Smallpox in the Viking era complicates the story of virus evolution

The evolution of the worst virus, humanpox, in human history is only partially understood. Like the novel coronavirus and many other disease-causing viruses, smallpox probably originates in rodents and appears to have leaked into humans probably thousands of years ago. In the 20th century alone, hundreds of millions died.

To date, the earliest confirmed cases of smallpox have been found in mummified bodies of smallpox. 17th century Lithuanian children.. On Thursday, an international team of researchers put the date back 1,000 years ago, Report in Science Journal Recovered smallpox DNA from the bodies of Scandinavian people during the Viking era.

The virus they discovered is now extinct and has not been found in any other more recent skeletal remains. It is not an ancestor of the modern smallpox virus, but an evolutionary dead end. It has more genes than modern viruses, and scientists have observed that among many different poxviruses in nature, fewer genes tend to mean more deadly viruses. Taken together, these facts suggest one prominent smallpox expert suggested that modern viruses may become more deadly as they evolve. Most viruses become less deadly over time.

Poxviruses are not closely related to coronaviruses, and research has not been directly applied to the current coronavirus epidemic. But even the idea of ​​some viruses, which have evolved to become more deadly in the midst of a pandemic, is clearly offensive.

According to experts, the early date of the new smallpox virus is important, but not surprising. Like other poxvirus experts, the authors believe that smallpox almost certainly traces back to the past, although DNA evidence is currently lacking.

According to historical sources, Terry C. Jones, one of the senior authors who studied the evolution of disease-causing organisms at the University of Cambridge, judged by historical sources: It seems, or perhaps China, 1,000 or 1500 years ago in the Common Age.”

Most interesting about the discovery was Dr. Jones’s genetic composition of smallpox virus recovered from the bones of 11 people who lived between 600 and 1050, and the fact that older virus strains are now extinct. Said. As the author calls it, the modern version was eradicated from the population by 1980.

The smallpox virus Latin name is Variola, and other strains of Variola are known. The small variola, which was eradicated with smallpox (major Variola), caused a mild illness with a mortality rate of less than 1%, while smallpox killed about 30% of infected patients. I don’t know why it was less deadly.

The difference in Viking variants is significant enough that the virus constitutes a new group or clade of Variola. It’s not an earlier version of a modern virus. Both modern smallpox and the newly discovered subspecies originate from a common ancestor, but diverged at least 1700 years ago. Dr. Jones said: “The Viking virus was on another evolutionary path that did not lead to modern viruses.”

Klaus Osteriader, a poxvirus expert at the City University of Hong Kong and who had not participated in the study, said the analysis of the Viking virus and the establishment of new clades was very compelling.

The genetic details of the Viking virus probably prompted speculation that the smallpox virus may have become more deadly. Barbara Mulemann, The general understanding of poxviruses, both Cambridge virologists and the first author of the paper, is that viruses with fewer genes aimed at fooling the host’s immune system are actually more deadly. The reason is not clear, but in viral infections, a very strong immune response often kills the victim.

“The pattern we have seen in this paper is that with modern smallpox virus, there is a loss of gene over time, compared to the viking virus, which has a more active gene than the modern smallpox virus.” That’s what happened,” she said. But she and her colleagues warned that there was no direct evidence that the Viking version of the virus was less deadly.

Antonio Alcami, a smallpox specialist at the Autonomous University of Madrid, wrote a commentary that raised the hypothesis of smallpox on the same issue of science. Actually evolved and became more deadly..

He said that the standard view of virus evolution that a virus is less virulent may not always be true. The variola virus has evolved in humans over time. “Maybe it was a mild illness for some time,” he said.

The idea was previously proposed, Dr. Jones said, He suggested that smallpox might once have been a relatively benign disease.

The way this kind of evolution might have happened “is counterintuitive,” said Dr. Genes inactivated by modern smallpox and other deadly smallpox viruses help to weaken or evade the immune response of infected hosts. But why should you lose those genes because they should help the virus?

Somehow, the loss of those genes seems to help the virus, Dr. Alkami said. Perhaps with fewer active genes, the virus can replicate faster, resulting in an increased chance of infecting others despite causing an uncontrolled immune response. He stressed that he raised the idea only as a hypothesis to facilitate discussion and further investigation.

Dr Osterrieder said the idea was still speculative, but thought it made sense. “I think it’s a very convincing hypothesis,” he said.