CWD’s long-term effects studied
St. Paul — Researchers for the first time have shown that declines in free-ranging deer populations occur as a result of high prevalence of chronic wasting disease.
The eight-year study, conducted in east-central Wyoming, showed a 10.4-percent annual decline. The results were published in the journal PLOS ONE.
“The decline was caused directly by CWD lowering annual survival of female deer, which have the biggest impact on population growth rates,” said David Edmunds, the recent University of Wyoming Ph.D. graduate student who led the study. “This was because CWD-positive deer died both directly from the disease and were more likely to be killed by hunters than CWD-negative deer.”
While researchers know deer, elk, and moose that contract CWD invariably die, this study marks the first time they’ve been able to say at what rate the population actually declines.
“We show that a chronic disease that becomes endemic in wildlife populations has the potential to be population-limiting, and the strong population-level effects of CWD suggest affected populations are not sustainable at high disease prevalence under current harvest levels,” according to the abstract of the study.
During the study, which took place from January 2003 to February 2010, researchers captured 112 deer as fawns and 63 as adults. They recaptured each deer every year. During the study period, there were 118 mortalities.
Of those animals, 50 had CWD, 64 did not, and it was unknown whether the remaining four had the disease.
“The difference in survival by CWD-status and the high proportion of CWD-positive deer in this population help explain the declining population trend,” according to the study. “The CWD-positive deer were 4.5 times more likely to die annually than CWD-negative deer. These results support concerns of wildlife managers, wildlife disease experts, and conservationists that this endemic (chronic) disease can negatively impact deer population sustainability at high disease prevalence.”
Lou Cornicelli, DNR wildlife research manager, said the results of the study – that CWD has a population-level effect on deer – are fairly obvious, but that it’s the first time the level of decline has been quantified.
“We didn’t know the long-term effect of CWD on deer populations,” he said. “We hadn’t been able to truly quantify it.”
A couple of reasons for that, Cornicelli said, are time and the availability of technology.
“It takes a long time to detect trends in populations,” he said.
But even then, researchers have to have the ability to determine when an animal dies and what killed it.
“Being able to get on the animal and determine definitively what killed it was really difficult until about 10 years ago,” Cornicelli said.
The Minnesota DNR has been aggressive in trying to ensure CWD doesn’t get a foothold in the state. When a hunter in the fall of 2010 killed a CWD-positive deer near Pine Island in southeastern Minnesota, the agency quickly moved to reduce deer densities in the area. To date, that animal remains the lone wild one to ever test positive in Minnesota.
The agency’s goal is to stamp out the disease before it becomes prevalent in the population.
“We hammer it with the hopes that we get the last one,” Cornicelli said. “One clear (management application of the study) is that we want to do whatever we can to not be in this position. That’s the take-home message. That’s why we do what we do, even though it’s sometimes unpopular. We don’t want to be in a position where we are talking about a 20 percent prevalence rate in male white-tailed deer in Minnesota.”
Even if the deer population declines at 10 percent a year, when there are high densities to begin with it may take hunters a long time to notice the effects of CWD.
But, Cornicelli asks, what happens when in 100 years, for example, the population has dropped from 65 animals per square mile to 10 per square mile?
Too, managing in the face of CWD generally puts managers at odds with hunters, who want to see more deer and older deer on the landscape.
“You manage for a lower density and a population that’s much younger,” Cornicelli said. “Even though the prevalence is going to be high, those deer are going to be killed before they are symptomatic of the disease.”
After not conducting CWD surveillance in the southeast in 2015, the DNR will conduct it again this year, the result of disease discoveries in Iowa and Wisconsin. While most of the testing will occur during the firearms season, which opens Nov. 5, the DNR also is partnering with some taxidermists in that part of the state to collect samples beginning with Saturday’s archery season opener.
“We don’t want to deal with it, but if it’s out there, we want to find it,” Cornicelli said. “And if we find it, we want to find it early so we can do something about it.”
There are taxidermists who have gone through training – or currently are going through it – so they can collect samples from deer that hunters bring in to have mounted. The taxidermists will collect lymph nodes while they’re caping deer, and DNR officials will pick up the nodes for testing.
While Cornicelli doesn’t expect to get a large number of samples from archery hunters, the ones the DNR does receive likely will be the right ones.
“A good chunk of our archery harvest in the southeast is mature males,” he said. “It’s not going to be a lot of deer, but it’s going to be the right type of deer.”