Bull semen frozen for several years before being used to inseminate a cow may have sparked an ongoing bluetongue outbreak among farm animals in Europe, according to a genetic analysis of the virus strain.
“It’s the most likely explanation,” says Massimo Palmarini of the University of Glasgow, UK.
Bluetongue is a viral disease spread by biting midges. It can cause a variety of symptoms, from fevers to the bluish tongues that give the disease its name. All ruminants – such as cattle, goats and deer – and camelids – llamas and alpacas – can get the disease, but it is most serious in sheep, killing many animals. It is not a threat to human health.
In 2006, an outbreak began in the Netherlands. It spread to 16 countries and cost billions of euros before a vaccination effort brought it to an end in 2010. “By 2011, no cases were detected,” says Palmarini.
But in 2015, the disease re-emerged in France and this latest outbreak is still ongoing. To try to work out the source of the infection, Palmarini and colleagues analysed the genetic sequences of 150 samples of the virus from both outbreaks.
Viruses accumulate minor mutations as they replicate, so if the bluetongue virus had somehow circulated undetected in wild or farm animals in Europe between 2010 and 2015, it should have acquired lots of mutations.
Instead, the researchers found that the 2015 strain was almost identical to strains circulating in France and Germany in 2007 and 2008 – as if it had been somehow frozen in time.
That is probably exactly what happened. The virus is known to be present in the semen of infected males and can be passed to females during mating, so the team think a farmer froze semen from an infected bull around 2007 and stored it until 2015. It may have been another species, but artificial insemination is most common in cattle.
“The paper is convincing,” says Chris Oura of the University of the West Indies in Trinidad and Tobago. There are other possible causes, such as frozen colostrum, the milk produced immediately after a mammal gives birth, but this is rarely stored for a long time. Frozen semen is the most likely explanation, Oura says.
Companies that export animal semen have to follow strict standards to avoid contamination, says Palmarini. However, there’s far less regulation of what farmers and companies do locally.
Two other animal disease outbreaks, of Venezuelan equine encephalitis in Colombia in 1995, and of foot-and-mouth disease virus in the UK in 2007, have also been attributed to accidental releases of viruses.
The 1977 flu epidemic is also thought to have been caused by the accidental release of a strain frozen since the 1950s. Fortunately, it had only mild effects in most people.
There have also been claims that the current coronavirus outbreak began when a sample escaped from a lab, but its genetic sequence is quite distinct from previously known coronaviruses, so there’s no evidence to support this, Palmarini says.
Journal reference: PLOS Biology, DOI: 10.1371/journal.pbio.3000673
More on these topics: