By Michael Le Page
Stuart Black / Alamy
The DNA of ancient smallpox viruses has been found in the bones and teeth of a dozen or so people who lived in northern Europe during the Viking age. Unexpectedly, these smallpox strains are quite different to the strain that was eliminated in the 20th century – and possibly far less deadly.
Historical accounts and lesions found on Egyptian mummies suggest that the Variola virus, which causes smallpox, has plagued people for thousands of years. Barbara Mühlemann at the University of Cambridge and her colleagues now have the first unambiguous evidence.
They started by looking for viral sequences in previously sequenced DNA from nearly 2000 individuals who lived in Eurasia and the Americas between 30,000 and 150 years ago. “Presumably many people died of the virus,” she says.
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In these people, viral DNA might be present in their remains and could have been sequenced along with their own DNA. Sure enough, the team found signs of Variola DNA in 26 individuals.
They then looked for more viral DNA in the original samples. They found it in 13 individuals, 11 of whom died between AD 600 and AD 1050 – overlapping with the Viking age from AD 793 to AD 1066.
Most of these individuals died in Scandinavia or what is now western Russia. Three were found on an island in the Baltic Sea called Öland, one in a boat burial around AD 700 and two others in separate burials around AD 1000. The last two probably died in the same outbreak.
The Variola virus was also found in a man in a mass grave in Oxford, UK, which is odd given that all 35 men in this grave were violently killed. They are thought to be Viking warriors killed in AD 1002 after Ethelred the Unready ordered the death of all Danes in England – the St Brice’s Day massacre.
In four cases, Mühlemann recovered near complete viral genomes. What these reveal was unexpected.
The ancestor of the Variola virus probably had about 200 genes, similar to some pox viruses still circulating in animals. The strain eliminated by vaccination in the 20th century – which killed 1 in 3 people – had lost around 30 genes.
The strains that Mühlemann sequenced had lost only half of these 30 genes. They derive from the same ancestor as the 20th-century virus, but didn’t give rise to it. Instead, they are a now-extinct side branch.
“It’s more complicated than anyone imagined,” says team member Terry Jones, also at the University of Cambridge.
Strains with the full set of 200 genes typically cause only mild disease, says Antonio Alcamí at the Autonomous University of Madrid, Spain, who was not part of the team. He thinks the Viking age virus type was less deadly than the 20th-century one. “It was probably able to kill but was not as terrible,” he says.
This flies in the face of conventional thinking, which is that viruses are most deadly when they first jump to humans and evolve to become less deadly, because viruses that cause severe disease are less likely to spread.
“Why it became more virulent does not make sense,” says Alcamí.
The team itself is not making any claim one way or another, because they aren’t immunologists like Alcamí. “We feel we can’t say with certainty that that virus was less virulent in the past,” says Mühlemann.
One explanation for this diversity of strains is that the smallpox virus jumped from animals to people more than once. That might mean it is more likely to happen again than we thought, says Jones.
Increasing numbers of people are being infected by monkeypox viruses – whose normal host is unknown despite the name – but so far there has been no sustained human-to-human spread. The increase in cases could be due to the fact that people are no longer being vaccinated against smallpox following its eradication, says Mühlemann.
Journal reference: Science, DOI: https://doi.org/10.1126/science.aaw8977
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