Hot fall triggers disease outbreak in some Montana big-game herds

Above-average September heat led to a widespread disease outbreak in Montana’s southeast big-game herds that has killed thousands of wild animals across the Great Plains.

Incidents of bluetongue and Epizootic hemorrhagic disease, or EHD, in deer and antelope have started to decline with the onset of overnight hard frosts, according to Chrissy Webb, communications officer for Montana Fish, Wildlife and Parks Region 5 in Billings. That’s good news for the start of general rifle-hunting season on Saturday, but won’t help the archery season hunters who had to endure abnormally high temperatures.

“That [frost] really reduces midge activity,” Webb told Montana Free Press, referring to the flying insect that spreads the disease to ungulates. “In the past few years, we’ve definitely seen more intense outbreaks with high numbers of dead deer in the river bottoms. We didn’t have quite that extent this year.”

Surrounding states haven’t been so fortunate. South Dakota’s Game, Fish and Parks Department offered license refunds to big-game hunters in 16 counties to reduce pressure on deer herds after disease reports spread across the state. Wisconsin’s EHD outbreak is its worst on record, according to the Boston University’s monitoring site BEACON, and has also affected domestic dairy cattle.

“The 2026 EHD season has been unusually widespread across the northern Great Plains and upper Midwest,” the Oct. 19 BEACON notice observed. “The overall toll does not yet appear to rival the historic 2007 and 2012 epizootics.”

Wisconsin officials have reported at least 2,300 sick or dead deer from EHD, while South Dakota’s 407 reported deer EHD deaths has surpassed its entire 2025 total. Webb said Montana game wardens have not conducted an in-depth count of diseased game animals this year. 

In Montana, cases have been most reported in Yellowstone and Big Horn counties along the Yellowstone, Big Horn and Clarks Fork Yellowstone river corridors. The Clark Fork River on the west side of the state is not currently affected.

Both diseases affect ungulates such as whitetail and mule deer, and antelope. They are spread by insect bites from the Culicoides midge, which breed and cluster in the moist soil and standing water of riparian areas. Sick animals suffer dehydration, disorientation, swollen heads, bluish tongues and severe fever, and often die within days of infection. Many are found dead near water sources, where they seek relief from fever. The diseases are not transmissible to humans.

The cases come as Montana logs its warmest 12-month period on record, according to the National Oceanic and Atmospheric Administration. September’s average temperature of 53.8 degrees was 3.5 degrees above normal in the state, with the southeast counties recording their 13th warmest September in 130 years. 

“That’s compounded by the consistently warmer-than-normal temperatures we had all summer long,” said Montana State Climatologist Kelsey Jencso. “The southeast also is still currently in D3-level drought conditions. That’s extreme drought. Out of the last 100 years, only three or five years are worse. Animals are really water-stressed.”

Additional statistics for all states are available on NOAA’s Climate at a Glance website.