Bluetongue is an insect–transmitted, viral disease of sheep, cattle, goats, and other ruminants, such as white–tailed deer and pronghorn. It is particularly damaging in sheep; half the sheep in an infected flock may die. In cattle and goats, however, bluetongue viruses cause very mild, self–limiting infections with only minor clinical consequences. A bluetongue virus infection causes inflammation, swelling, and hemorrhage of the mucous membranes of the mouth, nose, and tongue.
Inflammation and soreness of the feet also are associated with bluetongue. In sheep, the tongue and mucous membranes of the mouth become swollen, hemorrhagic, and may look red or dirty blue in color, thus giving the disease its name. Bluetongue viruses are spread from animal to animal by biting gnats. In the United States, the disease is most prevalent in the southern and southwestern States.
Animals cannot directly contact the disease from other animals. The bluetongue vaccine for sheep is only effective against certain serotypes, will not prevent the disease, and may cause adverse reactions. Pregnant ewes should not be vaccinated.