In 2021, Editor-In-Chief Christian Berg hunted an area of eastern Montana that had been devastated by a late-summer EHD outbreak that likely killed more than half of the area’s whitetails, including this fine 10-point buck.
August 20, 2024
By Dr. Dave Samuel
The dog days of summer are nearly upon us. That means plenty of heat and humidity, bucks with velvet-covered antlers feeding care-free in farm fields and a growing sense of anticipation for cooler days and archery seasons soon to come.
Unfortunately, in many parts of whitetail country, the period from mid-summer through early fall can also mean localized outbreaks of Epizootic Hemorrhagic Disease (EHD), a cyclic disease transmitted to whitetails by tiny insects called midges, of which there are many species. Marked by high fever that causes internal bleeding, EHD kills many infected deer within 24 hours of symptoms appearing. And in areas where outbreaks are severe, a very high percentage of the local deer herd can be wiped out in a very short amount of time.
The Perfect Storm EHD outbreaks have long been associated with periods of drought, and a recently revealed scientific study offers new insights on that connection. The study, “Climate Influenced Disease Expansion of Hemorrhagic Disease,” was conducted by researchers from Michigan State University, University of Georgia and the West Virginia Division of Natural Resources and presented in February at the Southeast Deer Study Group’s annual meeting. The study examined 181 EHD outbreaks in my home state of West Virginia between 1981 and 2019 in an effort to determine what kind of weather pattern might most predictably produce an EHD outbreak. According to the researchers’ analysis, EHD was most likely to occur in years when there was higher than average rainfall in July followed by dry conditions in August.
Looking at the way EHD is spread, the study’s findings make perfect sense, because the habitat conditions such a weather pattern creates are ideal for midges. Midges spend the winter as larvae in the mud of shallow puddles, ponds and similar terrain. Temperature is the trigger for midge larvae to turn into adults, which fly and bite deer and other animals. Adult midges don’t emerge until temperatures reach 51.6 degrees Fahrenheit, and before midges emerge, the temperature cannot have fallen below 31 degrees for at least seven days.
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The bottom line is this: midges are most active in warm weather, and the combination of a wet July followed by a hot, dry August is the perfect storm when it comes to EHD transmission. As water sources evaporate in the heat, deer concentrate around those water sources that remain. Meanwhile, the constant shrinkage of those water sources continually exposes fresh mud that is a prime breeding ground for midges.
The result is more midges and more deer getting bit by them, ultimately becoming infected with the EHD virus midges carry. Infected deer show signs of the virus within seven days of being bitten. Deer become lethargic and stop eating. They become weak, have bloody diarrhea and show excessive salivation. Sometimes, they have blueish-colored tongues. They have a rapid pulse and high fever, which is why deer that die from EHD are frequently found dead in or near water, where they go to slake their thirst and/or attempt to reduce their body temperature. Once an EHD-infected deer is obviously sick to an observer, it will likely die within a day.
Incidentally, this is why EHD outbreaks in areas with very high deer densities can be so devastating, both in terms of the number of deer killed by the disease and how the event is perceived by hunters. Although high deer density does not cause EHD outbreaks, areas with high deer numbers will of course result in large numbers of deer in close proximity to limited water sources. That, in turn, can result in an extremely rapid spread of the virus and sometimes results in situations where dozens of deer carcasses litter the edges of ponds, creeks and rivers.
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Survival, Immunity & More There are many strains of the EHD virus, with new ones being identified all the time. The most common strain of EHD identified in recent years has been EHDV-2. This strain has been responsible for numerous EHD outbreaks across the Midwest.
Although EHD kills many infected deer, not all die. In fact, a recent Indiana study showed that 20 percent of the deer that got EHD not only survived but developed bloodborne antibodies that provide immunity. Another study conducted in 2023 in Illinois showed that when the EHD virus enters immune cells in some deer, it triggers inflammation that leads to cellular changes. Researchers found the EHD virus caused a mutation in receptor cells that were significantly more common in uninfected deer. So, after an outbreak, you can sample living deer and those that have this mutation in receptor cells have been bitten but survived. For some reason, deer with these mutated cells remain less susceptible to future EHD outbreaks.
However, it is important to note that just because a deer is immune to one strain of EHD doesn’t mean it isn’t susceptible to another. It’s also widely accepted among EHD researchers that some strains are more deadly than others.
Further, EHD outbreaks have been documented with increasing frequency in the Northeast and portions of the Upper Midwest where the disease was previously uncommon. In these areas, the impact of EHD on local deer herds can be exacerbated since there are few, if any, previously infected deer with immunity. The reasons for EHD’s move into new areas is not entirely understood, though many researchers speculate that warmer than average summertime temperatures and increased precipitation — perhaps caused by climate change — could be at least partly to blame.
Obviously, we can’t control temperature or rainfall, but thanks to the West Virginia study we at least know some indicators explaining why outbreaks occur. If we have a July with lots of rain, followed by an August with low rainfall, we will probably see some deer die from EHD.
One good piece of news is that the strains of EHD identified in the United States thus far are not overly virulent, and in areas where outbreaks do occur, the local deer population typically recovers within three to five years. However, those who study EHD at the Southeastern Cooperative Wildlife Disease Lab at the University of Georgia warn that it is possible a stronger strain of the virus from another part of the world could show up here unexpectedly, potentially with devastating impact.
As with all wildlife diseases caused by viruses, there are many complicated and changing factors involved, leaving no simple answer for wildlife managers or hunters. Bowhunters who live in places where EHD outbreaks occur will need to be patient and know the deer herd will rebound in time. Until then, it’s probably best to plan hunts where EHD has not occurred recently.
