Ask Department of Natural Resources Secretary Scott Hassett … …about Chronic Wasting Disease in Wisconsin White-tailed Deer
Editor’s Note: This is the first of several summer and fall 2004 updates in which DNR Secretary Hassett answers questions the public has been asking him and agency staff about CWD since the end of last year’s deer hunting seasons.
Last year, I sent out eight bi-weekly columns on CWD, the fatal nervous system disease of whitetailed deer. At the end of my final column, I noted that if your editor was willing, I’d send out periodic updates in 2004 to keep you informed of our activities, our progress in stopping the disease, and to answer additional questions that have come up since December, 2003. The time to do that has arrived.
The discovery of CWD in 2002 in southwest Wisconsin sent shock waves throughout our state’s hunting community. Since then, our intensive testing program and your willingness to kill deer and bring them in for sampling have helped us identify two areas of CWD infection in our wild deer herd – one centered along the Iowa-Dane county line in southwest Wisconsin, and the other in the southeast, primarily in Rock and Walworth Counties along our border with Illinois.
DNR sampling and surveillance efforts have answered some questions, and researchers are getting a better understanding of CWD. But we still have a long way to go to eradicate this disease with the continuing help and cooperation of landowners and hunters. Let’s take a look at answers to some of the questions you’ve been asking:
How many wild deer were killed last year in the CWD Disease Eradication Zone (DEZ) and Intensive Harvest (IHZ) Zone?
Last year, hunters killed 13,501 deer in the DEZ/IHZ of southwest Wisconsin and 193 in the Rock County DEZ. The fall 2003 population estimate for the IHZ in southwest Wisconsin was 41,000.
What is the wild deer population estimate for the DEZ following the end of last fall’s hunting seasons?
The past two winters, we have used aerial surveys in the CWD zones to estimate deer populations. These surveys are experimental, but are likely the best available tools we have for monitoring deer populations in these areas.
Aerial survey estimates are not complete deer counts because observers can’t see all the deer present below the aircraft. Observing and counting deer is affected by many factors, including snow cover, amount and type of vegetation, light conditions, aircraft type, flight speed and elevation.
We used a helicopter to count the number of deer in square mile sections of land in the DEZ/HRZ. During January and February, 2004, we were able to fly over 201 of the planned 220 square mile sections before the snow melted in the DEZ. By extrapolating the number of deer seen in the 201 square mile sections sampled to the entire DEZ, we estimated the population at about 21,000 deer or about 35 deer per square mile of habitat. This translates into about a 10 percent reduction in the DEZ’s whitetail herd through hunting efforts during the 2003 season. About a 30 percent reduction was achieved in the “core area” of the DEZ, the area where CWD prevalence is the highest.
What is the population goal in the DEZ?
The deer population goal in the DEZ is less than five deer per square mile. Scientists are continuing to research how far the population should be reduced. For now, they believe that the deer population must be brought to a very low number to control CWD, but that something higher than zero might do the job.
How many wild deer tested positive for CWD in 2003? As of August 17, 115 wild deer tested positive for CWD from the 2003 seasons. The breakdown was 106 from the DEZ/IHZ in southwest Wisconsin, five from the DEZ in Rock County, and two from the Herd Reduction Zone (HRZ) in Rock County. Two deer were shot outside of a CWD Zone, one each in Kenosha and Walworth counties.
Based on last year’s sampling results, we extended the DEZ boundary in southwest Wisconsin to highways about four to six miles from positives in the Cobb and Highland areas. We also increased the size of the Rock County DEZ and surrounding HRZ. The expanded HRZ in southeast Wisconsin includes all of Deer Management Unit (DMU) 77B and the southern portions of DMUs 77C and 76A.
Where has DNR found CWD in Wisconsin’s wild deer herd?
To date, we have tested more than 56,400 wild deer in Wisconsin for CWD. So far, 326 wild deer have tested positive for CWD with the majority from western Dane and eastern Iowa Counties. Last fall, all wild deer that tested positive were in a CWD zone except for one in Kenosha County and one in Walworth County.
What are the hunting season dates for Fall 2004?
Hunting seasons for the DEZ and HRZ are:
- CWD Disease Eradication Zone
- Archery: September 18-January 3 (Archers must wear blaze orange Oct. 28-Jan. 3)
- Gun: October 28-January 3
CWD Herd Reduction Zone
- Archery: September 18-January 3 (Archers must wear blaze orange Oct. 28-31 and Nov. 20-Jan. 3)
- Gun: October 28-31 November 20-January 3
What was behind published accounts in spring 2004 indicating that tests at the University of Wisconsin’s Veterinary Diagnostic Laboratory (WVDL) had found “preliminary indicators” of CWD in wild deer in 12 additional Wisconsin counties?
I was very disappointed with some of those press reports. They were misleading and did not describe the full process used to identify CWD-infected deer.
Last summer, WVDL, which tests Wisconsin wild deer for CWD, chose a test called the IDEXX ELISA to quickly screen thousands of samples for evidence of CWD and identify suspect positive lymph node tissues that need further testing. This preliminary, low-threshold screening test is not approved as a stand-alone test.
All suspect lymph node tissues identified by this screening test must be examined further using the CWD “gold standard” immunohistochemistry (IHC) test to positively identify whether there is evidence of the CWD prion. The U.S. Department of Agriculture (USDA) still relies on IHC as the definitive test for CWD. This strategy of using a screening test (IDEXX ELISA) with confirmatory follow-up (IHC) is also the same strategy USDA currently uses for mad cow disease.
Like any good screening test, IDEXX ELISA has a cut-off for positive results set low enough to not miss any positive tissues. This also means that this test is designed to produce a certain level of suspect positive tissues that turn out to be negative. These are known as false positives.
News accounts indicating that a false positive is equivalent to a preliminary indicator of CWD were simply not accurate and caused a great deal of unwarranted concern for many hunters in those 12 counties outside of the established CWD Zones where suspect positives were found. None of the suspect positives in those 12 counties were confirmed as positives by the IHC, so there is currently no evidence of CWD in those 12 counties.
The “gold standard” IHC test continues to be the USDA-approved standard for CWD confirmation in whitetail deer, although USDA is evaluating other tests. The WVDL is following IDEXX manufacturer guidelines and USDA national standards for CWD in only reporting as positive those deer that have an IHC positive follow-up result.
I’ll answer more questions in my next column. Thanks for taking time to find out more about chronic wasting disease in Wisconsin.