One in Three: Wisconsin's CWD, Recalculated

Wisconsin reports that about 5% of the deer it has ever tested for chronic wasting disease came back positive. The number is true, and it is the wrong number to hunt by. It averages twenty-six years of testing into a single figure and buries the fact that the disease is accelerating: in the 2025 sampling year, statewide prevalence was 11.1%, and in the southern core counties roughly one deer in three tested positive. The gap between the lifetime number and the this-year number is the whole story.

Every state that tests for chronic wasting disease publishes a cumulative positive rate — total positives divided by total deer ever analyzed. Wisconsin's is 4.65%: 16,402 confirmed positives against 352,752 deer sampled since 1999. It is an honest number and a reassuring one, and it is built to mislead by construction. A cumulative rate pools twenty-six years of testing, most of it done early and in places where the disease had not yet arrived. It answers the question “how much CWD has Wisconsin ever found,” which is not the question a hunter is asking. The question a hunter is asking is: how much is out there now.

Restrict the same records to the 2025 sampling year (April 2025 through March 2026) and the rate is 11.1%, more than double the lifetime figure. It is the same disease, measured without the dilution of history. The cumulative number and the current-year number diverge because prevalence has been climbing the entire time, and averaging over the climb flattens it.

The statewide 11.1% is itself an average across a state that is mostly clean in the north. In the southwestern farmland core the 2025 rates are among the highest documented anywhere in North America: Richland County returned 427 positives out of 1,197 deer analyzed — 35.7%. Sauk was 33.0%, Lafayette 31.0%, Columbia 27.4%. These are not lifetime figures accumulated over decades; they are single-season snapshots. In this country, a checked deer coming back positive is now closer to a coin-toss than a rarity.

The mechanism is simple arithmetic. CWD was first confirmed in Wisconsin in 2002, in a handful of southern counties, and the state has tested heavily every year since. In the early years almost every sample was negative, and those hundreds of thousands of negatives are still in the cumulative denominator. A county like Rock, at 3.3% lifetime but 23.1% this year, has not gently drifted upward — its recent seasons are seven times its own historical average, and the historical average is doing the hiding.

The cumulative rate answers how much CWD Wisconsin has ever found. It cannot tell you how much is in the woods this fall. In the core, the honest answer is one deer in three.

None of this changes what a hunter should do — test, and follow local carcass-movement rules — but it changes what the numbers are saying. If you hunt the southern core, forget the state's tidy 5%. Look up your county's current-year rate instead, which is very likely in the twenties or thirties. Prevalence at that level is not a reason to stop hunting; several of these counties still produce excellent harvests. It is a reason to stop reading the lifetime average as if it described this season. The right number is the recent one, and it is much larger.