How dangerous is smallpox

Sometimes it was allegedly the crusaders who brought the pathogen to Europe. Or the Arabs when they invaded Spain in the 8th century AD. Or at least the Normans who reached England in the 13th century. Most recently, Canadian researchers isolated gene fragments of the dreaded smallpox virus from a children's mummy in Lithuania and dated the last common ancestor of the modern smallpox virus to the 17th century. An international team of researchers is now showing that the pathogen circulated much earlier: "The smallpox viruses spread in Northern Europe as early as the 7th century," says Barbara Mühlemann, bioinformatician and one of the two first authors of the current study in the specialist journal Science. Namely among the Vikings.

Mühlemann and colleagues discovered the traces of the old smallpox virus when they sequenced the remains of 1,867 people from Eurasia and America, between 31,630 and 150 years old. Among them, the researchers identified 13 individuals, all of whom came from Northern Europe, eleven of them from roughly the Viking Age, between 600 and 1050 AD. For four of the old samples, they were able to almost completely reconstruct the genetic makeup of the viruses - and compare them with modern strains of smallpox.

To their surprise, Mühlemann and colleagues discovered that they were dealing with a previously unknown and now extinct strain of the smallpox virus. "We assume that the tribe was widespread throughout Northern Europe at the time," says Mühlemann. Amazingly, the ancient smallpox viruses had a very different pattern of active and inactive genes than their modern relatives. "The activity pattern of the genes in the smallpox virus of the Viking Age could mean that the virus could not only attack humans, but also animals," says Mühlemann.

500 million people died of smallpox in the 20th century

How high the mortality was or what symptoms the old virus caused, however, the researchers cannot read from it. At least the genetic data suggest that the viruses also caused fever in the Vikings. "Above all, this gives us an important insight into how smallpox viruses can develop," says the bioinformatician, who until recently worked on the study at the Center for Pathogen Evolution at the University of Cambridge and now works at the Charité Institute for Virology.

Researchers still know surprisingly little about the history of the smallpox virus, which killed up to 500 million people in the 20th century alone. People had a high fever and developed a rash, especially on their face, arms and legs. The pustules filled with fluid, formed crusts that eventually fell off but left ugly scars. About 30 percent of the sick died. As early as 1796, the English doctor Edward Jenner had vaccinated a boy with cowpox for the first time in medical history. Smallpox has been considered eradicated since 1980, the world's first infectious disease.

"But we still don't know how and where the smallpox virus originated," says bioinformatician Terry Jones, also co-author of the study. The researchers do not even know in which animals the infectious agents occur primarily in nature. "We assume that it is a rodent," says Jones, who works as a working group leader at the Charité as well as at the Center for Pathogen Evolution at Cambridge University.

Even if the smallpox viruses, which are highly dangerous for humans, have been eradicated, there are still other strains that infect humans or animals. Cowpox, for example, and camelpox, those from gerbils and monkey pox. In West and Central Africa there are regular cases of monkey pox that spread to humans, in Central Africa ten percent of those infected die from it. "Our study shows how much more we can learn about the history of the smallpox virus," says Jones. In order to better understand the development of the virus strains - and to recognize at an early stage when new infectious agents that are highly dangerous for humans arise.