Epizootic Hemorrhagic Disease in Deer

By Daniel Barringer, Preserve Manager.

Photo: Devon Funt

Chester County last had an outbreak of Epizootic Hemorrhagic Disease (EHD) in deer in 2018. In 2024 there were also a somewhat unusual number of dead deer found in the area. Some of those have been tested and the Pennsylvania Game Commission has determined that a few of them had EHD.

EHD is caused by a virus that is spread by midges that are not native to Pennsylvania but blow in occasionally from the south. Outbreaks don’t normally happen often enough here for deer that exhibit some resistance to pass that trait on to subsequent generations, so this disease has a high mortality rate for deer in our area.

The midges breed in mud so the disease is sometimes worst in dry years, when deer congregate near the little remaining surface water and are exposed to them in the diminishing mud flats. This year was about as dry as any in memory. There is no vaccine that will inoculate deer. There is no cure once infected, though some exposed deer will survive. The disease ran its course until the vector—the midges—were killed by frost.

The extreme outbreak in 2018 resulted in much smaller deer populations and this was noticeable in the reduced amount of browse in the forest. There was distinctly more forest regeneration in the few years after that until deer numbers gradually rose again.

Humans are reportedly not affected by the virus, though people who hunt should avoid exposure to outwardly sick deer as there could be secondary pathogens to which they could be exposed. Deer exhibit symptoms within seven days of exposure, according to the PA Game Commission, and succumb 8 to 36 hours after that.

This year was not like 2018, but until frost we were monitoring deer populations around Crow’s Nest Preserve and requesting dead ones be tested. (Note that the deer in the photo above is just one that walked past our trail camera; there’s nothing to suggest it was a deer with EHD.)