Frozen Bull Semen May Have Sparked Outbreak of Bluetongue Virus in Europe

Frozen semen from a bull infected with bluetongue virus (BTV) may have been behind an outbreak that started in France in 2015, scientists have said.

By analyzing the genomes from two seperate outbreaks that started in 2006 and 2015, researchers found surprisingly little differences between the two virus strains. They say the most plausible explanation for the virus reemerging was that semen from a bull infected during the original outbreak was used to artificially inseminate a cow years later.

BTV is a viral disease that affects sheep, cattle, deer and goats, among other species. While sheep are worst affected by the virus, cows are the main mammal reservoir. According to the U.K.'s National Animal Disease Information Service, symptoms in cattle include a fever, swelling of the head and neck, conjunctivitis and drooling from the mouth.

The 2006 outbreak had spread to most of Europe by 2009. It had had a huge economic impact, with large numbers of livestock lost. In Germany, the outbreak is estimated to have cost up to 203 million Euros ($217 million). Across Europe in total, it is believed to have cost billions, researchers say. It was brought under control a year later as a result of a mass vaccination program.

However, in 2015, the virus re-emerged in France. Cases were later detected in Germany, Switzerland, and Belgium. How and why the virus had reapparead was unknown, but it is thought the virus had continued to circulate at low levels in wild animals in the five year gap.

In a study published in PLOS Biology, researchers led by David J. Pascall and Kyriaki Nomikou, from the U.K.'s University of Glasgow, have looked at the genomes of the virus from both outbreaks to understand if the two were linked and if so how.

Their findings showed that during both outbreaks, the viruses evolved rapidly. However, the virus that emerged in 2015 was strikingly similar to the one from the 2006 outbreak. Had it been circulating in the period between 2010 and 2015, there should have been mutations, the authors say. The team say that during both outbreaks the virus displayed "clock-like evolution," but not in between the outbreaks. They found the virus that started the second outbreak was most similar to the virus from 2008 samples.

"This is inconsistent with the prevalent view of undetected low-level circulation of the virus in wild or domestic ruminants between 2010 and 2015, and instead points to another mechanism of emergence," they wrote.

This other mechanism, they believe, is frozen bull semen. Artificial insemination using frozen samples is widespread in the livestock industry. "Due to specific animal husbandry procedures, there are important potential sources of frozen virus that apply to viruses of livestock and not viruses of most other animals, specifically the widespread use of bull semen for artificial insemination and embryo transfer in cows," the study said. Researchers note BTV has previously been detected in the semen of bulls and can lead to infection of the mother and embryo.

"We stress that the link between bull semen trade and embryo implantation in France and the BTV-8 re-emergence in 2015 is only speculative," they conclude, but say their research suggests new areas of surveillance are required for the control of infectious diseases in livestock.

Stock photo of a bull in a field. Researchers say the 2015 outbreak of BTV may have come from bull semen frozen in the height of an earlier outbreak. iStock