Smallpox spread from person to person via infectious droplets, killed around a third of sufferers and left another third permanently scarred or blind. Around 300 million people died from it in the 20th century alone before it was officially eradicated in 1980 through a global vaccination effort – the first human disease to be wiped out.
Now an international team of scientists have sequenced the genomes of newly discovered strains of the killer virus after it was extracted from the teeth of Viking skeletons from sites across northern Europe. The findings are published today in the journal
Science
.
“We already knew Vikings were moving around Europe and beyond, and we now know they had smallpox. Just as people travelling around the world today quickly spread COVID-19, it is likely Vikings spread smallpox. Only back then, they travelled by ship rather than plane,” said Professor Eske Willerslev in the University of Cambridge’s Department of Zoology and St. John’s College, and Director of The Lundbeck Foundation GeoGenetics Centre at the University of Copenhagen, who led the study.
The team found smallpox - caused by the variola virus - in 11 Viking-era burial sites in Denmark, Norway, Russia, and the UK. They also found it in multiple human remains from Öland, an island off the east coast of Sweden with a long history of trade.
They were able to reconstruct near-complete variola virus genomes for four of the samples. The genetic structure of this earliest-known smallpox strain is different to the modern smallpox virus eradicated in the 20th century.
“There are multiple ways viruses may diverge and mutate into milder or more dangerous strains. This is a significant insight into the steps the variola virus took in the course of its evolution,” said Dr Barbara Mühlemann, formerly at the Centre for Pathogen Evolution at the University of Cambridge, now based at the Institute of Virology at Charité – Universitätsmedizin Berlin, and one of the first authors of the report.
Historians believe smallpox may have existed since 10,000 BC but until now there was no scientific proof that the virus was present before the 17th century. It is not known how it first infected humans but, like COVID-19, it is believed to be a zoonotic disease - one that originated in an animal.
Smallpox was eradicated throughout most of Europe and the United States by the beginning of the 20th century but remained endemic throughout Africa, Asia, and South America. The World Health Organisation launched an eradication programme in 1967 that included contact tracing and mass communication campaigns - all public health techniques that countries have been using to control today’s coronavirus pandemic. But it was the global roll-out of a vaccine that ultimately enabled scientists to stop smallpox in its tracks.
While it is not clear whether these ancient strains of smallpox were fatal, the Vikings must have died with smallpox in their bloodstream for the scientists to detect it up to 1400 years later. It is also highly probable there were epidemics earlier than these findings.
“While written accounts of disease are often ambiguous, our findings push the date of the confirmed existence of smallpox back by a thousand years,” said Dr Terry Jones at the Centre for Pathogen Evolution at the University of Cambridge and the Institute of Virology at Charité – Universitätsmedizin Berlin, and one of the senior authors who led the study.
He added: “To find smallpox so genetically different in Vikings is truly remarkable. No one expected that these smallpox strains existed. It has long been believed that smallpox was in Western and Southern Europe regularly by 600 AD, around the beginning of our samples. We have proved that smallpox was also widespread in Northern Europe. Returning crusaders or other later events have been thought to have first brought smallpox to Europe, but such theories cannot be correct.”
“Smallpox was eradicated, but another strain could spill over from the animal reservoir tomorrow. What we know in 2020 about viruses and pathogens that affect humans today is just a small snapshot of what has plagued humans historically,” said Willerslev.
This research is part of a long-term project sequencing 5000 ancient human genomes and their associated pathogens. It was made possible thanks to a scientific collaboration between The Lundbeck Foundation, The Wellcome Trust, The Nordic Foundation, and Illumina Inc.
Reference
Muhlemann, B. et al. 'Diverse variola virus (smallpox) strains were widespread in northern Europe in the Viking Age. Science, July 2020. DOI: 10.1126/science.aaw8977
Adapted from a press release by St John’s College, Cambridge.
