MADISON – Testing of deer shot last year within the “core area” of chronic wasting disease (CWD) infection in southwest Wisconsin showed an increase in the rate of infection or disease prevalence.

Prevalence is the proportion of animals that test positive for CWD, the always fatal nervous system disease of white-tailed deer and other members of the cervid family.

In 2008, “estimates of prevalence (based on deer tested) in the South Central Wisconsin core area of infection showed increases in yearling and adult males,” said CWD project leader Davin Lopez.

The prevalence rate for adult bucks (2.5 years and older) in the western core area, which covers mostly western Dane County and eastern Iowa County, went from 10 percent in 2007 to 15.5 percent in 2008. The prevalence for yearling bucks went from 3 percent to 6 percent.

Recent studies of regional deer populations in Colorado and Wyoming — states where CWD likely has infected wild deer for several decades — are documenting high prevalence rates (20 to 40 percent) and lower survival of CWD-infected deer when compared to other deer in the populations. Authors of these studies suggest that CWD may be limiting deer numbers in these populations, according to Lopez.

“Five to ten years in the future, we will know better whether this was just a one year blip on the chart or the beginning of a trend of increasing disease prevalence in Wisconsin,” added population ecologist Robert Rolley.

Infection Rates
Epidemiological analyses of the prevalence data from the western core were conducted by Dennis Heisey of the U.S. Geological Survey-National Wildlife Health Center, Madison, and colleagues. Sophisticated statistical techniques that adjust for factors such as age and sex were used to estimate infection rates, more technically referred to as the force-of-infection.

They noted that the infection rates appear to show substantial random variability from one year to the next, but that there is evidence of a general underlying trend of increase at about 4 percent per year.

The statistical techniques also observed infection clusters and depended on age and sex. Their analyses suggested that the cause for the clustering is due primarily to when the disease arrives in an area.

Since 2002, DNR has analyzed almost 152,000 deer with a total of 1,172 free-ranging deer testing positive for CWD. All the positive deer were found within the CWD-MZ. Wisconsin has two separate epicenters of disease, one in the southwest part of the state, one in the southeast. The southeast CWD area is contiguous with a CWD area in northern Illinois where 256 CWD positive deer have been found since 2002.

In it for the Long Haul
“After seven years, it’s clear that there’s no easy answer to managing CWD, but we continue to believe that the stakes are high, it’s a statewide issue, and we take seriously our responsibility to manage the disease,” emphasized Lopez.

“We need to minimize the extent and spread of the disease in our treasured deer herd. Science tells us the only practical tool to do that is to reduce deer density and, therefore, deer to deer contact. To make that happen, everyone – DNR, landowners in the CWD zone, hunters, and people who cherish having robust, healthy wildlife in our state – needs to take a long term view of disease management,” he added.

CWD is an always fatal nervous system disease known to naturally infect white-tailed deer, mule deer, moose and elk. It belongs to the family of diseases known as transmissible spongiform encephalopathies (TSE) or prion diseases. Though it shares features with other prion diseases, like mad cow disease in cattle and scrapie in sheep, it is a distinct disease known to only affect members of the deer family. CWD has been discovered in wild deer or elk herds in 11 states and two Canadian provinces.

FOR MORE INFORMATION CONTACT: Davin Lopez, CWD Project Leader, Madison: 608-267-2948; Robert Rolley, Population Ecologist, Madison: 608-221-6341; Dennis Heisey, CWD Research Biologist, National Wildlife Health Center, Madison: 608-270-2478; Greg Matthews: (608)275-3317