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FAQ: Bluetongue

What is Bluetongue?

Bluetongue is an infectious disease that affects domestic (i.e. Cattle and sheep) and wild (i.e. Deer) ruminant animals. Cattle are the main reservoir but do not always show signs of disease. It is sheep that are most severely affected by the disease. The disease is characterised by inflammation of the mucus membranes around the mouth and nose, this inflammation causes the classic ‘blue tongue’ that gives the disease it’s name.

What causes Bluetongue disease?

Bluetongue is caused by the Bluetongue virus (BTV). The virus is a double stranded RNA virus and belongs to a group of viruses within the genus Orbivirus (Orbi- is latin for ‘ring’ which describes the virus’s characteristic shape when viewed with an electron microscope), which sits within the family of viruses known as the Reovirades (The name Reovirades is derived from the term Respiratory Enteric Orphan virus). BTV is related to other orbiviruses that also cause disease in animals like African horse sickness and epizootic hemorrhagic disease viruses. To date 24 distinct serotypes of bluetongue viruses have been identified, which have been shown to circulate throughout different geographical areas.

How is the virus transmitted?

The virus cannot be transmitted directly between animals. Biting midges from the family Culicoides carry the virus between animals, picking up the virus in infected animals in its blood meal and passing it to new animals (Culicoides are therefore known as the disease ‘Vector’). It is for this reason that Bluetongue outbreaks are intrinsically linked to the geographical areas of the world where Culicoides midges exist. The prevalence of the disease also peaks in late summer, which is associated with the peak in population of the midges.

Has global warming affected the distribution of Bluetongue disease?

Possibly. Culicoides imicola midges (the main BTV vector) only live in temperate climates, which is why bluetongue disease is seen in countries within the tropics and sub-tropics. Traditionally the cooler European continent has been seen as being ‘Bluetongue-Free’. However, since 1998 there have been outbreaks of at least one serotype of BTV on the continent every year, and in 2006, Dutch authorities reported the first ever case of BTV in northern Europe. In 2007 the first UK case of BTV was also reported. It has been suggested that this spread of the disease has been due to the northward expansion of Culicoides imicola, that has come about due to changes to the European climate in recent years.

What are the clinical signs of bluetongue disease?

The disease will present differently in different animals. It is possible that cattle will show no signs of clinical disease. Clinical signs have included: Nasal discharge, Swelling of the neck and head (especially around the eyes and muzzle), Conjunctivitis, Swelling inside and ulceration of the mouth, Swollen and/or reddened teats and vulva, tiredness, lameness, drooling, and fever. In sheep the disease is much more serious and clinical signs include: Eye and nasal discharges which become thick and crusty, Drooling as a result of swelling and/or ulcerations in the mouth, raised body temperature, Swelling of the neck and head (especially around the eyes and muzzle), Severe lameness affected sheep are reluctant to rise, Haemorrhages into or under the skin, Inflammation and pain at the junction of the skin and the horn of the foot – the coronary band, Respiratory problems – difficulty breathing, Sheep will ‘look sick, tired and lethargic’. Interestingly a ‘blue tongue’ is rarely a clinical sign of infection.

How is Bluetongue disease diagnosed?

The World organisation for animal health (OIE), will only confirm the disease when there is evidence that the virus is circulating between animals and vectors in an area. In regions where there is known transmission, BTV infection can be diagnosed based on the characteristic lesions in susceptible animals. The diagnosis of BTV infection is usually carried out using serology (detecting antibodies to the virus in infected animals), virus isolation or viral nucleic acid detection by the Polymerase chain reaction (PCR).

How can it be prevented?

The most effective preventive measure to protect livestock from BTV is through vaccination with a modified live virus vaccine. For the vaccine to be effective the vaccine must be specific for the serotype that is currently circulating in an area. There is currently no effective treatment available for infected animals, so control is based on quarantine, serological surveys of animals and vector control.

Are humans at risk from the disease?

No. The virus does not infect humans.

References

  • WILSON, A., DARPEL, K. & MELLOR, P. S. (2008) Where Does Bluetongue Virus Sleep in the Winter? PLoS Biology, 6, e210. URSE, B. V.,
  • MELLOR, P. S., ROGERS, D. J., SAMUEL, A. R., MERTENS, P. P. C. & BAYLIS, M. (2005) Climate change and the recent emergence of bluetongue in Europe. Nat Rev Micro, 3, 171-181. 

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