Smith: CWD-positive deer die at three times rate of non-diseased animals in Wisconsin study

Deer with chronic wasting disease died at three times the rate of uninfected animals, according to first-year results of a groundbreaking study of white-tailed deer in Wisconsin.

The research found a 75% annual mortality rate of CWD-positive deer compared to 25% in those without the disease.

The higher mortality of CWD-positive deer was attributed directly and indirectly to the disease, according to a Wisconsin Department of Natural Resources report.

Not only did some deer die from the prion disease, other sickened animals - which are less aware of their surroundings, less responsive to stimuli and less physically coordinated - become more susceptible to other causes of death, including hunting, predators and vehicle collisions.

The research, called the Southwest Wisconsin CWD, Deer and Predator Study, is the first to assess the impacts of the fatal disease in a dense population of whitetails in North America.

It is being conducted by the DNR in two study areas, primarily in Iowa County.

Initial results of the project, started in January 2017, were released Wednesday during a presentation to the Natural Resources Board in Madison.

Scheduled to run for five years, the project is examining factors that could impact deer survival and deer population growth in southern Wisconsin.

To perform the work, researchers randomly capture deer, take tissue and blood samples for CWD and other testing, attach a GPS-tracking collar and release the animal.

The deer are then monitored at least daily for movements. If a mortality signal is received, researchers travel to the site as quickly as possible in an effort to determine the cause of death.

The first year of results span January 2017 to January 2018. 

Researchers captured and collared 138 deer, 122 of which were successfully tested for CWD, said Dan Storm, DNR deer and elk research scientist.

Of the 122 whitetails tested, 12 (9.8%) were CWD-positive.

The CWD-positive animals included seven females (five adults and two yearlings) and five males (three yearlings and two juveniles).

For purposes of the study, adults are defined as age 2 and older, yearlings are less than 2 years old and juveniles are 8 to 12 months of age.

Nine of the CWD-positive deer died, including four directly from CWD, Storm said.

A complete list of causes of mortality will be completed in coming months, as several animals are still being evaluated. The Wisconsin Veterinary Diagnostic Laboratory in Madison is conducting all necropsies for the project.

Storm noted the sample size of infected animals (12)  was smaller than desired; the agency is conducting additional trapping and collaring efforts this winter and hopes to more than double the number of animals, including CWD-positive deer, in the study.

However, despite the small sample size, the difference in mortality rates of CWD-positive and CWD-negative was statistically significant, Storm said.

Chronic wasting disease is caused by a misfolded protein, or prion, and is fatal to deer, elk and moose. The disease is a transmissible spongiform encephalopathy (TSE), a family of diseases that includes scrapie, mad cow disease and Creutzfeldt-Jakob.

No case of human prion disease has been associated with CWD. However, health experts, including with the Centers for Disease Control and Prevention and Wisconsin Department of Health Services, recommend humans not consume meat from a CWD-positive animal.

CWD was first identified in 1967 in mule deer at a captive research facility in Colorado.

It slowly spread in the wild deer and elk populations in western states and jumped to other states, likely through transport of infected animals to captive facilities or movement of infected carcasses, according to many wildlife experts.

It has been detected in 25 states and two provinces of Canada.

It was first documented in Wisconsin in wild deer killed in November 2001 in eastern Iowa and western Dane counties.

After several years of efforts to contain CWD, Wisconsin officials adopted a policy of monitoring the spread of the disease. Other Midwestern states continue to employ special seasons, sharp-shooters or other measures in an effort to control CWD.

The disease has been found in 28 of the Wisconsin's 72 counties, either in wild deer or in captive cervids.

In 2017, 593 of 9,731 deer tested statewide were CWD-positive, including 584 of 5,436 in the southern farmland zone where the study is being conducted.

Among the recent CWD findings were 11 animals killed last fall at a Waupaca County shooting preserve after being trucked from a captive facility in Iowa County.

The Wisconsin Department of Agriculture, Trade and Consumer Protection continues to allow movement of deer from CWD endemic areas.

Wisconsin is now likely home to more CWD-positive deer than any other state or province, in the opinion of many wildlife managers.

CWD experts found the first-year study results meaningful and concerning.

"It's illuminating to see this work, in Wisconsin's deer-rich habitat, showing the high rates of mortality associated with the disease," said Bryan Richards, CWD project leader at the U.S. Geological Survey's National Wildlife Heath Center in Madison. "If anyone in Wisconsin questioned whether CWD killed deer, they now have proof."

Richards said it was likely the disease was already impacting herd growth.

"I find it virtually impossible that CWD is not depressing population performance in north-central Iowa County," Richards said.

The CWD-prevalence in bucks in parts of Iowa County is near 50%, according to DNR figures.

The Wisconsin results are similar to those in a recent study of mule deer in Wyoming. In that work, conducted from 2010-'14 and led by Melia DeVivo of the University of Wyoming, the mean annual survival rates of CWD-positive and CWD-negative deer were 32% and 76%, respectively.

Due to relatively low productivity in the Wyoming herd, researchers estimated CWD was contributing to a 21% annual population decline. In the absence of the disease, the population would be stable.

Recruitment rates are higher in Wisconsin, however, and Storm said it will take more years of study to determine the effect of CWD on the whitetail population in the Wisconsin project.

"This is our first glimpse at one puzzle piece, deer survival," Storm said. "We'll be adding more as move through the coming years."