Farmers urged to be on alert for bluetongue re-emergence

Farmers are being urged to remain vigilant as temperatures rise and the threat of bluetongue virus re-emerges.  

The Animal Health and Plant Agency (Apha) has raised concerns that mildly affected calves could “fly under the radar”, potentially reactivating the virus.

See also: Bluetongue virus: What to look out for and how to manage it

Recent investigations by Apha have also shed light on how the virus causes “dummy” calves – foetuses infected in utero that are born with brain malformations, which contribute to significant economic losses.

In affected herds, the appearance of these calves was the first obvious signs of infection, say vets.

In January this year, local vets in Suffolk raised alarms over unusually poor scanning rates and widespread fertility issues among suckler clients.

In these herds, a small number of dummy calves presented at birth. These calves struggled to stand, were weak, displayed poor suck reflex and had a passive demeanour.

Post-mortems

Post-mortem examinations were conducted on four calves from three beef farms, in Suffolk, Norfolk and East Sussex – within the bluetongue virus (BTV) restriction zone.

Results confirmed the calves had brain abnormalities associated with cysts or cavities replaced with fluid within parts of the brain.

“The signs were consistent across the calves we saw. More severe cases had a star-gazing activity, head pressing and seizures,” veterinary investigation officer Hayley Wighton revealed on a recent AHDB webinar.

Other causes of brain cavitation were ruled out, and follow-up tests confirmed BTV-3 in the affected herds.   

“This increased our confidence brain lesions were attributable to BTV-3,” added Hayley.

Timing of infection

On affected farms, the number of dummy calves was low. However, Hayley warned that “any number” of calves could be affected if the virus infected pregnant cows at the wrong time.

She said the dams of affected calves were likely to have been infected in mid-gestation (around 70-130 days).

“Only one herd had described clinical signs of BTV infection in the months prior. In economic terms, this might be the hardest-hitting effect of BTV, with the combination of early losses, abortions, stillbirths and non-viable dummy calves causing up to 50% losses in these beef herds,” she added.

Dummy calves are typically non-viable and must be euthanised if they do not die naturally.  

Bull fertility

Another concern is the impact on bull fertility. Bulls from all three suckler herds tested positive for current or past BTV infection.

Evidence from previous European outbreaks indicates that BTV can reduce bull fertility, potentially compromising an entire breeding season.

“The virus can be retained in semen, and this is a route through which cows can become acutely infected. If conception was successful, it could result in embryonic loss,” Hayley said.

However, it remains unclear how long the virus persists in the testes.

Long-term risks

A growing concern is that the virus may overwinter and re-emerge during the warmer months.

Hayley warned that mildly affected calves might appear healthy but still harbour the virus, reactivating midge-borne transmission and transmitting infection to other cows.

This underlines the importance of early detection and vaccination.

Farmers are being advised to speak with their vet to determine if vaccination is appropriate for their herd.

Vaccines can be used in England and Wales, and as of 30 April, the Scottish government approved the use of BTV-3 vaccines.

In the meantime, farmers are urged to remain vigilant and report suspicious cases to Apha immediately.