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Lofty 1,000-deer study indicates significant CWD impacts on populations

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Chronic wasting disease (CWD) has become a big topic in recent years as the disease has spread among the white-tailed deer population across the state and into the North Woods.

A new, in-depth study was recently released by the Wisconsin Department of Natural Resources (DNR) that looks at a large sample size of more than 1,000 captured adult deer and fawns as part of its Southwest Wisconsin CWD, Deer and Predator study.

While the study was performed in the southwestern part of the state, the information gathered will have significant importance for any portion of the state where CWD is found, both now and into the future. First discovered in 2001, CWD is now found in wild deer populations of at least 48 of the state’s 72 counties, including Vilas and Oneida. CWD is a prion disease that affects the brain and central nervous systems of deer, elk, moose and reindeer. It is a very contagious disease that is passed through saliva, feces, urine, and other deer-to-deer contact.

Staff at the DNR has been collecting data for the study since 2017. So far, that data has been analyzed in order to learn the estimated survival rates as a function of time, CWD infection status, age and sex, as well as document the breakdown of causes of mortality (i.e. starvation, disease, predation and hunting) and how those differ by CWD infection and status.

According to the DNR’s analysis of the study, the sheer size of the study has been unprecedented in size. It involved collaborations between the Wisconsin DNR’s Office of Applied Science, the USGS National Wildlife Health Center, the Wisconsin Wildlife Cooperative Research Unit, the UW-Madison Department of Forest & Wildlife Ecology, and the Montana Wildlife Cooperative Research Unit.

The sample size of this project is notable as well. While there have been previous research projects to estimate deer survival, relatively few have ever focused specifically on how CWD might affect it. Those that have were focused in the Western U.S. and had sample sizes ranging from 136 to 217 collared cervids. To put that into perspective, Wisconsin’s study deployed 1,089 collars on white-tailed deer, as well as additional coyote and bobcat capture.

“The model used to estimate survival is novel in that it can account for an animal’s changing disease status, such as deer that were CWD-negative at capture but later acquired the disease. It must also consider how survival differed over time and among sex and age groups,” the DNR’s analysis said.

“Before this study, survival models had never before incorporated all of these elements of disease, and to do so here required tremendous patience and innovation.”

Survival results

For this study, staff collared 766 deer with GPS collars, as well as 323 newborn fawns. Deer capture began in January, 2017, and concluded in March of 2020, with monitoring occurring since that time. While no new collars are being added, there are still deer with functional collars today, noted Daniel Storm, DNR Deer Research scientist.

With samples captured at the time of collaring and then at the time of death, Storm said combining that information with advanced statistical modeling, they are able to compare the annual survival rate of deer that did have CWD versus those that did not.

“We can take that information — the survival rate of our infected female versus uninfected females, then the prevalence of CWD in the population and recruitment, and we can put that into a population model which allows us to show how population growth changes with changing chronic wasting disease prevalence.”

Results showed the survival rate for uninfected does was about 83%, and for bucks it was 69%. In contrast, does that were infected with CWD had a 41% survival rate, and bucks had only a 17% survival rate.

“The primary point of interest with regards to how CWD is going to affect deer populations is going to be the extent to which CWD reduces the survival of adult female deer, because adult female deer survival is mostly what drives population. There are other factors that you must consider as well — which we did — such as recruitment and harvest,” Storm said.

Fawn survival in the study area was high enough to help sustain the population. Coyote predation was the highest source of mortality, followed by disease (pneumonia or enterocolitis), hunter harvest, and human-related causes (car collisions, haying equipment, domestic dogs).

“It is important to note that researchers do not expect CWD-affected deer herds to become extirpated (locally extinct). Deer populations have a strong ability to increase reproduction when deer abundance is lower, due to less competition for food and space,” the analysis notes.

Mortality effects on population

Storm said CWD indeed has a drastic effect on deer survival. “Infected female survival is half the survival of uninfected deer, and male survival is lower still. What we see is that CWD, through its mortality effect on females, immediately chips away at population growth potential of a deer herd, but at low prevalences we still have really robust growth potential.”

In short, CWD does substantially reduce deer survival and suppresses deer population growth rate. Where CWD prevalence is high, populations are likely declining.

“If prevalence continues to grow, the impacts of the disease are going to spread as well,” Storm said.

The southern landscape obviously has some differences between the northern portion of the state, and Storm was reluctant to speculate what these numbers mean, specifically for deer herds here. But one thing he was certain about is what happens with CWD — it grows. “So that’s today, but in the future, if you don’t do anything different where we have CWD, it’s going to get established and increase its prevalence, which is what always happens.”

Learning from CWD deaths

According to Storm, in order to learn what they could, the DNR collared a large number of deer, then tested them for CWD at capture and death in order to monitor how, when and where they died.

One of the ways the DNR was able to capture the deer was by using drop nets, Storm said.

Food was put out to attract deer to the net, then the net was dropped. Staff were then able to physically restrain them and chemically immobilize them to study and put on a GPS collar. Live tests were then done by rectal biopsy, which allows staff to get the CWD test before the deer is dead.

The collars also alert staff via email or text when the deer dies so they can have a veterinarian perform a mortality investigation. “We get a lot of information on those deaths,” Storm noted.

CWD can take down a healthy adult deer in as little as 18 months from infection to death.

“Deer can then very quickly lose tremendous amounts of condition in a very short time. They essentially die of starvation. You see it (starvation) in June, July, August, when there are no nutritional limitations — the disease itself causes that,” Storm said. “It’s a neurological disease that has all these cascading effects on the physiology of deer going down.”

According to the DNR’s analysis, CWD substantially reduces their survival rate and is closely associated with severe starvation and loss of bone marrow fat. Additionally, 51% of necropsied deer in the study had pneumonia, with bronchopneumonia more commonly found in CWD-positive deer. 

More intel to come

The results released by the DNR are only the primary findings of the Southwest Wisconsin CWD, Deer and Predator Study, and staff expects there to be more information gleaned as they work through the data collected to learn more about the deer herd and CWD’s impact on it.

Some areas of analysis are still underway, including the bobcat and coyote movement and habitat use, that will inform scientists which areas each species favors or avoids, as well as provide a better understanding of how predators fit into the bigger picture of deer survival.

Additionally, seasonal deer movement and habitat use studies will tell researchers how deer respond to seasonal changes in the environment and where there are likely to be higher rates of CWD transmission. The impact of CWD on deer movement will shed light on how deer movement changes once they are infected with CWD. The analysis will also look for sex, age, and seasonal patterns.

Finally, studies of deer contact rates will look for contact networks among deer and how CWD is likely being spread between them.

“These results will inform population management in currently infected areas and the agency’s future CWD management decisions,” the analysis says. “For areas currently seeing low or undetected CWD, this research may provide a glimpse of the future should CWD continue to spread as it has over the past 20-plus years.”

According to Jasmine Batten, DNR Wildlife Health supervisor, one of the biggest challenges with CWD is that it’s a very long-term game.

“With CWD, there are no easy answers and there is not just one answer. One thing we do know is that to affect CWD spread over time, it takes a multifaceted solution. There is not one silver bullet that can change the future of prevalence over time,” Batten said.

Chronic wasting disease, CWD, disease, north woods, white-tailed deer population, Wisconsin Department of Natural Resources, DNR, Southwest Wisconsin CWD, Deer and Predator study

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