Findings consistent with anecdotal reports
A re-emerged strain of bluetongue virus (BTV) circulating in France still poses a threat to susceptible animals in Europe, research by the Pirbright Institute has found.
The 2006-09 European outbreak of BTV-8 was the most costly epidemic of the virus ever recorded, according to Pirbright. The original BTV-8 virus and the current BTV-8 outbreak, however, have not been directly compared. Therefore the impact on susceptible animals and BTV-8’s ability to be transmitted is not yet understood.
Pirbright scientists assessed several properties of the re-emerging BTV-8 strain to see how it could affect animals and the economy should it spread to countries that had not experienced the original outbreak.
“We found that sheep infected with the re-emerged strain had lower concentrations of virus in their blood for a shorter period of time when compared to an original strain from the 2007 UK outbreak of BTV-8. Midge infection was also reduced, which is important as they transmit BTV between animals,” said Dr Carrie Batten, head of the non-vesicular reference laboratory at Pirbright.
“Milder clinical signs were observed in sheep infected by the re-emerged strain, although one developed acute lameness later during infection, which shows that the re-emerged strain can still severely impact sheep that have not been vaccinated or previously exposed to the virus”, Dr Batten continued.
Researchers say their findings, published in Transboundary and Emerging Diseases, are consistent with anecdotal reports of milder clinical signs in susceptible animals, and the relatively slow spread of the re-emerged BTV-8 strain since circulation was first detected in France in August 2015.
They note that many virus infections, including BTV, often lead to milder clinical disease under highly controlled experimental conditions.
Increased disease may be seen on farms that have different breeds and age groups which are exposed to environmental stress and other infections, researchers said. Development of more chronic clinical signs such a lameness and weight loss late in infection is a key feature of BTV and often more severe under natural farming conditions.
Dr Batten continued: “As the World Organisation for Animal Health (OIE) and National Reference Laboratory for bluetongue, it is important we understand the risks associated with circulating bluetongue strains that could affect our livestock.
“Our labs have already detected several imports of bluetongue infected animals to the UK in the past year, as part of the UK’s robust post-import checks. Although none of these have led to an outbreak in the UK, it is crucial that we understand how the current strain may affect the wellbeing of susceptible animals, in addition to helping us prepare appropriate control measures should an outbreak occur in the UK.”