By Vet Emma Stuart
Over the last few years, there have been increasing cases of Bluetongue reported in the UK.
Bluetongue is a viral infection that is not directly contagious between animals but is spread by infected midges. However, it can be transferred from a pregnant animal to its young and can also be spread by the movement of infected animals which then infect the local midge population. Once a midge becomes infected, it can transmit virus for the rest of its life (about a month).
There are 29 different serotypes (strains) of Bluetongue, with BTV-1, BTV-3, BTV-4, BTV-8 and BTV-12 now present in Europe. These numerous serotypes make vaccination difficult as there is limited cross-protection between strains. Bluetongue vaccines are available in the UK under a licence for BTV-3, although BTV-4 and BTV-8 vaccines are available commercially in Europe. There is currently no vaccine for BTV-12.
The serotype that has been identified in most Bluetongue outbreaks in the UK is BTV-3. The proximity of the Netherlands to the United Kingdom was always going to mean a high risk of incursion due to the spread of midges on the wind, as well as travel of infected animals, germplasm and midges through shipping ports.
For the 2024-25 vector season, there were 256 cases in Great Britain. A restricted area (controlling animal movements) was declared which covered much of the east and south of England, with Yorkshire (East Riding), Lincolnshire, Norfolk, Suffolk, Essex, Kent and East Sussex considered to be high-risk counties.
Midge populations peak mid-to late summer, and spreading of the disease can occur well into autumn if conditions are above 12°C to 15°C. Infected midges can be carried up to 100 miles over land. The high-risk period for Bluetongue is April to November although milder winters may mean animals are at risk for longer. There is also a risk that the virus has overwintered in the UK, either in infected animals, or having spread across the placenta in pregnant animals, or in midges that have survived the winter.
The symptoms of Bluetongue vary between different strains, but sheep are more likely to show outward clinical signs than cattle. Sores in the mouth, drooling, discharge from the eyes and nose, swelling of the lips, tongue, head, neck and around the coronary band are all commonly seen. Fever, lameness, respiratory distress and death are all features of the disease. Clinical signs in cattle can be more subtle, with lethargy, inappetence, milk drop and fertility issues being most common. Cattle can be infectious for several weeks even in the absence of clinical signs and can therefore infect many midges. If adult animals are infected while pregnant, abortions or stillbirths may occur, and lambs and calves can be born small, weak, deformed, blind or die within a few days of birth.
Bluetongue control relies on responsible sourcing of purchased animals, being aware of the clinical signs, maintaining good hygiene and biosecurity, limiting animal exposure to midge populations especially at dawn and dusk and vaccinating susceptible animals with an authorised vaccine that covers the important serotypes (BTV-3, BTV-8). Pre-movement testing of animals from high-risk areas is highly recommended.
There are now three approved Bluetongue vaccines which are available under a licence – Bultavo-3, Bluevac-3 and Syvazul. Onset of immunity is 3-4 weeks after completing the course. The vaccines do not prevent infection, but they reduce clinical signs and economic losses. However, they do not provide cross-protection against other serotypes, so animals will still be susceptible to other strains such as BTV-1, BTV-4, BTV-8 and BTV-12. BTV-8 appeared to be spreading north across Europe last year, while BTV-12 was identified for the first time in Great Britain in 2024, thus it remains to be seen what will happen with these serotypes in the 2025 season.