Epizootic Hemorrhagic Disease (EHD) is a viral disease that significantly impacts deer populations, particularly white-tailed deer. It is caused by the Epizootic Hemorrhagic Disease virus (EHDV), an Orbivirus. Characterized by internal hemorrhages, this infectious disease can be fatal for deer. Outbreaks typically occur seasonally, most often in late summer and early fall, coinciding with specific environmental conditions. The disease can lead to substantial mortality events in affected deer herds.
The Role of Biting Midges
EHD is transmitted exclusively through the bite of infected biting midges, tiny flies often called “no-see-ums.” These insects, primarily Culicoides species, act as vectors for the EHD virus. A female midge acquires the virus by feeding on an infected deer. The virus then replicates within the midge’s body, specifically in its midgut, before disseminating to its salivary glands. This process, known as extrinsic incubation, can take several days, and its speed is influenced by temperature.
Once the virus has replicated sufficiently, the midge can transmit EHDV to a healthy deer during a subsequent blood meal. As few as four infected midge bites can be sufficient to transmit the disease. Midges are highly active during warm weather, which facilitates the transmission cycle. This vector-borne transmission means direct contact between deer, such as nose-to-nose interaction or sharing food, does not spread the disease. The midge life cycle is therefore directly linked to the spread of EHD among deer.
Environmental Conditions and Disease Spread
Environmental factors significantly influence EHD prevalence and spread by affecting midge populations and deer behavior. Warm temperatures are crucial for Culicoides midge breeding and survival; adults emerge only when temperatures reach approximately 51.6 degrees Fahrenheit. Extended warm weather, especially in late summer, prolongs the midge breeding season and accelerates virus replication within the insects, leading to an exponential increase in midge abundance.
Stagnant water bodies and muddy areas are ideal breeding grounds for midge larvae. Drought conditions can exacerbate outbreaks by causing larger water sources to shrink, leaving numerous shallow, warm, and often muddy puddles. These concentrated water sources also force deer to congregate in smaller areas to drink and cool off, increasing their exposure to infected midges. This combination of increased midge populations and concentrated deer activity intensifies EHD transmission. Outbreaks typically cease with the onset of a hard frost, which kills adult midges and ends the transmission cycle.
Understanding What Doesn’t Spread EHD
EHD is not transmitted through direct contact between deer. A sick deer cannot directly infect another through shared food, water, or physical interaction. The virus requires the biting midge as an intermediary. The virus does not persist in the environment in a way that allows direct deer-to-deer transmission.
EHD is not a threat to human health. There is no evidence humans can contract EHD by midge bites or by consuming venison from an infected deer. While some EHD strains can infect livestock like cattle or sheep, these animals typically experience mild or inapparent infections, rarely showing severe symptoms or dying. Domestic pets like dogs and cats are not at risk from EHD.