Clinical disease in British sheep infected with an emerging strain of bluetongue virus serotype 3.

Background: Bluetongue virus serotype 3 (BTV-3) was detected for the first time in cattle and sheep in southern England in 2023, the first UK BTV incursion for more than 15 years. Clinical signs were not observed, yet severe clinical disease and mortality were reported during recent BTV-3 outbreaks in northern Europe.

Methods: To investigate the clinical disease and infection kinetics associated with this UK BTV-3 strain, five British sheep were infected with a UK BTV-3 isolate using Culicoides biting midges. Clinical signs, pathology, infection dynamics, immune responses and Culicoides infection rates were assessed.

Results: All sheep were infected with BTV-3 and developed mild to moderate clinical bluetongue disease, characterised by fever, haemorrhagic diarrhoea, lameness, depression and widespread petechial haemorrhage. Three sheep reached clinical humane endpoints and were euthanased. Clinical signs/severity, infection kinetics and immune responses were highly variable. Infectious BTV-3 was isolated from sheep blood up to 28 days postinfection.

Limitations: The impact of BTV-3 infection on British cattle and infection rate in UK Culicoides require investigation to fully determine the risk of this strain to UK livestock.

Conclusions: This study confirms the potential impact of a BTV-3 incursion/outbreak on the UK sheep population, highlighting the need for an effective vaccine.