Frequently Asked Questions

Data Collection

How many deer were captured for this study?

810 adult deer and 323 fawns.

How does this compare to most sample sizes for other research projects?

Many research projects have estimated deer survival. However, relatively few have focused specifically on how CWD affects survival. A study in Wyoming collared 175 white-tailed deer (84 females, 91 males) over 7 years. Similar studies with mule deer and elk have had sample sizes between 136 and 217 collared individuals.

Why collar and track predators like bobcats and coyotes, since they aren't specifically listed in the primary objectives?

The original intent of the bobcat and coyote work was to inform how bobcats' and coyotes' movements and habitat preferences might affect the mortality risk of white-tailed deer. Collaring coyotes gave us further insight into coyote survival and cause-specific mortality.

Why use different collars for adult deer (GPS) vs. fawns (VHF)?

The GPS collars used for bucks and does are heavier and designed to send automatic location updates to researchers between 2 and 24 times per day. The VHF collars used for fawns required researchers to use telemetry equipment to manually check location once per day. However, these collars are lighter and cheaper and designed to expand as the fawn grows, eventually releasing after 18 months.

How did you track CWD infection?

We can estimate this based on data from our CWD surveillance program and from pre- and post-death testing of deer in the study. More background about the study's data collection methods can be found on the project's Phase 1 and Phase 2 webpages.

Results

Based on this study in Southwest Wisconsin, what is the threshold for CWD prevalence where the deer population begins to decline?

When the CWD prevalence of females surpasses about 29%, deer populations are expected to decline. Depending on local population recruitment and adult deer survival rates, this threshold will vary across the state. This does not mean that CWD does not affect deer survival or abundance until prevalence reaches 29%. CWD reduces deer survival and population growth at lower densities, although the general expectation is that a deer population could still grow at prevalences lower than 29%.

What was the CWD prevalence in the study area?

CWD prevalence estimates come from Wisconsin DNR's surveillance data.

How is CWD changing how deer die?

For female deer that don't have CWD, hunting and vehicle collisions comprise about 75% of mortalities, typical for rural midwestern deer populations. Other sources of mortality for uninfected female deer include bacterial infections such as pneumonia and coyote predation; however, these are relatively insignificant.

Deer infected with CWD have a fundamentally different mortality composition. Over half of CWD-infected female deer mortalities were from end-stage CWD, which often includes starvation and bacterial infections. What's unique about starvation among CWD-infected deer is that it occurs during times when there is plenty of food (i.e., summer and fall). Hunting and coyote predation were the 2nd and 3rd most frequent causes of mortality, respectively, among CWD-infected deer.

More information on cause-specific mortality will be provided soon. Also, see the fawn survival results on the project webpage for more information on sources of mortality.

Could this CWD prevalence threshold change over time?

As deer abundance declines due to the effects of CWD, we anticipate higher recruitment (the number of individuals being added to a population in a given time frame). This is due to reduced competition for food, which has a presumed positive impact on pregnancy rates and fawn survival. While we do expect this to happen, exactly to what extent is unknown.

Are there deer management practices that can prevent the decline of deer populations due to CWD?

At high CWD prevalences, management options are limited. One option would be to reduce antlerless harvest; however, we believe this is unlikely to have long-term effectiveness for several reasons. One, reducing harvest will increase the number of deer succumbing to CWD; thus, the overall mortality rate will not be as affected as expected. Two, antlerless harvest is already much lower than in years past (Iowa County Harvest Trends). Third, reducing harvest will likely worsen CWD transmission because infected deer are not removed from the landscape.

How does a relatively high recruitment rate affect how CWD impacts deer populations?

Wisconsin has a higher deer herd productivity than western deer populations; thus, our deer herd is comparatively more robust. However, our research does show that at high prevalences, mortality from CWD can exceed the number of new deer recruited into the population each year.

Is there any interaction between epizootic hemorrhagic disease (EHD) and CWD?

EHD outbreaks have been observed in Wisconsin; however, they have not been of the size and severity seen in other states. Therefore, it remains unclear if EHD will affect CWD prevalence. 

Did this study determine the role predators play in the spread and/or reduction of CWD?

Predation is not a primary source of mortality for infected or uninfected adult deer. Additional analyses are underway, which utilize the data gathered from the Southwest Study's predator collars. Two of these analyses include documenting the home range size and habitat use of bobcats and coyotes, as well as their estimated survival and cause-specific mortality. In time, the results from these analyses could help paint a bigger picture of how predators might be contributing to the spread of CWD. View results on coyote survival and cause-specific mortality.

How long will it take for a deer herd in a DMU to reach the prevalence threshold?

This is difficult to predict and depends on several factors, including hunter-harvest rates, recruitment rates, disease transmission and habitat conditions. However, results from this study confirm that if left unchecked, CWD prevalence rates will likely rise and eventually impact overall deer population growth in the local area. 

Once prevalence reaches the threshold, how quickly will the population decline?

This is difficult to predict and mainly depends on local population recruitment, adult deer survival rates and hunter-harvest rates.

Looking Forward

Will the Wisconsin white-tailed deer population become endangered due to CWD?

It's not likely due to high recruitment (the number of fawns surviving long enough to be added to the population each fall). However, the overall population's health will be severely affected, as will the number of deer available for harvest by hunters in areas of the state with high prevalence. High CWD prevalence herds in the western U.S., although severely impacted by CWD, have not become extinct.

Do we know the CWD prevalence in any areas or counties outside of this study? Have any reached the threshold already?

CWD prevalence is estimated annually for several areas within Wisconsin; these results can be found on the Wisconsin DNR's CWD prevalence dashboard.

Should predators be reduced to help the deer population?

Predator removal/reduction, when implemented as a large-scale management action such as with ungulates or gamebirds, has proven ineffective in the past. In the case of this study, fawn recruitment (the number of fawns surviving long enough to be added to the population each fall) is already relatively high. Coyote harvest is also currently quite high, with nearly ½ of coyotes harvested yearly. Thus, it is unclear whether coyote harvest could be meaningfully increased further.

What is being done to prevent other areas of the state from experiencing the prevalence rates we see in the southwest?

The department's current approach to managing CWD in Wisconsin can be found in the CWD Response Plan.

Is CWD responsible for the decline in hunter harvest?

Chronic wasting disease is not the sole reason for declines in hunter harvest; social trends, such as fewer hunters, also contribute to these declines. 

What should we do to prevent the spread of CWD?

Read DNR guidance on reducing the spread of CWD.

What follow-up research is still ongoing?

The scope of this study provides us with a rich dataset from which we can continue to learn about our deer herd and CWD's impact. As such, there will be more results moving forward, and information will continue to be released to the public as it becomes available. Current ongoing analyses are at the bottom of our Phase 2 webpage under "Additional Analyses."

Will this information be used to inform the CDACs?

Because County Deer Advisory Councils (CDACs) work with local department staff to develop population-objective recommendations and harvest quotas, the results of this study will be shared and considered during future decision-making processes.

Why release this information now?

A key focus of the DNR's mission is "to protect and enhance our natural resources," a large part of which relies on working with and maintaining the public's trust. By releasing these results, we aim to ensure timely transparency and that all Wisconsinites are privy to information that could inform future decision-making processes of which the public is a part.