Epizootic Hemorrhagic Disease (EHD) is a common viral illness caused by a virus from the Orbivirus genus. EHD primarily affects white-tailed deer across North America, though other cervids like mule deer and elk can also be susceptible. EHD outbreaks lead to substantial, localized mortality events, making the disease a major focus in wildlife management. Understanding how the virus moves through a population is the first step in recognizing its impact.
How EHD Spreads Among Deer
EHD is not a contagious disease that passes directly from one deer to another through contact, shared food, or water. The virus requires a biological intermediary to move between animal hosts. An infected deer cannot pass the virus to a healthy one by touching noses or feeding in the same area.
The virus is spread exclusively by the bite of the Culicoides midge, a tiny flying arthropod often called a “no-see-um” or gnat. These midges act as a vector, picking up the virus when they feed on the blood of an infected deer. The virus replicates inside the midge before the insect transmits it to a new, susceptible deer during a subsequent blood meal.
The cycle of infection depends on the presence and activity of this biting midge. Midge populations thrive in warm, moist environments and often breed in the muddy edges of ponds, streams, or wet soil areas. When environmental conditions, such as drought, concentrate deer and midges near limited water sources, the transmission rate increases rapidly, leading to outbreaks.
Recognizing EHD Symptoms and Seasonality
Clinical signs of EHD typically appear five to ten days after a deer is bitten by an infected midge. The virus attacks the lining of blood vessels, causing internal bleeding and swelling. Infected deer often exhibit a high fever, which is why they are frequently observed near or in water sources, attempting to cool their bodies.
Acute signs include lethargy, difficulty breathing, and dramatic swelling of the head, neck, eyelids, or tongue. Deer may also lose their natural fear of humans and become unresponsive or disoriented. Death can occur quickly, sometimes within 8 to 36 hours of the onset of severe symptoms.
Deer that survive the acute phase may develop a chronic form of the disease, characterized by growth interruptions and breaks in the hooves. This damage can cause severe lameness, affecting the animal’s ability to move and forage. EHD outbreaks are distinctly seasonal, peaking in late summer and early fall (August through October) when midge populations are most active. The outbreaks cease abruptly after the first hard frost, which kills off the adult midges and halts the transmission cycle.
Can EHD Affect Humans, Pets, or Livestock?
Epizootic Hemorrhagic Disease is not a zoonotic illness, meaning there is no known risk of transmission to humans. People cannot contract EHD from an infected deer, handling venison, or being bitten by an infected midge. EHD is also generally considered safe for common domestic pets like dogs and cats.
While EHD is closely related to Bluetongue virus (BT), which affects livestock, the EHD virus rarely causes significant illness in animals other than deer. Cattle and sheep can be exposed to the virus, but they typically show very mild or no clinical signs. Individuals should report any sightings of sick or deceased deer to their local state wildlife agency. This reporting helps authorities monitor the extent and spread of the outbreak.