The disease has been observed in Australia, the USA, Africa, the Middle East and Asia. It is transmitted by biting midges, and major symptoms are high fever, excessive salivation, swelling of the face and tongue and cyanosis of the tongue. The incubation period is about 1 week, and all symptoms usually develop within one month. Swelling of the lips and tongue due to the viral infection gives the tongue its typical blue appearance, though this sign is confined to only a minority of the animals. The disease may occasionally be lethal.
On the other hand, the tongues of human patients with some types of heart diseases may have a blue color. But this is a completely irrelevant condition with the disease described above. The cause of this condition is inadequate blood and oxygen supply to tissues by a deficiently working heart.