Bluetongue disease

Bluetongue disease is a noncontagious, insect-borne, viral disease of ruminants, mainly sheep and less frequently cattle, yaks, goats, buffalo, deer, dromedaries, and antelope. It is caused by Bluetongue virus (BTV). The virus is transmitted by the midges Culicoides imicola, Culicoides variipennis, and other culicoids. In sheep, BTV causes an acute disease with high morbidity and mortality. BTV also infects goats, cattle, and other domestic animals, as well as wild ruminants (for example, blesbuck, white-tailed deer, elk, and pronghorn antelope). Major signs are high fever, excessive salivation, swelling of the face and tongue, and cyanosis of the tongue. Swelling of the lips and tongue gives the tongue its typical blue appearance, though this sign is confined to a minority of the animals. Nasal signs may be prominent, with nasal discharge and stertorous respiration. Some animals also develop foot lesions, beginning with coronitis, with consequent lameness. In sheep, this can lead to knee-walking. In cattle, constant changing of position of the feet gives bluetongue the nickname the dancing disease. Torsion of the neck (opisthotonos or torticollis) is observed in severely affected animals. Not all animals develop signs, but all those that do lose condition rapidly, and the sickest die within a week. For affected animals that do not die, recovery is very slow, lasting several months. The incubation period is 5–20 days, and all signs usually develop within a month. The mortality rate is normally low, but it is high in susceptible breeds of sheep. In Africa, local breeds of sheep may have no mortality, but in imported breeds, it may be up to 90%. In cattle, goats, and wild ruminants, infection is usually asymptomatic despite high virus levels in blood. Red deer are an exception, and in them the disease may be as acute as in sheep. Bluetongue is caused by the pathogenic virus, Bluetongue virus (BTV), of the genus Orbivirus, of the Reoviridae family. Twenty-seven serotypes are now recognised for this virus.