In two months, we’ll start to hear hunters talking about epizootic hemorrhagic disease (EHD) killing deer. EHD is a sometimes fatal disease in ruminants, most importantly white-tailed deer. This is a viral disease and it is transmitted by a biting fly called a midge. We can’t get the disease, but there are several strains of EHD that deer get. An infected deer does not always show symptoms of the disease, but those that do will show symptoms within days of being bit by an infected midge.
There may be extensive internal bleeding especially along the digestive tract, from the mouth to the intestine. In some deer, there is swelling in the feet, and the hooves slough off, making walking painful. If the deer has a low grade infection, it can remain sick for weeks, but usually a deer that gets bit is sick right away and dies rapidly.
Deer get the disease in late August and September, but once a frost hits, the midges are gone, as is EHD. OK, but why is it then spreading further North into Pennsylvania, Michigan, and even New York? The answer is warmer weather. Warmer summers and warmer falls allow the midges to live and that creates the problem.
Because there is a high fever, deer are often found dead near water where they go to drink. Warmer weather also causes the deer to go to water to drink, and since the midges are there, deer get bit and then get the disease. However, deer do not spread it to other deer. A midge bit does it.
Another interesting bit of information is some deer that are exposed to EHD become immune and immune does can spread this immunity to their fawns, and the fawns to their progeny. Herd immunity is more common in the South and the further north you go the less immunity is found. My guess is immunity is found in a minority of deer in West Virginia.
Recent research has shown that drought is a big driver of EHD. Maybe drought doesn’t trigger EHD, but if it doesn’t trigger the disease it plays a role in the intensity of an outbreak. The recent study looked at the drought Index in counties in
23 eastern states from 2000-14. In this mix were states that have had EHD since 1980 and states such as Michigan, Pennsylvania and Wisconsin where EHD has only recently been documented and is now increasing.
What they did was compare the level of drought with the intensity of EHD. Results showed drought severity was a big predictor of EHD presence. However, the further south one goes, the less drought predicts the amount of EHD. This was probably because more deer are immune further south. What makes this immunity factor more complex is the fact that there are new strains of EHD entering the country from the Caribbean all the time, and just because a deer is immune to one strain doesn’t mean immunity to another. Thus, as a herd becomes immune to one strain, meaning few deer die, as soon as another strains enters the picture, more deer die until immunity builds up. It’s a cyclic phenomenon in the south.
Just when researchers felt they had EHD intensity related to drought, an outbreak occurred in 2017 in the Appalachian Plateau, which includes the western half of West Virginia. The perplexing thing about this outbreak is that it occurred during a time when there was no drought, and little herd immunity. It is not known what triggered this outbreak, but this outbreak shows that drought only indicates that an event is more likely to occur, not that it will. As I mentioned above, maybe drought is not the trigger for EHD, but rather a factor that causes the disease to be more intense. Only further research can tell us the answers.
And knowing what leads to EHD probably won’t help us prevent the disease. Once learned, it will just allow the DNR to let hunters and the public know what is probably going to happen. Will we see much EHD this fall in West Virginia? I checked the Drought Index for West Virginia for this summer and as of August 4, our area and the northern panhandle counties are listed as moderate drought. The rest of the state isn’t listed at all. Moderate drought sounds bad, but there are severe, extreme, and extreme categories listed above moderate, so I’m not sure if this will trigger midge bites or not. We’ve had some really hot July and early August days strung together, but then we get a little rain.
I Googled “Drought Index, United States, July 2020” and found there is no severe drought listed east of the Mississippi. Severe drought is found in the far west and southwest, so maybe we’ll get lucky and not see much EHD this fall.
Dr. Samuel is a retired wildlife professor from West Virginia University. His outdoor columns have appeared, and continue to appear, in Bowhunter magazine and the Whitetail Journal. If you have questions or comments on wildlife and conservation issues, email him at email@example.com.