Bluetongue Disease (BTD) is a viral illness primarily affecting ruminant animals, both wild and domestic. This infectious, non-contagious condition is found globally, impacting various species including sheep, goats, cattle, and deer. BTD is an important concern for wildlife health, particularly given its effects on deer populations.
How Deer Contract the Disease
Bluetongue is caused by the Bluetongue Virus (BTV), which belongs to the Orbivirus genus. The main way deer become infected is through the bite of tiny, infected insects called Culicoides midges, often known as “no-see-ums.” These midges acquire the virus by feeding on the blood of infected animals.
Once a midge bites an infected animal, the virus replicates within the midge before it can be transmitted to another animal. Deer cannot directly transmit the disease to other deer through contact. Outbreaks typically occur seasonally, most often in late summer and early fall, aligning with periods of high midge activity.
Recognizing Infected Deer
Deer infected with Bluetongue Virus often display clinical signs, similar to those seen in Epizootic Hemorrhagic Disease (EHD). Affected deer may develop a fever, accompanied by swelling of the face, lips, tongue, neck, and eyelids. Lameness can also occur due to inflammation of the hoof coronets or joint issues.
Internal hemorrhages contribute to the development of mouth lesions, ulcers, and sores on the nose, gums, or dental pads. Excessive salivation, sometimes tinged with blood, is another common symptom. While named “Bluetongue,” the characteristic blue discoloration of the tongue is rare in deer. Symptoms typically appear rapidly, within days of being bitten, and the outcome is often fatal. Sick deer may be found near water sources, seeking relief from high fevers and dehydration.
Impact on Deer Populations
Bluetongue outbreaks can lead to substantial mortality within localized deer populations, particularly affecting white-tailed deer. Despite these localized impacts, the disease generally does not cause widespread or long-term declines across an entire regional deer population. The severity of an outbreak is influenced by several factors, including environmental conditions.
For example, drought conditions can concentrate deer around limited water sources, which also tend to be prime breeding grounds for midges, thereby increasing transmission rates. The specific strain of the Bluetongue Virus and the immune status of the deer population also determine the disease’s impact. In some localized white-tailed deer populations, case fatality rates can reach as high as 80-90%.
Safety for Humans and Other Animals
Bluetongue Disease is not transmissible to humans. There is no public health risk associated with handling infected deer or consuming their meat. However, it is prudent to follow local hunting regulations and exercise personal judgment when encountering sick animals.
While deer are highly susceptible, other domestic ruminants such as cattle, sheep, and goats can also become infected. Cattle and goats often show milder or no clinical signs, even when carrying the virus. Sheep, however, can be severely affected. Infected cattle, even without symptoms, can serve as a source of the virus for midges.