Among Wisconsin's 21 Counties Without a CWD Detection, Median Testing Was 60 Deer for 2025

Twenty-one Wisconsin counties, out of 72 statewide, have never reported a CWD positive in county tables published by the Department of Natural Resources (DNR), whose records reach back to 1999. Testing during the 2025 CWD year covered 20 to 183 deer per county; among the 21, the median was 60. With random sampling and a test that detects every infection, a county with that median sample has a modeled prevalence ceiling of 4.9% at 95% confidence. Those results cannot distinguish 4% prevalence from 1%. Past negative results have not kept similar counties free of detections: 53 counties began the 2017 CWD year without a positive, and 32 have reported one afterward. The latest annual sample therefore says more than the total accumulated over a county's history.

A zero in county results means testing found no CWD among the deer examined. The sample's size determines how much prevalence that result can exclude. Suppose CWD infected 5 out of every 100 deer locally. A random group of 59 deer would then have a chance near one in 20 of producing nothing but negative results. That makes 5% prevalence unlikely after 59 negatives. For 1%, the required count becomes 299; for 0.5%, it becomes 598. Divide ln(0.05) by ln(1 - p), then round upward to obtain n. The symbol n counts the negative deer required, ln denotes the natural logarithm, and p represents the infected fraction. For that prevalence, 0.05 represents the probability of obtaining only negative results, matching the 5% allowance for a confidence level of 95%.

If a test cannot detect every infected deer, all of those sampling thresholds rise. In their CWD surveillance model, researchers in Illinois selected sensitivity values from 0.92 to 0.99, based on field research involving lymph node testing. They also explain that prions accumulate gradually in the tissue being examined, making deer infected recently harder to detect (Mori et al., 2026). With sensitivity set to 0.92, excluding 5% requires 64 deer, and excluding 1% requires 325. All calculations that follow assume perfect detection, giving a zero its most favorable interpretation.

At the close of the CWD year on March 31, 2026, testing totals for each of the 21 counties ranged between 371 and 3,146 deer. Together they accounted for 30,231 deer analyzed, and the county median was 1,253. Calumet had the fewest, 371. That total could still exclude prevalence greater than 0.8%, provided sampling was random and infection rates stayed constant. The second assumption, stable prevalence, is the shakier part. Testing before the 2010 CWD year accounts for 14,616 deer out of 30,231, or 48.3%. Back then, 12 counties had reported a positive; today, 51 out of 72 have. Much of the total therefore reflects a period when known CWD covered far fewer counties.

For a current view, use the 2025 CWD year, which ran from April 1, 2025 through March 31, 2026. Each county tested between 20 and 183 deer. Combined testing reached 1,422, while the 21 counties had a median sample of 60. For a county with that median count, the prevalence ceiling at 95% confidence is 4.9%. Ashland's sample included 20 deer, while Kewaunee's included 21; their respective ceilings are 13.9% and 13.3%. Marinette tested the most, 183, enough to exclude 1.6% prevalence. With 1 infected deer per 100, a sample equal to the median would nevertheless produce all negatives in about 55% of samples.

County results from DNR show what happened to counties without detections. As the 2017 CWD year opened, 53 out of Wisconsin's 72 counties had yet to report a positive. When 2025 ended, 32 had done so, leaving the 21 examined here. Testing in the preceding year had substantial samples. Of those 32 counties, 31 had analyzed deer; Menominee had analyzed none. All results were negative, and the sample median reached 154, exceeding twice the median of 60 for the 21 still without detections. The 59-deer threshold for excluding 5% was met in 27 of those 31 counties. The threshold of 299 for excluding 1% was met in 3.

La Crosse County illustrates this across two table rows. DNR identifies it as having its first wild-deer positive that season in its report on 2025 testing. Its county results show 140 deer analyzed during 2024 with no positive, followed by 154 during 2025 with one positive.

The initial detection years yielded few positives. County samples in those years had a median of 168 deer. Each county recorded 1 to 4 positives, with a median rate of 0.8 positives for every 100 deer tested. Testing 154 deer without detecting CWD produces an upper bound of 1.9% at 95% confidence. A negative year with that sample size therefore fits with a later first-detection rate of 0.8 per 100. These records cannot establish whether earlier sampling overlooked infected deer or infection arrived after that earlier sample.

Eighteen counties among the 21 touch a county with a reported positive along a boundary line. Ashland, Bayfield and Florence are the only exceptions. Every county bordering Pepin (61 deer tested during 2025), Ozaukee (65) or St. Croix (132) has reported a positive.

An earlier detection next door has accompanied more frequent first detections locally. Count all counties that entered any CWD year between 2017 and 2025 without a positive. Each annual county observation counts as one county-year, giving 345 total. Among 224 county-years with a neighboring positive already reported when the year began, 27 brought the county's first detection, or 12.1%. Without a previously positive neighbor, the corresponding count was 5 among 121, or 4.1%. These county-years do not represent independent draws. Most of the 21 lie in northern Wisconsin: 15 have a center north of latitude 45 degrees. We therefore treat the difference as a description of those nine years, without turning it into a prediction for a particular county.

The upper bounds require random testing within a herd far bigger than the sample. They count each deer once and assume every infection is detected. The Illinois researchers model deer encountered by hunters as a random draw from the local herd. We use that approach as well. However, the county tables used for this article do not break out hunter-harvest samples separately from samples obtained through other sources. DNR cautions on those same pages that, where CWD occurs within a county, it is likely to be concentrated in one or more localized pockets. A bound for prevalence across the county therefore reveals little about conditions within any of those areas.

The reference table in the Illinois paper addresses a separate task: determining how many deer need testing to estimate prevalence within 2 points after disease is present. We leave that table out of our calculations.

DNR assigns a CWD year's label from its April 1 opening date. Only wild deer count, and its reported totals reflect deer analyzed, slightly fewer than those sampled. For the lifetime figures here, we summed full annual tables covering 1999 through 2025. Data on the cumulative page extend through September 29, 2026 and include results to date for 2026. Four of these 21 counties have slightly higher totals there, while all remain without positives. For this analysis, neighbors must share a boundary line. We excluded the state's ten county pairs that touch at a corner alone.

We think the deer count behind a zero should guide its interpretation. In Wisconsin counties without detections, hunters should regard any most-recent annual count below 299 as insufficient to exclude 1% prevalence. They should also submit their deer for testing.

Use DNR's 2025 county CWD results for the most recently completed year, then compare the sample with three thresholds. A count below 59 cannot exclude prevalence of 5% with 95% confidence, though a zero still provides weaker evidence. With 59 to 298 tested, the sample excludes 5% while leaving 1% possible. Testing at least 299 excludes 1%; excluding 0.5% requires 598. Next, look for a bordering county with a positive in the county totals across years.

One submission barely changes a county's count. Where 60 deer have tested negative, adding a 61st negative lowers the bound from 4.9% to 4.8%. Broad hunter participation builds the sample. DNR's where to submit samples explains that sampling is offered throughout Wisconsin. Deer harvested in the state receive free DNR testing, and self-service kiosks operate 24/7. During the 2025 season, an emailed result arrived an average of 8.9 days after drop-off. For a closer look at counties with common CWD, read One in Three: Wisconsin's CWD, Recalculated and The CWD Epicenter.