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From the Freezer

Bluetongue virus, which infects livestock like sheep and cattle, caused billions of euros of damage to European farming before coming under control. But then cases stemming from a closely related strain re-emerged, leading researchers at the University of Glasgow and elsewhere to examine the genomes of the virus before and after that resurgence.

As they report this week in PLOS Biology, Glasgow's Roman Biek and his colleagues conducted a phylogenetic analysis that confirmed the viruses were closely related, but also noticed something odd. Prior to bluetongue virus coming under control in 2010, it accumulated mutations at an expected rate. But when it re-emerged in 2015, there were few additional mutations within the viral genome. 

Viral persistence over years with few genetic changes seems unlikely to the researchers and instead, they suggest that the virus was re-introduced from a contaminated frozen stock — such as material used for artificial insemination — which would explain the lack of additional mutations.

"In order to survive, to be transmitted and to find new hosts, viruses need to replicate. New mutations are an inevitable consequence of this, so viruses can't remain 'frozen in time,'" co-senior author Massimo Palmarini, also from Glasgow, says in a statement. "While there is still lots for us to learn about virus biology, the most plausible explanation for our findings is that exposure to infectious material, stored from the earlier outbreak, caused the most recent emergence of this virus in Europe."