Why Deer with EHD Seek Water

Epizootic Hemorrhagic Disease (EHD) is a viral illness that is one of the most common and significant diseases affecting white-tailed deer in the United States. Transmitted by the bite of tiny midges (gnat-like insects often called "no-see-ums"), EHD outbreaks are seasonal, typically occurring in late summer and early fall when midge populations are at their peak. The disease's severity can vary, but it often leads to sudden and widespread deer deaths, especially in regions that are experiencing their first outbreak.

A key symptom of the disease is a high fever, which causes infected deer to seek out water sources. This is why many carcasses are found near ponds, creeks, and rivers. The water provides a cool place for the deer to lie down and attempt to regulate their dangerously high body temperature. In addition to a fever, other symptoms include a swollen head, neck, tongue, and eyelids, a bluish or rosy color to the tongue, excessive drooling, and a loss of fear of humans. The disease is not contagious from deer to deer, and it is not a threat to humans, domestic animals, or pets. The spread of EHD abruptly ends with the first hard frost, which kills the midge vectors.

Dr. Bronson Strickland of Wildlife Investments was recently on a collaboration with Mossberg and he discusses why deer hunters often find deer in water or near water sources when deer contract EHD. 

EHD is often confused with Blue Tongue Disease (BT), and for good reason—they are caused by two different but closely related viruses (EHD virus and BT virus) and produce nearly identical symptoms in deer. In fact, many people use the general term "Hemorrhagic Disease" to refer to either illness because the clinical signs are so similar that only a laboratory test can distinguish between them. Both are transmitted by the same type of midge, and both lead to internal bleeding, fever, and swelling. However, a key difference lies in the primary species they affect and their distribution. EHD is a more common and deadly disease in white-tailed deer, while the BT virus is historically known for causing severe disease and death in domestic sheep and cattle, though it can also affect deer.

In contrast, EHD is fundamentally different from Chronic Wasting Disease (CWD). While EHD is a seasonal viral disease transmitted by an insect vector, CWD is a progressive, fatal neurological disease caused by an infectious protein called a prion. Unlike EHD, CWD is not spread by an insect. Instead, it is highly contagious among deer, elk, and moose and is spread through direct animal-to-animal contact or through a contaminated environment (e.g., prions in the soil). CWD is a slow-acting disease; a deer can be infected for years before showing any clinical signs, such as extreme weight loss ("wasting"), stumbling, and abnormal behavior. While EHD outbreaks are often acute and result in mass die-offs over a short period, CWD is a persistent, long-term threat that can lead to a slow decline in herd health and population over time.