The Wisconsin Veterinary Diagnostic Laboratory (WVDL) has been the official USDA certified Chronic Wasting Disease (CWD) testing facility in Wisconsin since 2002. WVDL conducted over 35,000 CWD tests in 2002-3 and more than 14,500 tests on samples collected during the 2003-4 hunting season. WVDL uses only USDA approved testing reagents and methods and has passed all inspections and proficiency tests related to CWD testing with high scores.
In 2002/2003, all CWD testing was done by immunohistochemistry (IHC) which is considered to be the most reliable test routinely available. This test is more time consuming and expensive than the plate-based screening tests which are now available. In 2003/2004, WVDL used a USDA approved CWD screening test made by IDEXX, Inc. Westbrook, ME to provide preliminary information on tissues from 9476 Wisconsin deer. This test is capable of identifying individual samples as negative or suspect for CWD, but like other screening tests is not considered a definitive tool for diagnosing the disease. Referring to a screening test result as positive can be confusing and should be avoided.
In the 2003-4 testing season, WVDL identified 159 of 9476 samples as suspect after first analysis. This designation is based upon a machine reading above a cutoff point which is purposely set to minimize the chance of missing any animals truly affected with CWD. Sixty eight (68) samples were consistently above this cutoff point when retested in duplicate using the same screening test. WVDL checked all 159 initial suspect animals with the IHC assay to be certain of their disease status. There were 46 positive deer by IHC , all of which were in the group of 68 that retested as suspects with the IDEXX test. These 46 deer were judged to be affected with CWD and that final result was transmitted to the Wisconsin DNR. Twenty two (22) initial suspects that retested as such were negative by IHC, as were all of the other 91 initial reactors.
All 46 IHC positive deer were killed in Wisconsin counties where CWD had been previously shown to exist. There is no current evidence based on this testing that CWD exists in other counties in Wisconsin despite recent implications or reports to the contrary.
The CWD testing strategy used at WVDL in 2003 is identical to the testing strategy for Bovine Spongiform Encephalopathy (BSE, Mad Cow Disease) announced on March 15, 2004 by the USDA. The interpretation of test results for this bovine disease which has critical economic and human health considerations is identical to that used for Wisconsin’s CWD testing. The screening tests only identify animals in need of further, more definitive testing by IHC. An animal with a screening test suspicious test result which is not confirmed by IHC will be considered negative.
In conclusion, WVDL stands behind its 2003 CWD testing strategy, the specific assay used, and the accuracy of the final results as reported.