Deer dispersal study offers CWD insight
University Park, Pa. — Between 2001 and 2005, when Duane Diefenbach was studying the dispersal of young white-tailed deer, he had no idea the research would prove useful in trying to contain an outbreak of chronic wasting disease in the Keystone State.
By 2008, when the results of the collaborative research project conducted by Penn State, the Pennsylvania Game Commission and the U.S. Geological Survey were published in an issue of Behavioral Ecology, it occurred to him that his work might have epidemiological implications.
The study, which involved 500 radio-collared deer from Centre and Armstrong counties that ended up in 10 other surrounding counties, was part of the Game Commission’s evaluation of changes to the state’s deer population resulting from antler restrictions aimed at allowing male deer to grow older.
The research took place before CWD showed up in New York, West Virginia and Maryland. The disease, which always is fatal to deer and elk, has persisted in the West for more than three decades. Wildlife biologists knew the malady slowly was marching east, but they had not yet been faced with the reality it had arrived here.
Recently the Game Commission announced that brain-tissue tests conducted on wild Pennsylvania deer taken by hunters last fall revealed three were infected with CWD. Those animals were killed in Bedford and Blair counties.
Last fall, tests revealed that two captive deer at a private game farm in Adams County had CWD. So, state wildlife officials now are focused on keeping the disease from spreading into other southern counties and to the Northern Tier.
In his four-year study, Diefenbach, adjunct associate professor of wildlife ecology in Penn State’s College of Agricultural Sciences and leader of the Pennsylvania Cooperative Fish and Wildlife Research Unit, documented deer dispersal behavior that provides insight into how far and how fast CWD could spread among wild deer.
“We learned that 70 percent of yearling males will disperse, and the average dispersal is six to seven miles,” he said. “Depending on the amount of forest, those yearling males may go just a mile or as far as 30 miles.”
In Pennsylvania, few young female deer disperse, Diefenbach noted, but when they do they usually go farther than the males – some much farther.
“On average, females go about 12 miles, but their behavior is different from males,” he said. “When males disperse, they go in one direction and are finished moving in 12 to 24 hours. When females disperse, they engage in strange, seemingly random movements – wandering over the landscape and often changing directions over weeks or months.”
Why all of this is important, he explained, is it gives deer managers like Game Commission biologists a solid idea how far CWD-causing prions might be carried by free-ranging deer.
Now, researchers at Penn State are using results from Diefenbach’s study to model how and where the disease will spread in Pennsylvania.
David Walter, adjunct associate professor of wildlife ecology and assistant leader of the Pennsylvania Cooperative Fish and Wildlife Research Unit, and Tyler Evans, a master’s degree candidate in Wildlife and Fisheries Science from Salem, Ohio, are collaborating with the Game Commission to conduct the research.
Walter is no stranger to chronic wasting disease – he studied its spread in the endemic region (where CWD originated) in Neb-raska, Colorado and Wyoming.
“We know now how deer move on the landscape, so we hope Dr. Walter’s research will help us make better predictions about how CWD will spread,” Diefenbach said.
“So far, most research has simply observed retrospectively where the disease shows up over time, but it is much more difficult to predict the future movement of disease. Our research defining how deer actually travel will make that much more possible.”
Scientists know that the disease spreads through the combination of an animal moving and either interacting directly with another animal, or contracting it indirectly through the environment.
Diefenbach’s research showed that deer in Pennsylvania are much more likely to disperse parallel to ridges than perpendicular to them. And larger, busy roads and rivers influence the direction they disperse and where they stop.
The research showed that in forested landscapes, deer did not disperse as far. So in areas with fragmented forest interspersed with fields and development, deer likely will move farther. Hence CWD would be expected to spread more quickly.
“Northcentral Pennsylvania is 90 percent forest, but we have other areas that are less than 30 percent forest – there is a huge variability in the amount of forest on the landscape,” he said.
“So, in parts of the state with less forest, the Game Commission may have to consider disease-management areas that are larger, and it also has implications on sampling efforts to try to get a handle on the prevalence of the disease.”