Bluetongue is primarily a disease of sheep and cattle and is considered to be the most economically damaging disease of livestock worldwide. There are 24 serotypes of bluetongue virus (BTV), with differences in severity of disease. Serotype 8, has been identified in northern Europe, previously found in Nigeria and now in the UK since September 2007.
BTV is entirely dependent on female Culicoides for its transmission, but the species involved varies in different parts of the world. C. imicola is the main vector worldwide; C. dewulfii is the vector in northern Europe; C. obsoletus & C. pulicaris are the native British midges and are potential vectors.
The ability of midges to acquire and transmit infection (vector competence) is temperature dependant. Once infected, midges remain so life long, but the infection is not transmitted vertically. It is not certain how disease re-occurs year to year (either in infected animals or survival of infected midges). Transmission is very efficient; a single bite will infect and can kill a sheep.
Midges are strong fliers but also passively dispersed by wind (‘aerial plankton’). Infected midges arrived in the UK due to wind. They feed on domestic livestock but prefer cattle to sheep. Horses can be targets for feeding and horse muck suitable for breeding. They are most active dusk to dawn but rarely enter confined spaces such as buildings.
After a susceptible animal is bitten by an infected female midge there is a latent period of 2-4 days before animals are infective to other midges. Infected ruminants do not transmit infection directly to other animals, but there is potential for transmission via contaminated needles. Outbreaks tend to occur late summer and autumn and cease in colder weather.
Clinical signs are seen 6-9 days after infection and associated with damage to vascular endothelium resulting in haemorrhage, ischemia, inflammation and oedema.
- Lesions are in areas subject to mechanical trauma and abrasion (feet, mouth, eyes)
- Marked fever in sheep (up to 42°C) in early stages
- Mucosal inflammation and oedema
- Necrosis of the hard pad
- Lips and gums affected with petechiae and ecchymoses
- Conjunctivitis and lacrimation
- Salivation (tacky and stringy)
- Respiratory distress
- Cyanosis of tongue is an inconsistent feature
- Lameness due to coronitis
- Abortion, stillbirths, weak lambs, congenital defects
- Sub-clinical (especially in cattle).
Diagnosis is made by virus detection (EDTA blood) or serology (clotted blood). Clinically affected animals usually are positive to both. Currently half the outbreaks have been identified by serology from clinically normal animals. However the disease can be fatal or require euthanasia, particularly in animals with severe coronitis. In northern Europe, heavy losses have been reported this year, so there is the potential for more severe disease in the UK.
- Nursing, gentle handling, move minimally, soft food, shade, water
Movement restrictions have been put in place, to limit the spread of the disease, but vaccination appears necessary for control and prevention. There are disadvantages to using a live vaccine, so a serotype 8 vaccine is required (there is no cross protection between serotypes).