EHD and CWD: Recognizing the Differences in Deer

Start with the date, the location, the animals' physical condition and whether the discovery involved a group. Look at harvest figures afterward. Hunters commonly refer to hemorrhagic disease as EHD. It kills groups of deer from late summer through the first hard frost, typically close to water. Chronic wasting disease (CWD) occurs without a seasonal window, and symptoms can remain absent for months to years after infection. County harvest records cannot identify the disease involved. Among the Illinois counties we compared, 63 out of 99 recorded harvest declines reaching 15% in 2013. Across Wisconsin's 11 counties with the highest CWD levels, comparable declines occurred during 13.4% of county-seasons. A county-season represents a county during a hunting season. The comparison group recorded a similar rate, 14.5%, across the remaining 50 counties.

According to Wisconsin's wildlife agency, hemorrhagic disease outbreaks generally occur between August and October. Activity fades late in fall after the first hard frost kills the virus-carrying midges. Iowa DNR lists July to September as the usual period for deer deaths and notes that deaths sometimes occur later during weather warmer than usual for the season. The description from CDC's CWD overview presents a different pattern. Prions pass through bodily fluids or contaminated food, water and soil. After CWD becomes established locally, those prions persist for years in water and soil.

The deer themselves provide different clues, too. According to Wisconsin DNR, hemorrhagic disease is detected only through discoveries of grouped deer carcasses, generally around water, with body condition otherwise good. CDC's guidance on infected animals explains that symptoms may remain absent for months to years in deer carrying CWD. Signs include severe weight loss, drooling and stumbling. Diagnosing CWD from symptoms alone is difficult because inadequate feeding and other diseases produce identical signs.

An outbreak is active in Wisconsin during the current season. On its page updated September 28, 2026, DNR identifies Washington and Waukesha counties as confirmed outbreak locations, with suspected cases across five additional counties. Estimates drawn from biologist and public reports put Washington County slightly above 1,200 deer and Waukesha County slightly above 400. Consult that page for current counts because the figures change frequently.

For hunters, whether the meat is safe is the leading concern. According to Wisconsin DNR, hemorrhagic disease viruses do not infect humans, including when someone eats venison from an infected deer. According to CDC, no human CWD infection has been reported, but whether humans can become infected remains unknown. Its advice is to consider having a deer tested before consuming it.

Since hemorrhagic disease can cause deaths among groups of deer, we examined whether county harvest falls when an outbreak strikes and whether a decline distinguishes it from CWD. Within the Illinois and Iowa series, we compared each county's registered or reported harvest with the preceding season, flagging decreases reaching 15%. Illinois recorded that decrease in 63 counties out of 99 during 2013, while the statewide total decreased 17.4%. Iowa recorded it in 47 counties out of 99, alongside a statewide decline of 14.0%. Across the remaining seasons, the median count was 1 county in Illinois and 9 counties in Iowa.

Those declines covered a broad area and occurred near the outbreak years identified by Iowa DNR. Iowa's July 2024 update identifies 2012, 2013, 2019 and 2023 as severe outbreak years. In its November 2023 update, the agency places 2023 third in severity, after 2012 and 2019. Harvest records cannot determine whether disease contributed to the 2013 decrease or how large any contribution was. The 2013 harvest season followed 2012, a year Iowa DNR includes among its two most severe outbreaks. The agency also identifies 2013 as a severe outbreak year. Permit availability, weather and hunting effort affect harvest, too. We attribute none of these declines to disease.

The outbreak severity ranking also does not match the geographic spread of counties meeting the harvest-decline threshold. At the 15% threshold or above, harvest decreases during the outbreak season affected 12 counties in 2012, 7 counties in 2019 and 24 counties in 2023. If the geographic spread matched the severity ranking, 2019 would rank ahead of 2023. Iowa DNR also reports that effects vary within a county: deer on one property can suffer severely while those on a neighboring property do not suffer severely. Combining those properties into a county total hides that difference.

According to CDC, months to years may pass before an infected deer shows symptoms, so CWD follows a slower course than hemorrhagic disease. Applying the same harvest test in Wisconsin, we examined whether declines occurred more frequently in counties with the highest CWD levels. For the 2025 CWD year, the county results published by Wisconsin DNR identify 11 counties where positive tests accounted for at least 15% of deer analyzed: Richland, Sauk, Lafayette, Columbia, Green, Iowa, Rock, Grant, Walworth, Dane and Juneau. From 2007 to 2025, harvest in those counties was at least 15% below the preceding season's total in 28 of 209 county-seasons, a rate of 13.4%. Across the remaining 50 counties, comparable decreases occurred during 136 out of 941 county-seasons, equivalent to 14.5%. Excluding 2009, a season with declines in 52 counties out of 60, leaves rates of 11.1% and 10.1%.

Across 8 out of 19 seasons, combined harvest in the 11 counties performed worse than combined harvest in the remaining 50 counties. That means a larger decrease or a smaller increase. The annual difference averaged 0.4 percentage points favoring the CWD group.

The Wisconsin decline during 2009 covered more counties proportionally than the Illinois decline during 2013. Harvest decreased at least 15% in 52 counties out of 60, while the series covering 61 counties decreased 26.2%. We did not investigate its cause; doing so is unnecessary for this comparison. A decline spread across many counties does not, by itself, identify disease.

Confirmed hemorrhagic disease locations listed by Wisconsin DNR complicate the comparison further. Its 2012 list includes Columbia, Dane, Jefferson, Iowa, Marquette, Rock, Sauk and Waukesha. County totals of suspected deer ranged from 2 to 132, and five of those eight counties also belong to the group of 11 CWD counties. For those eight counties, the median harvest decrease going into 2013 was 10.9%, compared with 4.8% across the remaining 53. The reported deer counts are small beside county harvest totals in 2012 ranging from 2,221 to 8,127. They also represent reports rather than a census. Harvest figures alone cannot attribute Dane County's decrease that year to a particular disease.

Harvest measures the deer hunters report or register. Deer abundance affects that count, but so do permits, weather, hunting effort and season structure. A decrease in harvest therefore cannot establish a population decline on its own, although both may occur together. Comparisons for Iowa begin with 2006 because its estimates from 2005 or earlier cannot be compared. For most years, Illinois lacks series covering Jo Daviess, Rock Island and St. Clair counties. Wisconsin's series covers 61 counties and excludes 11 counties in the northern forest. Indiana, Michigan, Missouri and Ohio lack paired records for 2012 to 2013, preventing that comparison.

Summed Iowa county counts match the statewide table during 16 out of 19 seasons. During the remaining three, they fall short by less than 0.4%. We selected the 15% cutoff. Using either 10% or 20% still puts 2013 highest among seasons in both states. Wisconsin DNR's positive tests divided by analyzed deer determine the CWD county group; that measure includes no adjustment for sex or age.

We think a decline in county harvest cannot establish a diagnosis of deer disease. Hunters should begin with timing, physical signs and the circumstances of discovery. Use harvest records solely to assess the geographic extent of a change.

If you discover several dead deer in good body condition from late summer through the first hard frost, typically close to water, notify the state of suspected hemorrhagic disease. Wisconsin accepts reports through its Sick/Dead Wildlife Reporting Form. Iowa DNR provides hunters and landowners with an online reporting tool. Carcass collection is beyond what Wisconsin DNR staff can provide. The agency discourages handling carcasses and advises wearing gloves when handling is necessary.

When hunting a county with CWD, focus on the individual deer you have taken. CDC's hunter guidance advises avoiding shooting, handling or consuming deer that appear ill or behave unusually. It also advises considering a test before consuming venison and avoiding meat from animals with positive results. Wisconsin's Department of Health Services, as relayed by Wisconsin DNR, recommends consuming venison only from deer that appeared healthy and whose tests did not detect CWD.

County harvest decreases, considered by themselves, cannot reveal whether either disease contributed. Declines reaching 15% occurred in roughly one out of seven Wisconsin county-seasons. Assess a change's reach by counting the affected counties, then consult the state wildlife agency about confirmed findings. For a closer examination of CWD, see our papers covering Wisconsin's CWD prevalence and the herd signals in the same 11 counties.