Bluetongue, Biology

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Bluetongue (BT), a viral disease, transmitted by Culicoides midges, is an infectious, non-contagious disease of ruminants including sheep, goat and cattle, is characterized by congestion, oedema and haemorrhage in the affected animals. The virus belongs to the genus Orbivirus in the family Reoviridae. There are 24 different virus serotypes identified so far.

Epidemiology: Bluetongue was first reported by Hutcheon (1881) as epizootic catarrh fever in Africa. From Africa, the disease spread to American continent and then moved toward Europe and Indian subcontinent. In Indian subcontinent, BT was first reported from Pakistan in 1959. Subsequently, the first outbreak of BT in India was recorded by Sapre in 1964 among sheep and goats in Maharashatra state. Although indigenous sheep are affected but exotic and crossbreds are severely affected. In India, BT is noticed at the end of southeast monsoon and in the beginning of northeast monsoon when the breeding and population of culicoides are at peak. BT virus is transmitted by biological and mechanical vectors. Out of about 1000 species of Culicoides in the world, 7 have been recognized as biological vectors of BT. Sheep ked (Melophagus ovinus) act as mechanical vector (without viral multiplication). There are at least 24 serotypes of BT virus in the world. At least 14 of these have been reported from India.

Clinical signs: The symptoms consist of fever, lameness, oral lesions, emaciation and death. The disease is of seasonal occurrence. Virus enters the blood following the bite of insect and localizes in endothelium of oral cavity, nasal cavity, lamina and coronary band. Due to stagnation of blood in the vessels, the tongue appears cyanotic i.e. blue in color, giving the name as Blue tongue. Later on, necrosis leading to sloughing of mucosa resulting in ulcers on lateral borders of the tongue can occur.

Diagnosis: The diagnosis is based on the clinical signs, virus isolation and detection of antibodies by virus neutralization, complement fixation and fluorescent antibody tests.Treatment, prevention and control: There is serological evidence of the occurrence of this disease in almost all the states in India. Virus isolation has been done in some outbreaks. A live attenuated chicken embryo vaccine is available for the control of the disease and its application is recommended in endemic zones. Vector control is a very important programme in disease prevention. Work is in progress in India for the development of polyvalent killed vaccine.

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