The surprise find of smallpox DNA in a child mummy from the 17th century could help scientists start to trace the mysterious history of this notorious virus.
Smallpox currently only exists in secure freezers, after a global vaccination campaign eradicated the virus in the late 1970s. But much about this killer remains unknown, including its origins.
Now scientists have the oldest complete set of smallpox genes, after they went hunting for viral DNA in a sample of skin from a mummified young child, probably a boy, that was found in a crypt underneath a Lithuanian church.
"While ancient pathogen research is kind of a booming field right now, most of what's been done is actually bacterial. There's very little viral work," says Ana Duggan, a researcher at McMaster University in Canada, who explains that the original goal of this study was just to see what kind of viruses might be detectable in this centuries-old sample.
The team was not expecting to find smallpox, but there it was — even though the remains of the child showed no signs of the disease, like pockmarks.
Henrik Poinar, director of McMaster University's Ancient DNA Center, says the smallpox virus is unusual because it doesn't infect other animals. "It seems really to be human specific," he says, noting that other pox viruses out in nature, like camelpox and gerbil pox, do infect animals.
So if smallpox originally jumped into humans from an animal, scientists have no idea what animal it was or whether that species is still out there. "I find that fascinating," says Poinar. "This is a major pathogen that's caused tremendous mortality and morbidity in humans."
And it has had a major effect on human history. For example, historians think that Native Americans were decimated by smallpox brought to the New World by Europeans.
Scientists have assumed the virus must have been infecting people for thousands of years. And there's some evidence to support that, like pockmark scars on the 3,000-year-old remains of Egyptians.
"Whether or not those are real smallpox or measles or chicken pox or something else remains, I think, a question," says Poinar.
So once he and his colleagues found that smallpox DNA from the child who lived around 1665, they were eager to compare it to DNA from more recent smallpox strains collected from the 1940s to 1970s.
What they saw suggests the timeline of the kind of smallpox eradicated in the 20th century may not go back as far as scientists have thought. The common ancestor among all these strains seems to be pretty recent, from around 1588 to 1645.
"That date is coincident with many outbreaks within Europe," says Poinar, who notes that historical records suggest that a shift occurred about that time, with smallpox seeming to infect more people and become more virulent.
Paul Keim, a biologist at Northern Arizona University who has used DNA of ancient plague bacteria to help trace the history of that disease, says the smallpox find is fascinating — but since they have only one ancient example so far, you can't draw too many conclusions.
"All we can really say is that the smallpox that was trafficking around the world at the end of its existence [in the 1970s] can be traced back to the 17th century," says Keim. "There were other models out there that were arguing that contemporary strains had gone back thousands of years, and that doesn't look like the case."
It's probable that there are other strains of smallpox that circulated way back when, says Keim, and as scientists find more ancient samples of smallpox DNA, they'll be able to better understand the history of this storied virus — as well as the way it has affected humanity.
ARI SHAPIRO, HOST:
We're going to look now at some of the mysteries behind one of the deadliest diseases humanity has ever faced. There's a lot about smallpox that remains unknown, like where it came from and how it became such a killer. Now scientists have found a clue in a 17th century mummy. NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE, BYLINE: In Lithuania, there's a church with a crypt that contains the remains of hundreds of people. A couple dozen of the bodies somehow dried out and became well-preserved natural mummies, including part of a corpse that was found there with no coffin.
ANNA DUGGAN: It's just the lower extremities, so there's no associated torso or head.
GREENFIELDBOYCE: Anna Duggin is a researcher at McMaster University in Canada. She says these legs belonged to a young child...
DUGGAN: ...Estimated to be two to four years of age...
GREENFIELDBOYCE: ...Probably a boy who lived around 1665. Her team got a bit of the mummy's skin and did DNA tests to see if they could recover any ancient viruses.
DUGGAN: Because while ancient pathogen research is kind of a booming field right now, most of what's been done is actually bacterial. There's very little viral work.
GREENFIELDBOYCE: Well, they turned up one ancient virus - smallpox. This is the oldest complete set of smallpox genes ever found. Scientists were able to compare this DNA to the DNA of more modern smallpox viruses, ones collected in a few decades before 1977. That's when a global vaccination campaign wiped out smallpox from nature. In the journal Current Biology, the researchers report that all of these strains - the ancient and more modern - seem to have shared a pretty recent common ancestor, a virus that was out there around 1588 to 1645. And biologist Paul Keim says that seems to kill some theories about smallpox.
PAUL KEIM: You know, there were other models out there that were arguing that contemporary strains had gone back thousands of years, and that doesn't look like the case.
GREENFIELDBOYCE: Keim is from Northern Arizona University. He didn't work on this study, but he has used DNA of ancient plague bacteria to trace the history of that disease. He says with only one sample of ancient smallpox DNA, you can't draw too many conclusions.
KEIM: All we can really say is that the smallpox that was trafficking around the world there at the end of its existence can be traced back to the 17th century.
GREENFIELDBOYCE: Still, the link to the early 17th century is tantalizing. Hendrik Poinar is director of McMaster University's ancient DNA center.
HENDRIK POINAR: That date is coincident with many outbreaks within Europe.
GREENFIELDBOYCE: Poinar says historical records suggest that a shift occurred about that time, with smallpox becoming more virulent, but there is evidence that some versions of the virus were around long before that. There's even signs of pockmarks on 3,000-year-old mummies from Egypt.
POINAR: Whether or not those are real smallpox or measles or something else remains, I think, a question.
GREENFIELDBOYCE: He's hoping to get more ancient DNA samples to find out. The biggest question is where smallpox came from in the first place. What's unusual about this virus is that it only infects people.
POINAR: So it seems really to be human-specific.
GREENFIELDBOYCE: Scientists think smallpox may have jumped into humans from an animal, but they don't know what animal or when or how.
POINAR: I find that fascinating. I mean, this is a major pathogen that's caused tremendous mortality, morbidity in humans.
GREENFIELDBOYCE: He says understanding its history is really important because there's other viruses out there that could pose a threat. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.