Confirmed: CWD Reduces Deer Population
October 11, 2016
Chronic wasting disease was first discovered in 1967 in captive mule deer in Colorado. It has since been detected in wild and free-ranging whitetails, mule deer, moose and elk in 23 states and two Canadian provinces.
The always-fatal neurological disease was recently discovered in wild reindeer in Norway, the first known case in free-ranging ungulates outside North America. There is no known cure and researchers determined CWD is currently impossible to eradicate from the environment, creating a potential management nightmare for wildlife biologists.
A new study confirms what many biologists suspected: Chronic wasting disease is leading to measurable declines in deer populations.
Research led by recent University of Wyoming doctoral research graduate and University of Wyoming veterinary science professor Dr. Todd Cornish found that the disease is contributing to a 10 percent annual decline in a whitetail herd in southeast Wyoming.
The study tracked 175 whitetails, including 63 captured as adults and 112 captured as fawns, between 2003 and 2010. Each deer was recaptured annually and tested for CWD. By the end of the study, 46 had been killed by hunters, 20 died from capture-related stress or injuries and 17 perished from CWD. An additional 35 died from accidents, other diseases, predation and unknown causes.
It found that mortality rates were 4.5 times higher for all CWD-positive deer than for CWD-negative deer. What's more, females that contracted CWD lived longer than bucks, and bucks that tested positive were more likely to be killed by hunters than bucks that tested negative.
"They may be less aware of their surroundings and have a decreased ability to avoid hunters, even in the early stages of the disease," he says, adding that many hunters were surprised to learn that the bucks they killed tested positive. "The deer they killed had none of the clinical signs of the disease, so they weren't targeting CWD-positive deer. They didn't know."
It can take a year or more before infected animals show any obvious physical effects, but most infected deer perish within 18 months.
What was most surprising to Edmunds, though, is that overall infection rates were higher for females than for males, something contrary to test results in other regions with relatively high infection rates.
"It could because of the habitat. The whitetails in the study area spend most of their lives in linear riparian habitat and therefore have smaller home ranges," says Edmunds. "Since does tend to be more social than bucks, they may be spreading the disease among each other faster."
It's not that infected does won't bear fawns. Edmunds found no correlation between the disease and pregnancy rates. However, females drive populations. As their numbers decline, so will the number of fawns entering the population each year.
"It can take a year or more before infected animals show any obvious physical effects, but most infected deer perish within 18 months."
That's what seemed to be going on in the study herd. What's more, CWD, combined with hunter harvest, has shifted the studied population's age structure to younger deer.
"We found that the population was unable to sustain itself through normal recruitment once CWD infection rates reached about 27 percent," says Edmunds. "The overall prevalence in our study was 35 percent for all deer and 42 percent for does."
So far, it's the only study that has confirmed an actual population decline in whitetails as a result of CWD. However, a recent study that examined the disease in mule deer found that it accounted for a 19 percent annual decline in mule deer in the study area.
The five-year project, conducted at the University of Wyoming in east-central Wyoming, determined that the region's mule deer decreased as a direct result of CWD and that the herd could go extinct in 41 years. Edmunds determined that whitetails in his study area could disappear in less than 50 years.
"There's really no way they would actually go extinct because new deer would likely move in from surrounding areas. It's not a closed population. However, we use that word to show the magnitude of the population decline," he explains. "It will likely lead to an unsustainable population where hunting will have to be reduced significantly."
So far, though, that hasn't happened. Officials with Wyoming Game and Fish are taking a wait-and-see approach before making any management decisions.
Edmunds conducted additional research, this time examining genetics and CWD. It appears as though some animals are less likely to contract the disease, although he has yet to find any animal that is actually immune to it. They may be genetically wired to live longer before succumbing the clinical symptoms. That study will be released soon.
Researchers in Wisconsin are also in the early stages of a study similar to Edmunds'. They want to know how much of an impact the disease is having on the population where it has become more common.
CWD prevalence rates in some regions of Wisconsin have gradually increased since it was first detected there in 2002. Testing in 2002 found about 8 percent of adult male whitetails had CWD. More recent testing determined that upwards of 30 percent are infected. Similar trends in those same regions in Wisconsin have been found in other age classes and in females.
While Edmunds' study shows what's going on, it doesn't offer any solutions. That's because there aren't any, he says.
"There have been cases where prevalence was stabilized through culling efforts, but those were only successful in regions where prevalence was already low. It hasn't worked where rates of infection are already high," explains Edmunds. "The best option is to protect non-infected herds."