Plague found in Siberian hunter-gatherers 5,500 years ago

Plague was killing people in small hunter-gatherer communities near Lake Baikal about 5,500 years ago, according to a study published in Nature. The finding pushes the disease’s lethal record far earlier than the crowded towns and rat-linked outbreaks usually associated with historic plague, the University of Copenhagen said.

An international research team analyzed ancient DNA from human remains at four cemeteries in East Siberia. By sequencing genetic material preserved in teeth, the scientists reconstructed bacterial genomes and identified early strains of Yersinia pestis, the bacterium that causes plague, according to the university.

The team found plague DNA in 18 of 46 people studied. The researchers said that rate, close to 40%, is higher than detections reported for some medieval plague burial sites.

Burials point to rapid outbreaks

The study combined bacterial DNA, human genetic relationships, archaeological evidence and radiocarbon dating to assess what happened in the prehistoric communities, according to the researchers. Ruairidh Macleod, the study’s lead author, carried out the work while at the University of Cambridge and is now a research fellow at the University of Oxford.

At the two largest cemeteries, the researchers found many children and young teenagers among the dead. Andrzej Weber of the University of Alberta, principal investigator of the Baikal Archaeology Project, said archaeologists had been trying to explain that burial pattern since the 1990s, according to the University of Copenhagen.

Radiocarbon results showed that many burials occurred within a short span, the researchers reported. In some graves, people identified as siblings or as parents and children appear to have died at about the same time and were buried together, according to the study.

Eske Willerslev, a senior author and professor at the University of Copenhagen and the University of Cambridge, said the findings show that these early plague strains were already highly lethal. The conclusion addresses a long-running question over whether the oldest known forms of plague caused mild infections or severe disease, according to the university.

An early strain with a potent genetic feature

Earlier work had shown that ancient Yersinia pestis strains did not have some traits later tied to efficient flea and rodent transmission, the University of Copenhagen said. That had led some researchers to question whether the earliest strains could drive deadly outbreaks.

The Nature study found that the Lake Baikal strains carried a distinctive superantigen, a toxin-producing genetic factor not seen in later historic plague strains, according to the researchers. Superantigens can provoke intense immune reactions and are associated with severe inflammation.

Martin Sikora, a senior author and associate professor at the University of Copenhagen, said the finding changes how researchers view the first plague outbreaks. According to Sikora, the ancient bacteria appear to have carried virulence factors that could make infection deadly even before the evolution of efficient flea-borne spread.

Possible route from rodents to people

The findings also support the idea that plague arose in Central or North-East Asia before spreading across Eurasia through wild rodent populations, according to the researchers. The Lake Baikal evidence places an early, lethal form of the disease in that broad region.

Archaeological evidence suggests the hunter-gatherers studied had close contact with marmots, large burrowing rodents that still carry plague today, the University of Copenhagen said. The researchers believe infected marmots may have passed the disease directly to people, setting off outbreaks in the communities represented by the cemeteries.

This story draws on original reporting from ScienceDaily.