Hemorrhagic Disease: Deadly Deer-Killing Viruses Push North

EHD is spreading north and infecting more deer herds by the season. Can anything be done to stop it?
Hemorrhagic Disease: Deadly Deer-Killing Viruses Push North

As Scott Moran headed home after guiding elk bowhunters in Colorado in mid-September 2012, his thoughts shifted to bowhunting early-season whitetails in southeastern Wisconsin. He had good reason to be eager about his whitetail hunts. He and several neighbors east of Madison control a 1,200-acre square, much of it woodlots and wooded marsh surrounded by agricultural fields. Further, as a member of the Quality Deer Management Association for more than a decade, Moran works with his neighbors to maximize the deer herd’s potential by controlling the doe population and passing up younger bucks.

Soon after reaching home, however, Moran got bad news from a neighbor — he had found dead deer in and around a nearby pond.

Could the deer have died from a hemorrhagic disease? They knew dead deer near water in late summer and early fall often means bluetongue or epizootic hemorrhagic disease. They had also heard forecasts that 2012 could be a record-setting year for these diseases, which are so closely related that experts simply lump them as hemorrhagic diseases, or “HD” for short. Then again, they’d never seen HD in their area. Wisconsin’s only known brush with HD viruses occurred in 2002 when at least 14 deer died of it about 60 miles west of them. Still, the signs looked ominous. An unusually mild winter in 2011-2012 was followed by a hot, dry summer across the Midwest. Such conditions often produce swarms of biting midges, gnats and black flies, which carry and pass HD viruses to deer. Moran could only hope the dead deer were a fluke, and that Wisconsin had again dodged HD, much as it sidestepped 2007’s record HD outbreaks that stretched from the Missouri River corridor all the way to the southeastern United States.

To learn if the problem went beyond the one pond’s dead deer, Moran and his friends walked crosswind routes to search the property. They turned into the wind each time they detected foul odors and followed the stench to its source. Eventually, they found and logged 82 rotting deer on the 1,200 acres, of which 39 were antlered bucks, some in the 3½- to 5 1/2-year-old age groups, with antlers scoring in the 150s.

2012: A Devastating Year

Moran didn’t know it at the time, but he was witnessing the nation’s second record-breaking HD scourge in five years. Nothing rivaling the 2007 and 2012 outbreaks had occurred since the HD viruses were first identified in 1955, a finding that helped launch the nationally recognized Southeastern Cooperative Wildlife Disease Study (SCWDS) at the University of Georgia in 1957. By the time early November frosts killed off the biting insects in 2012, Wisconsin documented at least 350 HD-killed deer. Although that was 25 times worse than its first outbreak a decade before, it was minor compared to Michigan, which recorded 15,000 HD deaths — 15 times more than its previous high of 1,000 in 2010. Further, Missouri lost at least 10,200 deer to HD in 2012, and Illinois, at least 2,100. In all, 28 states confirmed HD-related deer deaths in 2012, and Alabama and South Carolina also reported HD-suspected deaths.

Professor David E. Stallknecht at SCWDS said body counts aren’t accurate enough to proclaim if 2012 was worse than the 2007 outbreak, which documented HD in 31 states. “The real news is that the North had outbreaks like this five years apart,” Stallknecht said.

Clearly, things are changing for HD, and those changes are forcing some wildlife agencies to adjust their deer-management programs. Until recently, HD was something most states viewed as spotty, small-scale die-offs with few regional implications. The outbreaks typically start in mid- to late July, peak in September, fade as temperatures drop, and eventually end with the first killer frost. They also tend to run in two- to three-year cycles where they’ve been long established, such as Georgia.

But beyond that, increases in the diseases’ spread and prevalence caused several states to reduce antlerless harvest prescriptions or license sales to compensate for HD losses. In fact, North Dakota suspended license sales entirely in three deer management units, and South Dakota refunded deer hunting licenses and removed unsold licenses for several areas decimated by HD. Further, hunters in areas hit by HD often pass up antlerless deer in hopes of quickly rebuilding local herds.

Maps showing HD cases track such changes. From 1957 to 1980, HD appeared as an intermittent swath from eastern Montana to eastern Texas and eastern Virginia. Other than thin bands in Iowa and central Illinois, HD seldom appeared north of Missouri to Maryland. That swath started filling in and fattening during the 1990s, and then looked like someone used a paint roller rather than a pen to mark HD cases from 2000 to 2009. That’s when the disease rolled across southern portions of Illinois, Indiana, Ohio and southwestern Pennsylvania, with random shots into Wisconsin, Michigan and New York.

2008: Bigger Changes Yet

In fact, the disease changed dramatically in 2008 when a large section of Eastern Europe suffered an outbreak, causing widespread death and crippling in livestock. HD infects some livestock in the United States, but it primarily affects whitetail deer and, to a lesser extent, elk, mule deer, pronghorn, bighorn sheep, and domestic elk and deer in fenced facilities.

Since 2008, HD has also become an annual disease for many parts of the Midwest. Michigan rarely reported an HD case between 1955 and 2006, but has reported cases each year since 2008, including as many as 400 in 2013. Further, eastern Pennsylvania, southeastern New York and northern New Jersey now report HD more often.

What’s happening? For one, the disease itself appears to be changing. The number of HD strains has jumped. Before 2004, researchers knew of only two types of epizootic hemorrhagic disease (EHD) viruses and five bluetongue viruses in North America. Since then, researchers report at least six EHD strains and 24 bluetongue strains. They identified one of those new EHD strains in 2006. The EHD-6 strain has been found every year since, and across a broad geographic range. Stallknecht said EHD-6 also “re-assorted” itself with the traditional EHD-2 virus to create yet another new strain. Further, all of these 30-some viruses have killed deer. The EHD-6 strain proved particularly lethal in 2012, turning up as the primary deer killer in Wisconsin and Michigan, as well as Louisiana and Alabama. The new strain was also found in Indiana, Iowa, Missouri, northern Virginia, Arkansas and northern Florida/southern Georgia. “EHD-6 is the new kid on the block, and it’s really interesting because it was the predominant virus in Wisconsin and Michigan, areas where we don’t normally see these viruses; and in a part of the Gulf Coast that generally does not see a lot of diseases,” Stallknecht said.

Further, researchers are no longer confident the Culicoides family of midges is the only biting insect capable of spreading HD. Eastern Europe’s outbreak was caused by a new bluetongue strain, but that region isn’t supposed to have any biting insects from the Culicoides family. Or it’s possible this family of midges is expanding its range. “What’s really interesting about Europe’s 2008 outbreak is that if you look at a map of the United States’ 2007 outbreak, the scale of the outbreak on both continents is exactly the same,” Stallknecht said in February when addressing the 2014 Southeast Deer Study Group meeting in Athens, Georgia. “This is a complex system. We have a lot of new viruses entering the system, and we apparently have vectors (biting insects) we didn’t know could transmit these viruses. And so now we have at-risk (deer) populations being infected in areas where HD has not historically occurred.”

Some Survive, Most Don’t

Deer in areas with little or no history of HD are especially vulnerable because they don’t have antibodies to fight the diseases. That might explain why deer in a swath from Montana to Nebraska to Missouri, Illinois, Kentucky, Pennsylvania and northward typically get HD and die within two weeks. Meanwhile, Texas deer shake off HD with little effort, while deer in coastal areas from Louisiana to eastern Virginia get sick but often survive. In fact, some states suffer die-offs in one area but only illness in others. Whitetails in northern Georgia typically die while those farther south struggle but survive with hoof lesions and other scars. Likewise, deer in eastern Kansas typically die during HD outbreaks while those to the west survive. Researchers, however, believe HD attacks all deer equally, no matter their age or sex.

Even so, hunters like Moran worry that bucks could be especially vulnerable. After all, 48 percent of the dead deer he and his friends found were antlered bucks, even though the Wisconsin DNR estimates the herd generally has one antlered buck per four antlerless deer (adult doe, yearling doe, doe fawn and buck fawn). Moran notes that HD kicks in each year while antlers are growing beneath thin-skinned, lightly-haired and blood-rich velvet, making them easy targets for biting flies.

Tom Cooley, a biologist and pathologist with the Michigan Department of Natural Resources, said he knows of no research showing adult bucks are at higher risk. Neither does Stallknecht. “We hear that from time to time, but I don’t know of any real proof of it,” Stallknecht said. “The antler-velvet idea makes sense, but (most searches follow) a relatively random method. From the reported information we get, I always worry that the public is more likely to report a dead buck than a doe.”

In all cases, though, HD-caused deaths are not pretty. They’re called hemorrhagic diseases for a reason. Stricken deer bleed from their tongue and from all internal organs. They also build up fluids in the sac around their heart, as well as under their skin. They become feverish and seek water to cool off. Eventually they go into shock, become prostrate and die, often in or near the water.

One question researchers are trying to answer is whether HD is changing on its own or if recent environmental factors simply unleash disease factors not previously known. After all, recent outbreaks seem triggered by a perfect storm of factors. Mild winters kill fewer midge eggs and larvae in the soil, enabling more midges to become biting adults, especially if they find favorable breeding conditions in summer. And that’s what they often found in 2007 and 2012: wet, low-lying areas that turned muddy as heat and drought dropped water levels. Further, when warm weather stretched into fall, more midges lived to spread disease.

Meanwhile, deer often concentrate near dwindling water sources, further increasing their vulnerability. Stallknecht said U.S. maps comparing drought conditions and HD outbreaks in 2007 and 2012 showed similar traits. “Several studies have looked at weather factors, and moisture and rainfall always come up as important factors in the outbreaks,” Stallknecht said. “In 2007, the drought was centered pretty much over Tennessee and Kentucky, and so was EHD. In 2012, the drought centered in Missouri, and so did EHD. And when you look at maps showing the counties reporting drought, you see a pretty good relation to EHD, especially after 2004.”


Can anything be done about HD? And how will it affect future deer seasons? Stallknecht said even after 55 years of study, it remains impossible to predict outbreaks with certainty. “There are simply too many random factors that influence the disease,” he said. “We don’t suggest anyone run out and change their management strategies, but keep your eyes open. EHD’s expansion to the North is probably the biggest issue we face. It’s expanding into ‘naïve’ deer populations. They’ll probably continue to have big (die-offs) and more of them. Whether that has long-term effects on deer populations is debatable, but there will be impacts.”

Mark Ruder, a research veterinarian with the USDA’s Agricultural Research Service, agrees the disease requires close monitoring. “We’re in a changing environment, and we don’t know if … there could be other disease threats that move in and tip the scale,” he said in an article by The Wildlife Society News.

Meanwhile, hunters like Moran hope their areas won’t be hit again. “The deer are coming back to some degree now,” Moran said. “We definitely have deer around, but the number of older deer is down. It’s kind of like a good fishing hole. If you have a good area and you remove the big ones, other fish start to fill it back in. It doesn’t happen overnight, though. It will take some time to rebuild our herd’s structure.”


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