Smallpox was one of the most dangerous diseases of mankind – in the 20th century alone, an estimated 300 to 500 million people died from this viral disease. However, where and how the smallpox, which is now believed to be eradicated, began, has not been clarified. A genetic study now reveals that Vikings carried the smallpox virus within themselves 1400 years ago and may have spread it in Europe. This shows for the first time that smallpox was rampant among people in the early Middle Ages. The Variola viruses found in the dead from the Viking Age, however, belonged to a different strain than the modern smallpox virus, as DNA comparisons showed.
The eradication of smallpox 40 years ago is one of the great success stories of modern medicine. A worldwide vaccination campaign has resulted in the pathogen, the variola virus Orthopoxvirus variolae, no longer circulating in the human population. The last samples of the virus are only kept in two high-security laboratories in Russia and the USA. But that doesn’t mean that smallpox can never occur again. Because like the Coronavirus Sars-CoV-2, the Variola virus also jumped from an animal to humans. To date, various types of animal pox viruses exist, including in camels, rodents, monkeys and cows. “Smallpox has been eradicated, but another tribe could make the art leap again from one of these animal reservoirs tomorrow,” says co-author Eske Willerslev from the University of Copenhagen. It is therefore all the more important to know the origin and development of the pox virus.
At this point, however, it has so far failed. Because where and when a person first contracted smallpox and from which animal these viruses came is unknown up to now. The earliest confirmed genetic evidence of smallpox so far came from a Lithuanian mummy from the 17th century and two Czech deaths from the 19th and early 20th centuries. “However, historical reports of possible smallpox diseases go back at least 3000 years,” report the researchers led by Willerslev and first author Barbara Mühlemann from the University of Cambridge and the Charité Universitätsmedizin in Berlin. In addition, skin lesions of the Egyptian pharaoh Ramses V, who died in 1157 BC, could be due to smallpox. According to a common theory, smallpox may have first developed in Africa and was then introduced to Europe from the Middle East by commercial travelers or returning crusaders.
Detection of smallpox DNA in dead Vikings
In order to clarify the early spread of smallpox, Mühlemann and her colleagues searched the genetic makeup of 1867 people for traces of smallpox virus DNA that died in Eurasia and America 31,630 to 150 years ago. They found what they were looking for in 13 of these samples. Eleven of these come from Viking tombs that were created between 603 and 1050 in Northern Europe, Great Britain and Western Russia. Two other smallpox virus carriers died in Russia in the 19th century. From four of the Viking samples, the researchers were even able to reconstruct almost the entire genome of the variola virus found in these dead. “The 1,400-year-old genetic information from these skeletons is extremely important because it tells us the evolutionary history of the human pox virus,” explains Willerslev.
The smallpox virus genome shown in these analyzes shows for the first time that this infectious disease was rampant in Northern Europe 1400 years ago. “The sequences from the Viking Age shift the date of the earliest detectable smallpox virus infection by 1000 years,” the researchers report. The detection of this virus in Northern Europe at this time also contradicts the assumptions that the virus came to Europe through returning crusaders, the invasion of the Moors or the conquest of England by the Normans. Instead, some Vikings already carried a smallpox virus, which they may then spread to Europe during their travels. “We know that the Vikings moved across Europe and beyond, and now we know that they had smallpox,” says Willerslev. “Similar to the way travelers spread Covid-19 today, the Vikings did with smallpox – only that they traveled by ship instead of by plane.”
Sister group of modern pox viruses
However, the DNA analyzes also showed that the pox viruses that were rampant among Vikings belonged to a different strain than the modern variola viruses. “This early version of smallpox is more like animal pox than camel pox or taterpox than modern poxvirus,” said co-author Lasse Vinner from the University of Copenhagen. DNA comparisons showed that the Viking viruses are a sister group to the modern smallpox virus, which apparently circulated in Northern Europe for around 450 years, but then died out. “Finding smallpox viruses in the Vikings that are genetically so different is really remarkable,” says Mühlemann’s colleague Terry Jones. “Nobody expected such smallpox virus strains to exist.”
According to the analyzes, both this virus variant and the tribal line of modern smallpox could have arisen from a common ancestor around 1700 years ago and then have developed independently of one another. The comparisons suggest that the ancient Viking tribes still had some genes that opened up a broader range of hosts, but may make them less aggressive and deadly, as the researchers explain. “We don’t know how the disease manifested in the Viking Age. But it could have been different from the virulent modern tribe that disfigured and killed hundreds of millions of people, ”says Vinner. Even if not all questions have been answered yet, the results of this study provide valuable new insights into the evolution of smallpox and its spread in Europe.
Source: Barbara Mühlemann (Charité Universitätsmedizin, Berlin) et al., Science, doi: 10.1126 / science.aaw8977