Scientists at The Pirbright Institute in the U.K. have shown that the strain of bluetongue virus (BTV) currently circulating in France poses a threat to susceptible animals in Europe despite causing less clinical disease than the original BTV-8 strain responsible for the first U.K. bluetongue outbreak in 2007.

The 2006-09 European outbreak of BTV-8 was the most costly epidemic of the virus in recorded history, the institute said.

However, the two BTV-8 strains — the original strain from the 2006-09 outbreak and the newer strain that re-emerged in France in 2015 — have not been directly compared, so the transmissibility among susceptible animals is unclear.

Pirbright scientists assessed several properties of the re-emerging BTV-8 strain that could help demonstrate how the virus might affect animals and the economy were it to spread to countries that had not experienced the original European 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 U.K. 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 Pirbright Non-Vesicular Reference Laboratory.

“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,” Batten added.

These findings, published in Transboundary & Emerging Diseases, are consistent with the anecdotal reports of milder clinical signs in susceptible animals, Pirbright said, and relatively slow spread of the re-emerged BTV-8 strain since circulation was first detected in France in August 2015, although the strain has recently been found in Germany.

That said, Pirbright noted that many virus infections — including BTV — often lead to milder clinical disease under highly controlled experimental conditions than when seen in the field. Increased disease may also be seen on farms that have different breeds and age groups that are exposed to environmental stress and other infections. Development of more chronic clinical signs such a lameness and weight loss late in infection is a key feature of BTV and often is more severe under natural farming conditions, the institute explained.

Pirbright pointed out that recent reports suggest the re-emerged BTV-8 strain might be able to cross the bovine placenta and cause birth defects in unborn calves, which could increase the disease threat of this strain.

“As the World Organization for Animal Health and National Reference Laboratory for bluetongue, it is important we understand the risks associated with circulating bluetongue strains that could affect our livestock,” Batten added. “Our labs have already detected several imports of bluetongue-infected animals to the U.K. in the past year as part of the U.K.'s robust post-import checks. Although none of these have led to an outbreak in the U.K., it is crucial that we understand how the current strain may affect the well-being of susceptible animals, in addition to helping us prepare appropriate control measures should an outbreak occur in the U.K.”

North America. According to the U.S. Department of Agriculture's Animal & Plant Health Inspection Service (APHIS), BTV-infected sheep are most likely to exhibit clinical disease, and most infections in cattle are subclinical. In North America, white-tailed deer, pronghorn antelope and desert bighorn sheep are also susceptible to BTV infection.

APHIS said at least 26 BTV serotypes have been identified worldwide. In the U.S., 13 serotypes (1, 2, 3, 5, 6, 10, 11, 13, 14, 17, 19, 22 and 24) have been reported, with types 2, 10, 11, 13 and 17 being considered endemic.

Since there is no curative treatment for BTV-infected animals, APHIS said prophylactic immunization of susceptible species remains the most effective and practical control measure against bluetongue in endemic regions. In the U.S., the only vaccine approved for national use is a monovalent attenuated modified-live vaccine against serotype 10; APHIS said the vaccine is expected to provide little or no protection if the BTV serotype causing infection is a different strain.

APHIS suggested that control of bluetongue vectors by using insecticides or protection from vectors may lower the number of midge bites and, subsequently, the risk of exposure to BTV infection.