Infection Dynamics and Clinical Signs of Bluetongue Virus Serotype 3 in Cattle and Transmission to Culicoides Vectors

Outbreaks of bluetongue (BT) virus (BTV) serotype 3 (BTV‐3) have been reported across Europe since late 2023, causing significant levels of disease in sheep, goats and cattle. The infection dynamics and clinical disease severity in the UK cattle and the infection rate of European Culicoides vectors to this BTV‐3 strain have yet to be investigated. Four Holstein‐Friesian calves were infected with BTV‐3 UKG2023/04 using a host:virus:vector transmission model with Culicoides sonorensis biting midges. Clinical signs, gross pathology, infection kinetics, duration of infectious viraemia in the blood and host immune response were assessed. The potential for onwards BTV transmission from infected calves to colony‐reared and the UK field‐caught Culicoides biting midge vectors was quantified over the study period. All four calves were infected with BTV‐3, even when few infected Culicoides had acquired a blood meal. Infected calves experienced mild clinical signs and transient elevation in rectal temperatures, presenting lower disease severity than previously reported in European cattle in the field. BTV infection, measured through the detection of viral RNA by real‐time RT‐PCR of the blood, peaked at 6–10 days post‐infection (dpi) for all individuals. Infectious BTV‐3 was isolated from the blood of three infected calves up to 56 dpi, although the consistency with which BTV was isolated declined after 24 dpi. Protective neutralising antibodies against BTV VP2 were first detected in calves from between 10 and 16 dpi. BTV RNA was detected in colony‐reared and the UK field‐caught Culicoides that had fed on infected calves at peak viraemia (6–8 dpi) and later in infection (42 and 55 dpi). Estimates of the vector competence of the Culicoides obsoletus group for BTV‐3 were low (0.68%). Our data indicate that whilst cattle are less likely than sheep to exhibit clinical disease following infection with BTV‐3, the duration of infectious viraemia observed may permit transmission to Culicoides vectors for several weeks post‐infection.