Disease hits Montana's Missoula Valley deer – 400 dead in a month


As fall began to settle into Missoula, Mont., and hunters got ready for deer season, a sudden, bizarre rash of deer deaths left carcasses decaying in the area. Montana Fish, Wildlife and Parks began receiving calls in mid-September reporting the deaths, and in less than two weeks, they confirmed more than 100 cases. All the evidence – otherwise healthy deer that suddenly keeled over, many in streams, bleeding from their mouths – pointed to one of a few viruses commonly found in Eastern states. By the time the lab reports came back this week, the count of dead deer had risen to roughly 400, and tests confirmed the cause was epizootic hemorrhagic disease, or EHD.

Epizootic hemorrhagic disease is most prevalent in eastern states, but showed up for the first time this year in Montana's Missoula Valley.

First discovered in 1955, EHD causes regular outbreaks in deer populations in the central plains and in the East. It also shows up in eastern Montana and further west in Idaho, Oregon and California. However, the discovery of EHD in the Missoula Valley is unusual; it has never crossed the Rockies into western Montana before. “We don’t have any idea of why it’s showing up,” says Vivaca Crowser, an information and education manager for FWP. “But we don’t know why not either.”

The midges that carry the disease are common in the Missoula Valley, and the virus has bordered the area for years. Still, when the tests came back from the lab confirming the cause of the deer deaths, “everyone was pretty surprised” that it finally showed up now, says Crowser. “For whatever reasons, the conditions were right.” Likely, those are environmental conditions that favor the midges, which thrive in warm, wet weather and are killed off by hard frost. Flare-ups of EHD often coincide with mild winters.

While EHD primarily affects whitetail deer, it can also show up in other ungulates like mule deer, pronghorn antelope and bighorn sheep. Once transmitted, the highly fatal infection causes internal bleeding, and feverish deer head to streams to cool down with a drink of water. That is often where they die. It’s alarming to people who come across a brook littered with decaying carcasses, but state managers say the stream-side concentration of dead deer makes the problem look more severe than it is. The state has stepped in to clear some of the carcasses, but Crowser says that scavengers have also made efficient work of them.

EHD causes internal bleeding and fever in whitetail deer and other ungulates, often causing the animal's death.

State officials have been reassuring people who have found these carcasses that EHD can’t be transmitted to humans. Montana FWP advises hunters that it’s probably best to avoid a live deer that shows signs of EHD, but also says there is no potential for harm in eating venison from an infected animal. Still, the impact to populations in north-central Montana has been severe enough that the region is considering more strict regulations on deer hunting this year.

Crowser says that with the recent frosts, wildlife managers do expect the rate of deer deaths to slow. She says that this year doesn’t necessarily indicate a growing overall trend of the virus appearing in western Montana, and could be a fluke. However, in other regions where the disease has taken a higher toll, states are trying more urgently to understand the causes of spread and flare-ups. Michigan has seen a marked increase in EHD in the past few years and confirmed 15,000 deer lost to the virus last year. The state is investing in studies of the disease to better understand how warmer winters will likely exacerbate EHD’s impact on deer populations and what that will mean for wildlife management. “Diseases are going to be a game-changer in wildlife management, and we’re going to see a lot more disease in the future,” Bill Porter, professor at Michigan State University, recently told Outdoor News.

As changes in climate continue, patterns of how viruses spread are shifting as well. A climate change-induced trend toward milder winters is one of the factors already confirmed in exacerbating some insect-carried diseases that affect humans, and researchers say it likely plays a role in the expansion of EHD as well. As with other diseases spread by insects, a hard frost that kills the insect carrier halts the spread of EHD. Let's hope for an early frost and a cold winter to make way for fewer midges to carry it next spring.

Katie Mast is an editorial intern at High Country News. Map courtesy New York Department of Environmental Conservation. Image of deer courtesy Michigan Department of Natural Resources.

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