A study of DNA sampled from skeletons from the Viking Age found the presence of smallpox in 11 individuals living in northern Europe, which is now Denmark, Sweden, Norway, Russia, and the United Kingdom, between 600 and 1050 AD.
“The 1,400-year-old genetic information extracted from these skeletons is enormously important because it teaches us about the evolutionary history of the variola virus that caused smallpox,” Professor Eske Willerslev of the University of Cambridge and Director of GeoGenetics at the University of Copenhagen. Center said in a statement.
As Covid-19 continues to rage around the world, learning about ancient epidemics gives us some perspective. Like us, the Vikings were also engaged in long-distance travel and trade.
“We already knew that Vikings were moving through Europe and beyond, and now we know they had smallpox,” Willerslev said.
“People traveling the world quickly spread Covid-19, and Vikings are likely to spread smallpox. Right then, they traveled by boat rather than plane,” he added.
“What is really exciting about this is that we can use these reconstructed genomes to track the steps these viruses and other pathogens took to become the disease they became,” said Martin Sikora, co-author of the study and professor at the Center for GeoGenetics at the University of Copenhagen, told CNN.
Delaying infection record older than 1000 years
While historical accounts suggest that smallpox existed more than 3,500 years ago, there is great uncertainty about the DNA sequence that could help clarify.
“Before the emergence of sequencing there is a long history of disease outbreaks, some of which sound like smallpox,” Terry Jones, co-author of the study and a computational biologist at the University of Cambridge and the Charité Institute of Virology. in Berlin, he told CNN.
“The further back you go back in time, the less certain things are.”
In particular, Jones noted, diseases like chickenpox and measles are difficult to distinguish from smallpox, and before 900 AD there was little understanding of these differences, he argued.
Thanks to the study, “we now return to about 600 AD in terms of concrete knowledge of the virus’s existence.”
The new evidence highlights earlier theories that smallpox may have been introduced to Europe through Spain during the Arab invasion, upon return to the Crusaders, or through England through the Normans.
The study shows that smallpox was already present in Europe during the Viking Age, and that it was “very widespread,” Sikora said.
The 11 Viking Age individuals who tested positive for smallpox were part of a larger sample of 500, a positivity rate of 2%.
Based on that rate, Sikora explained, “it can be assumed that smallpox was already endemic across Europe at least at that time, if not earlier.”
“That’s the kind of direct evidence that only ancient DNA can really give you,” he added.
New viral strains discovered
Scientists were able to reconstruct four nearly complete genomes for ancient smallpox strains, all different. The strains are also significantly different from the more modern smallpox genomes.
These ancient strains belong to a clade of their own in terms of their genetic makeup.
“It is an ancestral clade, but it is not the direct ancestor of variola,” Barbara Mühlemann told CNN. She is the first author of the study and participated in the research during her PhD at Cambridge University. Mühlemann is now based at the Charité Institute of Virology in Berlin.
As viruses evolve to perform better on their hosts, be they animal or human, some of the viral genes go through an inactivation process: they are deactivated, so to speak. Mühlemann explained that some genes inactive in Viking Age viruses are still active in modern variola.
“They cannot be the direct ancestors in a straight line. They are more a lateral branch,” added Mühlemann.
Viking Age viruses are extinct, but that doesn’t make them less compelling. Its gene inactivation patterns, unlike those of any previously known smallpox virus, offer a crucial conclusion.
“We now know that there are other combinations of genes that would result in a virus that would work in humans, that would be transmissible, and that might have other properties,” Jones told CNN.
This idea could not have been extracted from any of the smallpox genomes known to scientists, because that diversity had never been observed before.
“Knowledge from the past can protect us in the present. When an animal or plant becomes extinct, it does not return. But mutations can reoccur or reverse and viruses can mutate or spread from the animal reservoir, so there will always be another. zoonosis (disease caused by viruses that jumped from animal hosts to human hosts), “Jones said in a statement.
The newly discovered ancient strains probably circulated at the same time as the smallpox strains with which we became familiar in modern history, according to Jones, quoting with almost certainty that all the strains discovered so far descended from a common ancestor prior to all of them.
“Since we know that the other viruses still existed in the 20th century, they must have existed in the Viking Age in some way, presumably with more genes intact,” Jones told CNN.
“But we can’t know what host they existed in. Most people would assume they were in humans, but that might not be the case,” Jones added.
Finding smallpox in old teeth
Did all 11 Viking Age individuals really die of smallpox? That question remains open.
“We try to be careful when saying that these people have died from smallpox because we don’t know. There could have been trauma, murder; it could have been something else that we don’t know,” Mühlemann told CNN.
Sikora said she believes the individuals would have had to have some sort of active infection when they died in order for scientists to find the viral DNA in their remains, particularly their teeth.
“Virus genes are very small, so there is not much DNA unless there are many copies of that DNA present in the individual,” said Sikora.
“I don’t think we can get it out of a mixture of DNA extracted from a tooth that has been buried 1000 years ago,” he added.
For one of the individuals, who was found in what is believed to be a place of slaughter in Oxford, England, with a stab in the back, scientists do not believe that smallpox was the cause of death.
From the history of smallpox to the coronavirus
The study results left the researchers hopeful for the future and motivated to learn more about viral genomes.
“The past 40 to 50 years have been exceptionally quiet for us because we have made tremendous progress in medical research, combining vaccines and antibiotics,” Sikora said of the emergence of viral pandemics.
“Smallpox eradication gives us hope for the current situation,” Mühlemann told CNN, emphasizing how through vaccines we were able to eradicate this deadly disease.
“I think it shows what we are capable of,” he said.
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