MADISON – Researchers continuing to examine the results of the chronic wasting disease (CWD) sampling effort during the 2002 Wisconsin hunting seasons report that the data indicates older bucks have a higher prevalence of the fatal brain disease and that the disease is not uniformly distributed within the infected area.
“We examined the data from nearly 2,000 adult deer within the area where CWD is most prevalent. For yearling deer, we found chronic wasting disease at equal levels in male and female yearlings, 3.2 percent and 3.6 percent, respectively,” said Dr. Mike Samuel, lead CWD researcher with the U.S. Geological Survey (USGS) Cooperative Wildlife Research Center, housed at the University of Wisconsin. “But when the deer in our sample reach three years of age, males are showing double the prevalence — 16.3 percent in males compared to 8.1 percent in the females. This is similar to information reported from Colorado earlier this year.”
As for distribution of the disease across the landscape, testing of the deer removed from the intensive harvest zone last year show a core area in Dane and Iowa counties where the disease was found in about 7 percent of the deer.
“This kind of concentrated disease pattern is consistent with our understanding about chronic wasting disease and what we know about the home range and social behavior of white-tailed deer,” Dr. Samuel said. “It is not surprising to find a disease concentration given the social nature of deer. Studies have found female white-tailed deer often occupy about a 1-square mile area during their entire life and tend to live near related females. Once a disease gets started, the infection would tend to spread outward somewhat similar to ripples on a pond.”
Tom Hauge, director of the DNR Bureau of Wildlife Management, said the results suggest several possibilities about how the disease is transmitted that, with further analysis, may help identify new ways to manage the disease.
“As DNR Secretary Hassett voiced before a national review panel earlier this spring, we intend to ‘learn and adapt’ our CWD control efforts as new information points the way”, Hauge said. “The identification of the high CWD concentration area will help us communicate with the landowners and hunters in that area. Wisconsin will not be successful in controlling this disease without their help.”
Dr. Samuel said that researches on the CWD Taskforce had expected that the prevalence of the disease would increase with age in both sexes because older animals have been exposed for a longer period of time. The USGS Cooperative Wildlife Research Center is one of several CWD Taskforce partners, which also includes the USGS National Wildlife Health Center in Madison, sharing in new research on CWD.
“What is interesting in these results is the increasing prevalence of infection found in adult males compared to females, indicating that males have a higher risk of infection than females,” Dr. Samuel said.
Hauge said the CWD Taskforce has several research efforts underway or about to start up that look at the transmission question.
“We have more to learn about what factors cause the increased infection in males,” Hauge said. “For instance, if environmental transmission plays a role in spread of the disease, then it follows that males, who have larger home ranges than females, would logically experience more exposure. Another scenario that needs more investigation is the potential of females transmitting CWD to males during the breeding season. Once again, males cover a lot of ground during the breeding season and contact many different females physically as well as through urine scenting or other behaviors. A third possibility might be male-to-male transmission in the bachelor groups that form in late winter to mid summer.”
While the data indicates that older bucks are more likely to be infected than does, Hauge said, researches do not think that killing just all the older mature bucks would solve the problem.
“CWD is also present in other, more common, segments of the population. From a disease management viewpoint, the priority is to reduce the overall population numbers and reduce the opportunity for animal to animal contact. Landowners and hunters can really help control this disease by maximizing the harvest of does on their lands in the infected area,” Hauge said.
Researchers are also interested in learning more about both female-to-female and male-female transmission. Samuel notes that female deer form genetically-related matrilineal groups and these groups tend to exclude female deer from other matrilineal groups. Interaction between related females within a social group is much more intense than between individuals from unrelated groups.
Multi-agency research studies involving the DNR, USGS, and University of Wisconsin-Madison have already been initiated to better understand how CWD might be transmitted within and among related groups of females and among males and females during the breeding season.
“We have a large and valuable database from last year’s surveillance effort thanks to the great cooperation of landowners and hunters. We plan to use it to learn as much as we can about CWD,” Hauge said.