Wisconsin puts its overall chronic wasting disease positivity at about 5% for all deer tested since testing began. That figure is accurate. For hunt planning, though, we recommend the rate from the current sampling year. Combining twenty-six years of results conceals the increase in positivity among sampled deer. During the 2025 sampling year, 11.1% of tested deer statewide were positive. In the southern core counties, positives accounted for roughly one out of three deer tested. Positivity for 2025 sampling-year testing is substantially above the lifetime figure.
KEY FINDINGS
- Pooling Wisconsin's results from 1999 to present gives 4.65% positivity, with 16,402 positive deer among 352,752 analyzed. Isolating the 2025 sampling year gives 11.1%, exceeding twice the lifetime mean.
- Positivity this year is much greater in the southwest core: Richland, 35.7%, with 427 positives among 1,197 deer analyzed; Sauk, 33.0%; and Lafayette, 31.0%. That is about a positive result per three deer tested.
- The displayed counties each had at least 50 analyzed deer during 2025 sampling-year testing. Dividing each county's 2025 positivity by that county's all-years mean produces ratios with a median of 3.6×. The cumulative measure thus understates current prevalence by a factor of roughly three to four.
- When a hunter reads “5% of Wisconsin's tested deer”, that figure reflects two decades of early sampling with low prevalence. Those older results dilute the number; it does not describe the herd that hunter faces this fall.
ITwo numbers, one disease
A lifetime percentage measures something different from this year's percentage
To report cumulative positivity, Wisconsin's DNR divides all positive results by the number of deer analyzed since testing began. In Wisconsin, that works out to 4.65%, or 16,402 confirmed positive deer among 352,752 sampled beginning in 1999. Older tests and newer tests share the same calculation. This pooled measure spans twenty-six years. About a third of the samples, roughly 129,000 out of 352,000, were collected during 2002 to 2006, when positives accounted for less than 1 percent. It addresses “what share of all deer tested in Wisconsin tested positive,” rather than the question guiding a hunter: how much disease is present today.
Filtering those records for 2025 sampling-year testing, from April 2025 to March 2026, yields 11.1%, above twice the lifetime percentage. Only deer analyzed within that 2025 sampling year contribute to this rate. This comparison, by itself, neither demonstrates that prevalence rose continuously nor identifies why the rates differ. A different mix of counties or deer traits, or shifts in testing practices, could also change positivity in the sample.
IIThe core
Where the current positive rate is one out of three deer
Wisconsin's 11.1% statewide figure pools results across a state with positive tests clustered in its southern portion. Positive deer were absent from 32 of 73 counties' 2025 samples; most of those counties are northern. The southwestern farmland core's 2025 percentages were the highest county percentages within Wisconsin's 2025 sample. Richland County had 427 positive results among 1,197 analyzed deer, or 35.7%. Positivity was 33.0% in Sauk, 31.0% in Lafayette and 27.4% in Columbia. These percentages summarize 2025 sampling-year results, rather than lifetime results pooled across decades. Within these Wisconsin core counties, positive results accounted for 27.4% to 35.7% of tested deer.
Swipe table to see every column
| County | All-years % | 2025 % | × |
|---|---|---|---|
| Richland | 13.1 | 35.7 | 2.7× |
| Sauk | 10.6 | 33.0 | 3.1× |
| Lafayette | 7.8 | 31.0 | 4.0× |
| Columbia | 8.4 | 27.4 | 3.3× |
| Green | 6.8 | 26.6 | 3.9× |
| Iowa | 8.6 | 23.5 | 2.7× |
| Rock | 3.3 | 23.1 | 7.0× |
| Grant | 6.9 | 20.7 | 3.0× |
| Walworth | 2.3 | 20.0 | 8.7× |
| Dane | 5.8 | 19.3 | 3.3× |
| Statewide | 4.65 | 11.1 | 2.4× |
IIIWhy the average lies
Early low rates still pull down a mean spanning twenty-six years
The math explains the difference. Wisconsin confirmed CWD for the first time in 2002 in deer from the Mount Horeb area of Dane and Iowa counties. The state has tested deer each year since then. Nearly all deer tested in the early years were negative: 128,420 of 129,273 deer analyzed from 2002 to 2006 had negative results. Those deer still count toward the total used to calculate the cumulative rate. Rock County shows how much the rates can differ: 3.3% across all years compared with 23.1% for the 2025 sampling year. During its 2023 to 2025 seasons, rates were six to seven times the average across all years.
Wisconsin's cumulative percentage describes the CWD found across its testing history. It cannot provide either positivity among deer tested during the 2025 sampling year or prevalence across the entire herd. Among tested deer in the core counties during 2025, about a third were positive.
IVWhat it means for the hunt
Use this year's county rate when reading the numbers, instead of the statewide lifetime mean
The practical advice for hunters stays the same: test and obey local rules for moving carcasses. What changes is how to interpret the figures. When hunting in the southern core, set aside the neat statewide 5% figure. Check the rate for your county this year; it is very likely to be in the twenties or thirties. A rate that high does not, on its own, justify giving up hunting. It does mean you should quit treating the lifetime average as a picture of the current season. We recommend basing hunting decisions on the recent rate, which is much higher.
NOTES AND SOURCES
- Calculate prevalence by dividing confirmed CWD-positive deer by the number analyzed, using Wisconsin DNR's county CWD summary tables. The cumulative series covers 1999 to present at apps.dnr.wi.gov/cwd/summary/county. The 2025 sampling-year series starts Apr 1 2025 and ends Mar 31 2026; find it at apps.dnr.wi.gov/cwd/summary/YearCounty/2025.
- To limit noise from small samples, the displayed counties each had ≥50 analyzed deer during 2025 sampling-year testing. The statewide measures combine all counties.
- At retrieval, 2025 sampling-year testing was substantially complete, although not necessarily finished in full (June 2026). Late samples may shift county percentages modestly.
- Results labeled “All-years %” combine the period from 1999 to present. Core counties with many samples dominate this lifetime measure, which is not a yearly figure.
- Prevalence describes infection among sampled deer. It does not measure how the disease affects herd size; the companion paper about Wisconsin's core addresses that population issue separately.
Fully Drawn Research is an independent data analysis desk, not affiliated with any state wildlife agency, transportation agency, mapping provider or outfitter. Figures trace to Wisconsin DNR public CWD summary tables, retrieved June 2026.
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