Statewide Disease Outbreak Causes Localized Deer Mortality in Montana
Montana is experiencing localized deaths of deer and pronghorn antelope due to a disease outbreak linked to biting flies during this unusually warm autumn. The Montana Fish, Wildlife & Parks (FWP) big game chief, Brian Wakeling, recently informed the FWP commission of multiple hemorrhagic disease outbreaks affecting primarily white-tailed deer across the state.
Hemorrhagic disease refers to two viral infections: epizootic hemorrhagic disease (EHD) and blue-tongue virus. Both viruses are transmitted by biting midges, small, moisture-dependent flies that become active in late summer and early fall. These midges typically inhabit damp environments and transmit the viruses when biting deer and pronghorn coming to drink.
Localized Mortality Patterns and Surveillance Efforts
This fall, clusters of white-tailed deer mortality have been reported near the Clark Fork River west of Missoula and in Frenchtown. Additional suspected cases have surfaced near Potomac and Lincoln within the Blackfoot River Valley, with tissue samples currently being sent to the FWP laboratory for confirmation.
Further east, the Confederated Salish and Kootenai Tribes Wildlife Program reported dead deer—and potentially some elk—around Hot Springs and the Mission Valley. White-tailed deer in eastern Montana are also showing signs of hemorrhagic disease, specifically along the Milk River near the Hi-Line and downriver from Billings along the Yellowstone River. Landowners have reported significant losses, including one account of 60 deceased animals.
Historical Context and Environmental Factors
While hemorrhagic disease outbreaks in western Montana are not a new phenomenon, they first emerged west of the Continental Divide in fall 2013. That initial outbreak caused nearly 400 deer deaths, concentrated around the same geographic areas affected this year—from the Clark Fork River near Harpers’ Bridge downstream to approximately 10 miles and northeast through Mill Creek near Frenchtown.
In northwest Montana’s Region 1, no outbreaks were recorded until approximately 2016. Compared to this year, those first cases were less severe. This autumn, outbreaks are reported near lakes between Eureka and Trego, as well as in Pleasant Valley along the Fisher River west of Marion.
Biological and Climatic Considerations
- Biting midges, the primary vectors, are sensitive to frost, which typically curtails their activity.
- As of mid-October, Montana has not experienced a significant frost, allowing midges to remain active longer than usual.
- Hemorrhagic disease onset is rapid—around two weeks—and results in internal hemorrhaging, often making early detection difficult.
- Warm temperatures extending into autumn increase midge populations and enhance the spread of these viruses.
Former FWP biologist Tim Thier noted the delay in frost as a key factor in this year’s extended outbreak. Brian Wakeling also linked the warm September temperatures to heightened disease transmission, emphasizing how climate variability plays a critical role in the timing and severity of hemorrhagic disease events.
Additional reporting and sources from: Statewide disease outbreak causes localized deer deaths
