Our bluetongue virus specialist, Dr Carrie Batten, shares insights into a disease that threatens the livestock industry.
What is bluetongue virus?
Bluetongue does not affect people or food safety. It is a disease of sheep, cattle and other ruminants caused by the bluetongue virus (BTV).
Transmitted primarily by biting midges, it does not spread from animal to animal. It is a notifiable disease in Great Britain. Anyone suspecting a case of BTV must report it immediately to the Animal and Plant Health Agency (APHA).
Bluetongue mainly affects sheep, but some BTV strains affect cattle, goats, deer and camelids. BTV outbreaks leads to restrictions on animal movements and trade, affecting the UK’s sheep, beef and dairy industries. It is a World Organisation of Animal Health listed disease. For an outbreak of bluetongue to be confirmed, there must be evidence of virus transmission.
How do we spot bluetongue?
Signs of bluetongue disease include inflammation of the mucous membranes, congestion, swelling, fever, haemorrhages, lameness, in some instances a cyanotic or ‘blue’ tongue (from which the disease derives its name) and even death.
Animals that survive infection may show reduced milk yield, reduced weight gain and wool break.
How does bluetongue spread?
BTV is spread primarily by certain species of biting midges (Culicoides), which can be found in abundance on most UK farms. Midges infected with BTV may be transported to the UK on the wind from mainland Europe. Transmission usually takes place during the warmer months, between May and October, when midges are most active.
Is it controllable?
The diversity of BTV (with 29 known serotypes) means a vaccine for one serotype will not protect against another.
Several serotypes are currently circulating in Europe including BTV-3, BTV-4 and BTV-8. Vaccines (containing inactivated virus) are only readily available for two serotypes (BTV-4 and BTV-8). The serotype identified in Norfolk is BTV-3 and no vaccine is currently licensed for use in the UK.
Biting midges breed in damp organic habitats, including animal dung and leaf litter, and farms provide a fertile breeding ground. Biting midges can fly between farms allowing the disease to spread.
Animal trade can also drive the spread of disease if controls are not in place.
What’s the background?
Bluetongue was considered an ‘exotic disease’ prior to a European outbreak of BTV-8 from 2006 to 2009, resulting in the first incursion of the virus into northern Europe and the largest and most costly outbreak of BTV reported.
BTV-8 was first diagnosed in the UK in September 2007. UK vaccinations began in 2008 and, largely due to the high levels of vaccine coverage achieved, no BTV circulation was detected in the UK throughout 2008, 2009 and 2010. Due to the success of the vaccination campaign, the UK was declared free of BTV in July 2011.
Bluetongue returned to the UK in November 2023. In total, 126 confirmed cases of BTV3 were confirmed in England, affecting 73 premises in Kent, Norfolk, Suffolk and Surrey, compromising 119 in cattle and seven in sheep.
How does testing work?
The Pirbright Institute works with colleagues from the APHA and Defra to test animals for BTV. Two different molecular tests (detecting different parts of the BTV genome) are used to negate or confirm infection.
It is now possible to detect BTV in blood samples taken early in infection and, in some instances, for months post-infection.
Serotyping and additional molecular characterisation can also be performed, which is crucial for potential vaccine selection.
How can I report bluetongue?
Defra publishes guidance and advice for farmers and livestock holders about how to spot and report the disease, and further information is available on GOV.UK.
Monitoring the risk
The Pirbright Institute hosts the World Organisation for Animal Health (WOAH) Reference Laboratory for bluetongue with in-house expertise.
The Institute is committed to understanding BTV for the protection of animal health. Building on this expertise and working closely with international colleagues will underpin the response to BTV in the UK.
