Epizootic Hemorrhagic Disease in Deer: Can We Curb Devastating Outbreaks?

The first reports start circulating every year in August, like sketchy dispatches from a military front:

• "Seeing carcasses around water sources—bucks and does."
• "Localized mortality."
• "Hearing about big die-offs along our major rivers."

These are the leading indicators that whitetail deer are dying from epizootic hemorrhagic disease, better known as EHD and sometimes called "blue tongue." It’s a cruel virus spread by a biting midge that metamorphizes as an adult in the late summer, just as hunters are preparing for fall seasons, asking bosses and spouses for time off, and patterning the behavior of bucks they plan to target.

The disease is especially virulent in hot, dry summers following abnormally moist spring seasons. It can be so deadly that localized populations of deer can be nearly wiped out in especially intense epidemics. In my homeland of northeastern Montana, more than 90 percent of whitetails along the Milk River were killed in a single EHD outbreak in 2015. The herd has since rebounded, but for the following four years, it was rare to see a mature buck.

Last year North Dakota was hit hard by EHD, to the degree that the deer season was closed in several counties. Also last year, most counties in Ohio reported EHD mortalities, and in the southwestern region, hunters and landowners reported dozens of dead deer in scattered pastures and woodlots. In all, 24 states confirmed cases of EHD last year, according to the Southeastern Cooperative Wildlife Disease Study (SCWDS) at the University of Georgia.

The cruelty of EHD is both the speed at which it spreads and its unpredictability. It might kill hundreds of deer in a week along a single waterway, but 20 miles away, herds might be unaffected. There’s no vaccine for the disease, and it’s an equal-opportunity killer, affecting all age classes and sexes. While whitetails are by far the most afflicted species, it can also impact mule deer and pronghorn antelope. Interestingly, while EHD outbreaks are most common in the Southeast, the most deadly epidemics tend to occur in the Northern part of the whitetails’ range. That could be because Southern deer have developed some herd immunity to EHD.

While biologists and wildlife managers are getting a handle on the pathology of the virus, and generally can identify the factors that make an EHD epidemic likely, and even probable, they are still a long way from being able to contain the disease or minimize its deadly toll. Unfortunately, as the climate warms, as late summers become hotter and as midge-killing frosts occur later in the fall, EHD has the potential to become a more common part of our landscapes.

But simply stating the problem is neither satisfying nor helpful. Deer hunters have always been a solution-minded population, and there are a couple of points to consider as we look down the barrel of another hot, dry August. What can individual hunters and land managers do to minimize deer mortality? How can we examine, and potentially exploit, natural immunity to the virus?

UNDERSTANDING TRANSMISSION

The first step in combating EHD is understanding the conditions that create the most devastating outbreaks. The disease-carrying insect, called the Culicoides midge, is the very same "no-see-um" gnat that bites humans. While humans can’t contract the virus, it affects deer within days, inhibiting blood clotting, breaking down cellular walls and causing massive internal hemorrhaging. Deer can die within 48 hours of infection, and they commonly die in or near water. They have raging fevers and want to slake their insatiable thirst. It’s a horrible death to witness, with deer first becoming lethargic and careless, and finally drooling, helpless and hopeless as they bleed to death internally.

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The presence of water may be only incidental to the death of the deer, but it is an important clue to the life cycle of the disease. The midges spend their larval stages in mud under ponds and puddles where deer congregate during drought conditions. As these water sources dry up in late summer, the midge habitat is exposed and adult flies emerge to bite and infect deer. Importantly, the pathology can be interrupted by a soaking rain that covers shallow mud or by a cold snap that kills the adult midges.

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Given that disease pathway, efforts to discourage congregations of deer around shallow, muddy water sources can lessen mortality. Draining shallow impoundments by midsummer or steepening stream and pond banks to minimize the amount of muddy habitat can also lessen EHD severity, says Kip Adams, chief conservation officer for the National Deer Association.

The SCWDS, one of the primary investigators of EHD, notes that midges like shallow, murky, warm water with a muddy or silty bank. "Conversely, midge reproduction is minimal in clean, clear, deep water or where banks are vegetated or rocky," the group reports.

Adams says there’s no antidote, inoculant or potion that can protect deer from contracting the virus. "Some manufacturers claim their products protect against EHD, but that’s simply false advertising," he adds.

Infected deer can’t pass EHD on to other deer; they must be bitten by an infected midge. Efforts to reduce congregations of deer around feeders, 
mineral sites or artificial water sources can reduce mortality simply because those practices disperse deer in potentially midge-rich areas.

DISEASE IMMUNITY

As with most viral diseases, what doesn’t kill you makes you stronger, or at least less likely to get reinfected. That’s the case with deer that survive EHD. They build up resistance to the virus and can often withstand subsequent infections.

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The problem with that, especially in Northern and Western populations of whitetails, is that EHD is so virulent that it leaves few survivors. But researchers at SCWDS are finding that not all EHD is the same. They’ve detected it in three forms: peracute, acute and chronic. In its peracute form, EHD can kill deer in one to three days. Acute EHD can take up to a week to kill. Chronic EHD rarely kills its host. Instead, surviving deer grow sled-shaped hooves and infected does "may pass temporary immunity to their fawns through their milk," notes SCWDS.

What’s more, researchers at the University of Illinois College of Agriculture have detected subtle genetic differences that may protect some deer from EHD. While it’s hard to manage for genetic protection in wild herds, further research may help identify any behavioral cues that go along with genetic resistance.

The take-aways: Hope for cool, wet summers that keep ponds and streams full. Take solace in the fact that EHD outbreaks rarely strike the same population in consecutive years. And understand that even if EHD hits your area hard, deer are a resilient resource that rebounds in a few years. But, above all, join me in praying for an early frost.