase deer carcass disposal sites to limit the disease’s spread. Republican lawmakers stripped those items from the budget.

Since 2002, the DNR has sampled more than 333,000 deer. Of them, more than 14,000 have tested positive for the disease.