Bluetongue virus – your questions answered
Get the key facts on bluetongue virus (BTV), including how it spreads, vaccination, fertility impacts, vaccine protection and reporting requirements. For advice specific to your animals, speak to your vet, as the NFU cannot provide individual veterinary guidance.
How BTV spreads
Bluetongue virus is mainly spread by certain species of biting midge (Culicoides), many of which are found across Great Britain.
Female midges need a blood meal to produce and lay eggs. They become reproductively active when the average ambient temperature reaches about 5°C. If an uninfected midge bites an infected ruminant, it can pick up the virus in that blood meal.
When average ambient temperatures reach about 12-15°C, the virus can replicate in the midge. It may take 4-20 days to incubate and reach the salivary glands. The infected midge can then transfer the virus in a single bite to a susceptible animal. The ruminant host becomes infective after about 2-4 days, allowing the cycle to repeat.
Once a midge acquires BTV, it remains infected for life. Individual midges are short-lived, but their numbers and reproductive rate can be high in warmer conditions.
The midge breeding cycle at a glance:
- Midges are mainly active from April to November.
- Temperature, rainfall, wind speed and direction can influence transmission.
- Topography and the proximity and density of neighbouring farms with susceptible animals can affect how quickly and how far disease spreads.
There are other ways that BTV can spread, such as through blood and germinal products, including semen or embryos, and through the movement of infected animals.
Infected pregnant animals can, in some circumstances, transmit the virus to unborn offspring. After birth, infected offspring could provide a source of virus to uninfected midges.
Key points
- BTV does not pose a risk to people or the food chain.
- Biting midges are the main route of spread.
- Vaccinated animals can still become infected, so suspected cases must still be reported.
Vaccination timing and breeding
BTV transmission is seasonal because the midge lifecycle is seasonal. Culicoides midges are generally active in Northern Europe between April and November, with peaks around May and September. This window may change during a wet, mild winter.
It is considered highly likely that BTV will overwinter and that further cases will appear in 2027. Vaccination can provide an additional layer of protection for breeding animals and their newborns in spring. By reducing viral load in animals, vaccination can also help reduce the amount of virus available to midges when their breeding cycle resumes.
If vaccination is delayed until spring, allow enough time for immunity to develop. Full immunity generally takes about 3-4 weeks from the first dose. Animals should ideally be fully vaccinated by early May, or at least three weeks before the anticipated risk period, typically early June.
Vaccinating ewes during tupping
To protect conception and early pregnancy, vaccination should ideally be completed at least three weeks before rams are introduced.
There is no evidence that the vaccine itself affects fertility or pregnancy. Healthy rams and ewes may be vaccinated up to the day before mating if this is required for disease control.
As with any management intervention, avoid unnecessary stress in early gestation.
Where possible, minimise handling, nutritional changes and routine treatments during the first five weeks after mating.
Managing vaccination around block calving and lambing
All three BTV-3 vaccines can be used during pregnancy. As with other sheep and cattle vaccines, maternal antibodies can pass through colostrum and help protect newborn lambs and calves.
Maternal antibodies can also interfere with the young animal’s response to vaccination. Manufacturer recommendations therefore advise waiting 1-3 months before vaccinating lambs and calves, depending on the vaccine used. Discuss the timing for your herd or flock with your vet.
Vaccination and fertility
There is no evidence that BTV vaccination adversely affects fertility in ewes or cows.
By contrast, BTV infection is associated with significant reproductive effects, including reduced fertility, impaired semen quality and temporary infertility in rams, abortions, foetal abnormalities and stillbirths. Surveillance data also indicates increased fertility problems and abortions in affected cattle herds.
Vaccine protection and schedules
What is the vaccination regime?
| Vaccine | Species | Primary vaccination schedule |
| Bultavo 3 | Sheep | 1 injection from 1 month of age in naive animals |
| Bultavo 3 | Cattle | First injection from 1 month of age in naive animals; second injection 3 weeks later |
| BlueVac-3 | Sheep | First injection from 2 months of age; second injection 3 weeks later |
| BlueVac-3 | Cattle | First injection from 2 months of age; second injection 3 weeks later |
| Syvazul BTV3 | Sheep | 1 injection from 3 months of age |
| Syvazul BTV3 | Cattle | First injection from 2 months in naive animals, or 3 months in calves born to immune cattle; second injection 3 weeks later |
All three vaccines have a zero-day withdrawal period for meat and milk, including imported product when the correct conditions are met. Discuss with your vet the importance of confirming the zero-day withdrawal period, particularly for milk.
Vaccine-induced immunity is not expected to be long term and will wane over time. The NFU cannot provide individual advice on boosters or product choice. Speak to your vet about the most appropriate approach.
Costs and practical arrangements
Vaccine expense
Veterinary practices are private businesses with overheads, and an urgent vaccination rollout may require additional staff and administration. While the majority of vets are reported to be selling vaccine at cost, without a profit margin, these activities create extra costs for the practice.
The NFU will continue to monitor the situation.
The NFU has also asked the government to consider contributing to vaccine costs to encourage uptake and is awaiting a response.
Making the decision to vaccinate
Farmers may be able to use an Animal Health and Welfare Pathway funded vet visit to discuss vaccination. The Ruminant Health and Welfare Group also provides a vaccine decision-maker tool and cost calculators to help assess the potential costs and benefits for an individual business.
Visit the RHWG Battle Bluetongue campaign page for more information and resources.
The NFU encourages members to vaccinate because the cost of disease to a livestock business could be significantly greater than the cost of vaccination.
Wastage
Manufacturers are aware that current bottle sizes are not convenient for smaller livestock farms. Their stated priority has been to increase the overall volume of vaccine available. Speak to your vet about practical options.
Do not share BTV-3 vaccine directly with another animal keeper. Only a vet can arrange this, following veterinary guidance and the relevant legal requirements. Speak to your vet.
Imported BTV-3 vaccines are suitable for the strain circulating in the UK.
Breakthrough infection, testing and reporting
Bultavo 3 is the only vaccine with a claim to prevent infection, and that claim applies only to cattle. The other vaccines, and Bultavo 3 in sheep, reduce infectivity but do not prevent infection.
Vaccinated animals can therefore still contract BTV-3, although the clinical impact is expected to be lower.
You still have a legal duty to report suspicion of bluetongue in vaccinated animals to your private vet or APHA. Your vet will advise on treatment and testing.
Elevated somatic cell count results and subclinical mastitis in dairy cows
BTV is not known to cause mastitis directly. However, affected animals may be more susceptible to secondary infections such as mastitis because of the overall impact on the immune system and because of skin or teat lesions.
Somatic cell count testing detects inflammatory cells. These may be present during an active BTV infection even where there is no secondary mastitis infection.
Timescales for disease confirmation
APHA and Pirbright laboratories are operating beyond capacity because of the number of reported cases. The physical laboratory testing stage at Pirbright has a delay of around five days and cannot be expedited.
Other stages can add time. Samples must be collected on farm by the private vet and sent to the laboratory, and postal delays can occur. Animal keepers can help avoid preventable delays by ensuring submission information is complete and accurate.
