Diverse Strains of Smallpox Virus Were Widespread in Viking Age, New Study Shows
Smallpox, caused by the variola virus, is one of the most devastating human diseases. Smallpox killed millions of people but drove Edward Jenner’s invention of vaccination, which eventually led to the annihilation of the virus, declared in 1980. To investigate the history of smallpox, an international team of researchers led by University of Cambridge and University of Copenhagen scientists sequenced and analyzed DNA from 1,867 humans who lived in Europe and the Americas between 31,630 and 150 years ago. The smallpox virus sequences were recovered from 13 northern European individuals, including 11 dated to 600-1050 CE, overlapping the Viking Age.
Historians believe smallpox may have existed since 10,000 BCE but until now there was no scientific proof that the virus was present before the 17th century CE.
It is not known how it first infected humans but 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 program in 1967 that included contact tracing and mass communication campaigns. But it was the global roll-out of a vaccine that ultimately enabled scientists to stop smallpox in its tracks.
“Smallpox is the infection in the world that has killed most people. For that reason alone, it is very important and interesting to know how the disease developed,” said senior co-author Dr. Martin Sikora, a scientist in the Lundbeck Foundation GeoGenetics Center at the Globe Institute, the University of Copenhagen.
“It gives us a unique opportunity to understand the evolution of viruses: How did it change and become the pathogen that we know of today?”
“We already knew Vikings were moving around Europe and beyond, and we now know they had smallpox,” added senior co-author Professor Eske Willerslev, a researcher in the Department of Zoology and St. John’s College at the University of Cambridge and Director of the Lundbeck Foundation GeoGenetics Centre at the Globe Institute, University of Copenhagen.
Dr. Sikora, Professor Willerslev and their colleagues screened genomic data from skeletal and dental remains of 1,867 ancient humans for the presence of sequences matching the smallpox virus.
The authors identified sequences of the virus in 13 northern European individuals, including 11 dated to 600-1050 CE.
They were able to reconstruct near-complete smallpox 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,” said first author Dr. Barbara Mühlemann, from the Centre for Pathogen Evolution at the University of Cambridge, the Institute of Virology at Charité – Universitätsmedizin Berlin, and the German Center for Infection Research.
“This is a significant insight into the steps the variola virus took in the course of its evolution.”
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 1,400 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, also from the Centre for Pathogen Evolution at the University of Cambridge and the Institute of Virology at Charité – Universitätsmedizin Berlin, and the German Center for Infection Research.
“To find smallpox so genetically different in Vikings is truly remarkable. No one expected that these smallpox strains existed,” he added.
“It has long been believed that smallpox was in Western and Southern Europe regularly by 600 CE, 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.”
The results were published in the July 24, 2020 issue of the journal Science.
Barbara Mühlemann et al. 2020. Diverse variola virus (smallpox) strains were widespread in northern Europe in the Viking Age. Science 369 (6502): eaaw8977; doi: 10.1126/science.aaw8977