Bluetongue in Cattle and Sheep
Bluetongue virus serotype 8 (BTV-8) has recently re-emerged in Central France despite being undetected in mainland EU for at least five years. The two key routes of entry into the UK are the importation of infected animals, or much more likely the windborne spread of infected midges. Studies show the south and south east coast of the UK to be more exposed to wind patterns that would be sufficient for midge movement. With the likely temperatures, and assuming that disease does spread to Northern France, DEFRA estimates the probability of incursion into the UK via infected midges at 5-10% (Low) in May, 33-60% (Medium) in July and 80% (High) in September, however it should be noted that not all incursions will lead to an outbreak.
The most recent posting based upon expert opinion is on the DEFRA website
Bluetongue is a viral disease affecting sheep, cattle, deer, goats and camelids (camels, llamas, alpacas, guanaco and vicuña). Although sheep are most severely affected, cattle are the main mammalian reservoir of the virus and are very important in the epidemiology of the disease. Bluetongue is a notifiable disease in the UK and suspected cases must be reported immediately to the Divisional Veterinary Manager at the local Animal Health Office.
Fig 1: Early clinical case
of BTV showing depressed appearance.
Fig 2: There is a swelling
of the face and ears and the
sheep often appears stiff and reluctant to move.
Fig 3: Facial oedema and
nasal discharge and excoriation
of BTV infected sheep.
The geographic distribution of bluetongue is dependent upon the Culicoides (midge) host which was until recently restricted to the African continent. Bluetongue serotype 8 (BTV-8) was first found in the Netherlands, Belgium, Luxembourg, Western Germany, and in parts of North Eastern France in the summer of 2006. The last outbreak of bluetongue in the UK was 2007.
Fig 4: Typical foot lesion found in BTV infected sheep involving coronitis and inflammation of the whole claw region but no formation of blisters.
Fig 5: Affected cattle are febrile (up to 40.0°C) and appear stiff due to swelling of the coronary band at the top of the hooves
Fig 6: There is a serous to mucopurulent nasal discharge
Bluetongue virus infection has an enormous impact on sheep production in many countries on the African continent and elsewhere. Losses result primarily from mortality, reduced production during protracted convalescence including poor wool growth, and reduced reproductive performance including temporary ram infertility. The average mortality rate for sheep in the Netherlands during 2006 was 5 per cent in the flocks affected. Bluetongue virus has been identified in Australia but is not associated with overt disease in sheep.
The virus is transmitted by biting midges of the genus Culicoides. Bluetongue virus cannot naturally be transmitted directly between animals. Peak midge populations occur during the late summer and autumn and therefore this is the time when Bluetongue is most commonly seen. It has been estimated that a midge can travel up to 1.5 - 2 km a day in a local area. However, if caught in suitable meteorological conditions midges can be carried much farther distances, especially over water masses of more than 200 km.
Bluetongue is characterised by changes to the mucous membranes of the mouth and nose, and the coronary band of the foot. Clinical signs are generally more severe in sheep but cattle can show signs of disease. A veterinary surgeon must be contacted by the farmer where large numbers of sheep or cattle present with lameness, high rectal temperatures, salivation, lacrimation and ocular and nasal discharges. Bluetongue is a notifiable disease in the UK.
The clinical signs of Bluetongue, which vary depending upon viral strain and sheep breed, follow an incubation period of four to 12 days. Usually, only a small percentage of sheep develop clinical signs. In extensively managed flocks, unexplained sudden deaths may be the first evidence of disease. Affected sheep have a fever (up to 42.0°C) and appear stiff and reluctant to move. They often adopt an arched back stance with the neck extended and the head held lowered. There is swelling of the face and ears, and also pulmonary oedema which may cause breathing difficulties. Erosions may appear on the lips progressing to ulcers. There is often profuse salivation, and a serous to mucopurulent nasal discharge. There may be reddening of the coronary band, and around the muzzle and mouth. The tongue may become swollen. Bluetongue infection during the breeding season may result in a large percentage of early embryonic losses with sheep returning to oestrus at irregular intervals.
High rectal temperature
Eye and nasal discharges
Drooling as a result of ulcerations in the mouth
Swelling of the mouth, head and neck. A blue tongue is rarely a clinical sign of infection
Lameness with inflammation at the junction of the skin and the coronary band
Deaths of sheep in a flock may reach as high as 70 per cent. Animals that survive the disease can lose condition with a reduction in meat and wool production.
The most important differential diagnosis is foot and mouth disease but here a larger percentage of the flock may be affected with high temperatures, and erosions within the mouth and on the coronary band and interdigital skin. Orf is easily distinguished from bluetongue. Clostridial disease such as bighead may cause sick sheep with swollen heads and high temperatures but only unvaccinated sheep, and few sheep are affected.
Fig 7: There is lacrimation but no obvious eye lesions.
Fig 8: There is oedema (swelling) of the face
Fig 9: Extensive teat erosions may develop in some cattle.
Affected cattle are febrile (up to 40.0°C) and appear stiff due to swelling of the coronary band at the top of the hooves (coronary band) and are very reluctant to move. There is a serous to mucopurulent nasal discharge and erosions on the muzzle with sloughing of the mucosa. There is lacrimation but no obvious eye lesions.
Fever up to (40.0ºC)
Swelling of the head and neck
Conjunctivitis (runny eyes)
Swelling in, and ulceration of the mouth
Saliva drooling out of the mouth
Once again, the most important differential diagnosis is foot and mouth disease where there is profuse salivation, lameness and high rectal temperatures rapidly spreading to affect all cattle on the premises. The other differential diagnoses are infectious bovine rhinotracheitis (group or herd) and malignant catarrhal fever in individual cattle.
Diagnosis and treatment
Diagnosis is based upon clinical signs, virus isolation and/or seroconversion to bluetongue virus. Treatment is limited to antibiotic therapy to control secondary bacterial infections.
Control and prevention
Control of bluetongue is very difficult because of the large number of potential hosts and virus serotypes. While control is aimed at keeping susceptible animals away from the vector this is not always practical. Control of the Culicoides vector can be attempted with pour-on insecticides but this is expensive and does not achieve total freedom from the midge.
The BTV8 vaccines available in the UK are killed vaccines. It is important to realize that here is no cross-protection between serotypes; vaccination against BTV8 will not protect against BTV1.
In many countries the timing of vaccination will depend upon local factors, in particular the occurrence of high-risk challenge periods from infected midges. In the UK the traditional mating time is from October onwards which may be later than peak Culicoides activity (late summer/autumn).
Vaccination is thought to have played a key role in keeping the UK free from circulating disease in 2008/09 and will provide the greatest security against loss should there be a re-introduction of Bluetongue this year. For cattle in particular it takes six weeks and two doses to build immunity.
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