Symptoms and Transmission of Bluetongue

Clinical symptoms

The Bluetongue virus causes disease in sheep, cattle, and other ruminants. Although all ruminant species can be infected by the bluetongue virus the clinical signs of the disease are usually restricted to domesticated breeds of sheep. Other animals such as goats and cattle rarely show any symptoms. However, a relatively high number of cattle have been affected during the current outbreak in Northern Europe.

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 which may result in lameness. The animals may also have quickened breathing and nasal discharges, excess salivation and frothing.

The clinical signs of bluetongue can be seen here.

As infected animals may not show clinical symptoms, Bluetongue Virus may spread into new areas without necessarily being noticed.

Transmission

Like malaria or yellow fever the Bluetongue virus relies on a vector or carrier to spread from host to host. The vector of the Bluetongue virus is a small biting midge of the genus 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.

Range

Bluetongue viruses are generally found within the latitudes 40°N to 35°S (from Southern Spain to Southern Africa) (map, from Purse et al., 2005), 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.

Treatment

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.

Purse, B. V., Mellor, P. S., Rogers, D. J., Samuel, A. R., Mertens, P.P. C., and Baylis, M. (2005). Climate change and the recent emergence of bluetongue in Europe. Nature Reviews Microbiology 3, 171-181.