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Hemorrhagic Disease in white-tailed deer (often called "blue tongue") is caused by viruses transmitted by small, biting flies. The virus does not affect humans. Populations of these biting flies peak in summers with a lot of early rainfall followed by dry weather, creating ideal environmental conditions for their reproduction. Affected deer seek out water to drink or cool off, which is why many deer are found near streams or ponds. Hemorrhagic Disease is fatal to some deer, whereas others develop resistance and pass that on to their offspring. After the first hard frost kills the biting flies, reports of sick deer rapidly diminish and few are observed thereafter. Although elk have been exposed to these viruses, there is no evidence that elk are ever affected by the disease.
Hemorrhagic Disease is generally reported in late summer and early fall in Kentuck. Large-scale outbreaks typically occur every 5-10 years, although some reports of suspected cases are received by Kentucky Fish and Wildlife every year and localized outbreaks occur sporadically.
Clinical signs vary and range from rapid death to lingering effects in some individuals. Deer infected with these viruses may be depressed, feverish, emaciated (very thin), and remain near water in late summer or early fall.
Lesions vary depending on the virulence of the disease. They include edema (swelling) of the head, neck, tongue, conjunctiva, lungs, heart, rumen, and intestines. There may be ulcers in the dental pad, tongue, palate and stomach (rumen and omasum chambers in particular). More chronically-infected deer may have growth interruptions in the hooves or sloughing of hoof walls.
This disease is diagnosed from a blood sample, or from a refrigerated sample of spleen, lung, or lymph node tissue.
Hemorrhagic Disease is one of the most important diseases affecting white-tailed deer populations in Kentucky and throughout the Southeast. The disease affects deer throughout the Southeast and much of the rest of the country. It probably occurs at a low rate every year, but some years an outbreak may occur, resulting in a large number of deer deaths that are noticeable to the public. Outbreaks are more likely to occur in the northern and more mountainous areas of the range of the disease; however, they are still relatively uncommon.
Hemorrhagic Disease occurs frequently, but its severity and distribution are highly variable. Past occurrences have ranged from a few scattered cases every year or two, to dramatic regional or statewide outbreaks every 5-10 years, as in 1997, 2002 and 2003, 2007, 2012, and 2017. Death rates are usually are well below 25% of the population, and no deer population has ever been wiped out by Hemorrhagic Disease, even back in the 1950’s when it was first described and deer numbers were much lower!
There are no known treatments or controls for Hemorrhagic Disease.
At present there is little that can be done to prevent or control this disease. As with most diseases and parasites of white-tailed deer, impacts will be minimized in deer herds that are maintained at moderate to low densities. The best and only practical means of regulating deer populations is through recreational deer hunting, including the harvest of antlerless deer. Although die-offs of deer due to Hemorrhagic Disease often cause alarm, past experiences have shown that mortality will not decimate local deer populations and the outbreak will be curtailed by the onset of cold weather.
The viruses that cause Hemorrhagic Disease do not infect humans, and people will not contract these diseases from eating meat from infected animals. Deer with Hemorrhagic Disease may be more susceptible to other diseases, however, and consumption of a sick-looking animal is not advised.