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
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
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
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
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
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
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
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.