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Viking-age smallpox complicates story of viral evolution

The early date of the new smallpox virus, experts said, is significant but not surprising. Like other pox virus experts, the authors think that although DNA evidence is so far lacking, smallpox almost certainly goes much further back in time.

The smallpox virus is shown in this 1975 electronmicrograph from the US Centres for Disease Control.

The smallpox virus is shown in this 1975 electronmicrograph from the US Centres for Disease Control.Credit:AP/File

Terry Jones, who studies the evolution of disease-causing organisms at the University of Cambridge and was one of the senior authors, said that, judging by historical sources, “it seems quite likely that the virus was around in, let’s say, India or maybe China 1000 or 1500 years before the Common Era.”

What was most intriguing about the find, Jones said, was the genetic make-up of the smallpox virus recovered from the bones of 11 people who lived between 600 and 1050 and the fact that the old viral strain is now extinct. The modern version, as the authors call it, was eradicated from the human population by 1980.

The Latin name of the smallpox virus is Variola and other strains of Variola are known. Variola minor, which was eradicated along with smallpox (Variola major), caused a mild illness with less than a 1per cent death rate, whereas smallpox killed about 30per cent of those it infected. Why it was less lethal is not known.

The differences in the Viking variant are significant enough for the virus to make up a new group, or clade, of Variola. It is not an earlier version of the modern virus. Both modern smallpox and the newly discovered variant descended from a common ancestor but diverged at least 1700 years ago.

“The Viking viruses were on a different evolutionary path that could not have led to the modern viruses,” Jones said.

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Klaus Osterrieder, a pox virus specialist at the City University of Hong Kong who was not part of the research, said that the analysis of the Viking virus and the establishment of a new clade was quite convincing.

The genetic details of the Viking virus are what prompted speculation that perhaps the smallpox virus may have become more deadly. Barbara Mühlemann, also a virologist at Cambridge and the first author on the paper, said that the general understanding of pox viruses is that the ones with fewer genes directed at deceiving the immune system of a host are actually more deadly. The reason is not clear, although with viral infections, a very strong immune reaction is often what kills the victim.

“The pattern that we’ve seen in the paper,” she said, “is that there has been a loss of genes over time” in the modern smallpox virus compared to the Viking virus, which had more active genes than the modern virus. But, she cautioned, she and her colleagues have no direct evidence that the Viking version of the virus was less deadly.

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Antonio Alcami, a smallpox specialist at the Autonomous University of Madrid, wrote a commentary in the same issue of Science raising the hypothesis that smallpox actually evolved to become more deadly.

He said that the standard view of viral evolution, in which viruses become less virulent, might not always be true. Variola virus evolved in humans over time. “Maybe it was a mild disease for a while,” he said.

That idea has been suggested before, Jones said, by historians who proposed that smallpox may once have been a relatively benign illness.

The way this kind of evolution might have happened is “counterintuitive”, Alcami said. The genes that are inactivated in modern smallpox and other deadly pox viruses are ones that help weaken or evade immune responses of the infected host. But why lose those genes, since they should help a virus?

Somehow, loss of those genes seems to help the virus, Alcami said. Perhaps with fewer active genes the virus may replicate faster and therefore improve its chances of transmission to another person, even though it is provoking an out-of-control immune reaction, which in the end is what kills the host. He emphasised that he was raising the idea only as a hypothesis to promote discussion and further investigation.

Osterrieder said that even though the idea was still speculative, he thought it made sense. “I think it’s a very compelling hypothesis,” he said.

The New York Times

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