Ancient Danish Teeth Reveal Clues to the Evolution of the Plague

A recent study has discovered new information surrounding the arrival and spread of the plague in Scandinavia. The research, which involved the analysis of hundreds of ancient human teeth, is published in Current Biology.

Sampling ancient DNA

Caused by the bacterium Yersinia pestis (Y. pestis), the plague is estimated to have killed around a third of the European population in the 14^th century in a pandemic known as the Black Death. In this new study, researchers from McMaster University’s Ancient DNA Centre have documented the evolution of Y. pestis over an 800-year period within Denmark, helping us to understand more about the origins and persistence of the disease in northern Europe.

“We know that plague outbreaks across Europe continued in waves for approximately 500 years, but very little about its spread throughout Denmark is documented in historical archives,” explained Ravneet Sidhu, a PhD student and one of the lead authors of the study.

The research team, in collaboration with historians and bioarcheologists, examined historical samples from almost 300 individuals across 13 different archeological sites. Fragments of Y. pestis genomes were recovered from the teeth of these ancient individuals, then reconstructed and sequenced. In total, the researchers found samples from 13 individuals tested positive for Y. pestis, who together lived and died over a period spanning 3 centuries.

The Y. pestis strains they unearthed were analyzed to determine the similarities and differences between strains found across Denmark from 1000–1800 AD, as well as modern-day strains. Nine of the identified samples provided decent clues to draw conclusions on the plague’s evolution in Denmark, revealing how both rural and urban populations were impacted by repeated waves of infection from outside sources.

Waves of the plague

DNA recovered from Y. pestis samples showed that Danish sequences were interspersed with medieval strains from other areas such as Russia and the Baltic regions, rather than showing DNA from a single cluster in Denmark that re-emerged from natural reservoirs across the centuries.

“The high frequency of Y. pestis reintroduction to Danish communities is consistent with the assumption that most deaths in the period were due to newly introduced pathogens. This association between pathogen introduction and mortality illuminates essential aspects of the demographic evolution, not only in Denmark but across the whole European continent,” said Professor Jesper L. Boldsen, paleodemographer at the University of Southern Denmark and one of the study’s co-authors.

Additionally, the researchers detail the earliest known appearance of Y. pestis, dating back to 1333 in the western Danish town of Ribe, as well as its spread into rural areas and eventual disappearance by 1649. Most of the places affected by the plague were port towns and cities, but evidence of infection in rural, inland populations suggests it spread over the land, possibly facilitated by infected rodents or lice traveling with humans.

“The results reveal new connections between past and present experiences of plague, and add to our understanding of the distribution, patterns and virulence of re-emerging diseases,” said Professor Hendrik Poinar, senior author of the study and director of the McMaster Ancient DNA Center. “We can use this study and the methods we employed for the study of future pandemics.”

Reference: Eaton K, Sidhu RK, Klunk J, et al. Emergence, continuity, and evolution of Yersinia pestis throughout medieval and early modern Denmark. Curr. Biol. 2023;0(0). doi: 10.1016/j.cub.2023.01.064

This article is a rework of a press release issued by McMaster University. Material has been edited for length and content.