Disease Facts - Bluetongue

Bluetongue virus causes disease in sheep, cattle, and other ruminants.

In sheep, bluetongue disease is characterised by a fever that may last for several days. The virus mainly affects small blood vessels and this can lead to reddening (hyperaemia) and swelling (oedema) of the lips, mouth, nasal linings and eyelids. Animals may have quickened breathing. Nasal discharges, excess salivation and frothing are common (pictured left). Hyperaemia and oedema may result in lameness. Animals can lose condition rapidly, including muscle degeneration.

Although all ruminant species can be infected by the bluetongue virus, clinical signs of the disease are usually restricted to domesticated breeds of sheep. Other animals such as goats and cattle rarely show any symptoms. Bluetongue may therefore spread into new areas without necessarily being noticed. However, a relatively high number of cattle have been affected during the current outbreak in Northern Europe.

There is no treatment for bluetongue. Prevention may be possible by vaccination and by controlling midge populations (with insecticides or, where practical, by control of breeding sites), but neither is totally successful.

It is spread by Culicoides midges.

Like malaria or yellow fever, bluetongue relies on a vector or carrier to spread from host to host. The vector of bluetongue is a small biting midge called Culicoides.

Culicoides are rather like the mosquito in that it they are attracted to animals and people and feed on their blood. If a midge is infected with bluetongue, then it will transmit the disease to its victim. Similarly if the victim is infected and the midge is not, then the blood meal can carry the virus to the midge.

Midges carry a variety of diseases, including African horse sickness virus, that affect both animals and man. The rod-shaped piece protects needle-like stylets that act as scissors to open a wound in the skin of the animal. Saliva and an anticoagulant are then injected into the wound and blood sucked up to provide the midge with its food.

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There are at least 24 different varieties (serotypes) of the bluetongue virus.

The bluetongue virus (pictured left) is made up of three layers. In the centre, looking here like a pan scourer, is the nucleic acid (the genetic blueprint) which carries all the information needed to make more copies of the virus. This is surrounded by a layer made up of one protein. Outside this is another layer made up of two different proteins. There is a further outer coat that has been omitted from the picture. The outer coat is lost when the core, pictured here, moves into an animal cell.

Bluetongue viruses are generally found within the latitudes 40°N to 35°S (from Southern Spain to Southern Africa), although during the current outbreak the virus spread as far as 53°N. Not all serotypes occur throughout the whole area and the distribution of the viruses can vary within a given area from year to year. The current outbreak of bluetongue in northern Europe has been identified as serotype 8.