Southwest Wisconsin CWD study shows disease creates higher annual mortality probability

Earlier this month, Department of Natural Resources (DNR) deer research scientist Dan Storm spoke with the Natural Resources Board (NRB) about the results of the recently released Southwest Wisconsin CWD, Deer and Predator Research Study.

This study showed drastic effects of Chronic Wasting Disease (CWD) on decreasing annual survival rates, he explained.

CWD was first detected in Wisconsin in 2001. Since then, it has been detected in wild deer in at least 48 counties in the state, Storm said. In the highest prevalence ares, the prevalence of CWD has been seen up to 44% in adult female deer and 55% in adult males. 

Prevalence, of course, is highly variable across the landscape, however. 

Storm said the primary point of interest regarding how CWD will affect deer populations is the extent to which it affects adult, female deer and how the disease reduces the survival of those deer.

 Survival of adult female deer is the largest driver of deer population numbers, he explained.

The department’s approach to looking at this issue was to initially collar a large sample size of deer. When those deer were collared, a sample was taken to allow each of those animals to be tested for CWD at the time of capture. 

Each individual deer was then followed through time to see what happened to them including whether or not they died, when they died and how they died. Researchers would then take another CWD test at death. 

From that data, researchers were able to find the annual survival rate of deer, as a function of chronic wasting disease, he said. 

Basically, they were then able to compare the annual survival rate of deer that did have the disease and compare that to those that did not have the disease.

All of the data from both CWD-positive and CWD-negative deer was then put into a model, he said.

 That allowed researchers to show how population growth changed with changing CWD prevalence. 

Deer capture and collaring starting in 2017 and continued until March 2020, with that activity centering in northern Iowa County. The collared deer have been continually monitored since that time.

Twenty-six field staff and five full-time staff worked on the project annually and approximately 400 cooperating private land owners were also involved.

There were also thousands of fawn captures done by volunteers.

In all, Storm said, over 1,000 deer were collared. Seven hundred and sixty-six of those were GPS collared deer and 323 were newborn deer. The center of collaring activity was in the endemic area.

When results were tabulated, the annual survival rate of female deer that were not infected with CWD was approximately 83%.

For uninfected males, the annual survival rate was approximately 69%. When compared with annual survival rate of deer infected with CWD, the study showed there was a drastic effect of CWD on deer survival.

Annual survival rates for CWD-infected female deer was approximately 41%, which is half of the uninfected rate. 

For males mortality rates were much higher, with annual survival rates being only approximately 17%.

Storm said the research team took the survival information and prevalence and recruitment information and put it into a population model in order to see population growth across a gradient of prevalence. 

As one might expect, the model showed that, as prevalence increased, population growth stopped and then started to decline. At approximately 29% prevalence, the model showed that population growth would stop and deer numbers would start to decline.

 There are areas in Wisconsin, he said, where prevalence is higher than 30%, which would mean declining numbers. He did note, however, that there are always local factors at play. 

Some areas could have higher or lower recruitment numbers, just as some areas could have higher or lower harvest rates. While there may be variation in the actual outcomes on the landscape, the study showed that the general expectation would be for population growth to decline as prevalence of CWD increases, with 29-30% prevalence being the tipping point. 

While the impacts of the disease are being seen in the endemic area, as spread and prevalence continue to grow, he said, those effects will likely be seen in other areas. 

More information about CWD as well as this study, those interested in learning more can see the DNR website at dnr.wi.gov by inputting “CWD” or “Southwest Wisconsin CWD study” in the search box.

Beckie Gaskill may be reached via email at [email protected].