A new study highlights the potentially serious consequences of bluetongue virus serotype 3 (BTV-3) on the UK sheep population.
This BTV-3 strain emerged in Northern Europe in September 2023 and has since been reported in 16 countries across the continent, causing outbreaks of the haemorrhagic disease, bluetongue, in livestock.
In November 2023, BTV-3 was detected in cattle and sheep in the South of England through routine surveillance, making it the first UK BTV incursion for over 15 years. The virus has since spread and a Restricted Zone (RZ) is in place covering 28 ceremonial counties and unitary authorities (as of 16 December 2024) in England.
The virus is transmitted between livestock by Culicoides biting midges. While it poses no threat to human health or the food chain, outbreaks can have a severe economic impact on rural communities.
Severe clinical disease was reported in Northern Europe, whilst no clinical disease was reported in the few initial UK BTV cases in 2023, raising questions over the potential impact of a BTV-3 outbreak on UK sheep flocks.
Prior to the 2024 BTV transmission season, scientists at The Pirbright Institute carried out a study to assess the clinical signs of disease, viral infection dynamics, sheep antibody responses and onwards transmission potential to Culicoides midges in five British sheep infected with the BTV-3 strain.
The findings, published today in Veterinary Record, show the BTV-3 strain caused mild to moderate clinical disease in a common UK sheep breed. Clinical signs of disease varied in nature and severity between individuals and included fever, haemorrhagic diarrhoea, lameness and behavioural changes such as depression and isolation.
Senior Postdoctoral Researcher in the Non-Vesicular Reference Laboratory (NVRL), Dr Kerry Newbrook, who carried out the research, said: “The clinical disease we observed in BTV-3 infected sheep showed some parallels to that reported in the Netherlands in 2023 and we might also expect more severe clinical signs in more susceptible sheep breeds such as Dorset/Texel crosses.”
“Importantly, we found that infectious BTV-3 could be isolated in sheep blood up to 28 days after infection, suggesting the animals infected with this BTV-3 strain may be infectious to Culicoides midge vectors for longer than other studied European BTV strains.”
“Our study aimed to help livestock keepers and vets to spot clinical signs in BTV-3 infected sheep in the field and determine viral infection dynamics. Since our study, BTV-3 has now spread across the East of England in 2024, and during this time our findings have proven useful in providing guidance of expected clinical impact and infection risk of BTV-3 in UK sheep.”
“Continued vigilance in reporting clinical suspicions of bluetongue disease to the Animal and Plant Health Agency (which is a requirement by law) will prevent or at least slow the spread of BTV-3 across the UK livestock population in this and the following seasons.” added Dr Christopher Sanders who led the study.
Pirbright’s Reference Laboratories are working with APHA to play a key role in controlling the UK outbreak through screening ruminant blood samples to detect BTV-infected animals.
Read the paper
Newbrook, K., Obishakin, E., Jones, LA. et al. (2024) Clinical disease in British sheep infected with an emerging strain of bluetongue virus serotype 3. Vet Record https://doi.org/10.1002/vetr.4910
