Epizootic Hemorrhagic Disease (EHD)* is common to white-tailed deer, but rarely affects other species. It occurs in the driest part of the year when conditions are just right for biting gnats, the carriers of the disease.
The disease is not contagious from one animal to another, and it is not transferable to humans. It comes from a virus carried by biting gnats that live in or near water and wet, muddy areas. It is transmitted to deer that congregate at such watering holes during warm, dry weather.
The spread of the disease is usually cut short with colder, wetter weather that spreads deer out and away from gnat-infested areas, or the first hard frost, which will kill the disease-carrying gnats. Since the incubation period for the disease is five to 10 days, afflicted deer may be observed up to a couple of weeks after frost.
Deer in the early stages of EHD may appear lethargic, disoriented, lame, or unresponsive to humans. As the disease progresses the deer may have bloody discharge from the nose, lesions or sores on the mouth, and swollen, blue tongues. They become emaciated because they stop eating. Sometimes they even stop drinking, although many die close to or in water.
Other wildlife, like mule deer, elk, and bighorn sheep could be exposed to the disease but are usually not stricken like white-tailed deer. No evidence of an outbreak in these species has been found at this time nor in past outbreaks in recent years.
Domestic livestock could also be exposed, although cattle and sheep are usually only carriers, not victims, of the "Bluetongue" virus, which is very similar to EHD.
Since deer hunting season usually doesn't open until well after the first killing frost, deer hunters usually don't see live, infected animals. However, WDFW recommends hunters avoid shooting and consuming deer that show any EHD symptoms, even though the disease cannot be transmitted to humans.
EHD typically strikes in late summer and early fall during an unusually warm, dry year when wildlife concentrates at whatever water is available. Major outbreaks among white-tailed deer have occurred mid-August to mid-October in 1999 in northeast Washington (Spokane, Stevens, Ferry, Okanogan counties), 1998 in southeast Washington along the Snake River, and 1992 in northeast Washington.
* ("Epizootic" means an animal epidemic. "Hemorrhagic" means to bleed or hemorrhage.)