A Single Strain of Plague Caused Recurrent Outbreaks Over Many Centuries: Yersinia pestis

A new study has suggested that Yersinia pestis—the causative agent of plague—likely entered Europe only once, and later spread to China where it has become the source of modern plague outbreaks.

Maria A. Spyrou, The Max Planck Institute for the Science of Human History, Jena, Germany, and colleagues investigated the evolutionary history of this bacterium and tracked its route through Europe and Asia. The researchers published the findings of their study in Cell Host and Microbe.

“Our results provide support for (1) a single entry of Yersinia pestis in Europe during the Black Death, (2) a wave of plague that traveled toward Asia to later become the source population for contemporary worldwide epidemics, and (3) the presence of an historical European plague focus involved in post-Black Death outbreaks that is now likely extinct,”. the authors write.

The Black Death was an epidemic of bubonic plague, caused by Yersinia pestis, that killed up to half of the European population within just a five-year period between 1347 and 1353. Although the great epidemic of the Black Death ended in 1353, successive waves of plague followed in later centuries. More recent outbreaks of the disease have also occurred in China and India. In addition, plague eventually spread to squirrels and other small mammals in the Americas, Africa, and Asia, and these new species of carriers have allowed the disease to become endemic in some rural areas, including the western United States.

However, although Yersinia pestis has been linked to three great world plague pandemics—the Justinian Plague of 541 that affected the Roman Empire, the Black Death of 1347 that ravaged Europe, and the third in 1894 that emerged in China—how the disease spread through Europe and Asia has been unclear.

With this in mind, Spyrou and colleagues conducted a study to examine the evolutionary history of this deadly bacterium. They dug up ancient graves containing plague victims across Europe to obtain DNA samples from some of the victims: they collected DNA from individuals who died from plague in Spain (victims of the initial wave of the Black Death), Russia (14^th century victims), and Germany (16^th century victims). By studying three victims from separate waves of the second plague pandemic, they aimed to investigate multiple stages of the bacterium’s evolution in medieval Europe.

From the victims’ teeth samples, the researchers found DNA of Yersinia pestis, on 32 individuals from the three sites. Next, they obtained full genome sequences of three of these bacterial isolates, one from each location. They used the genomes to construct a bacterial family tree, comparing their findings to known previously sequenced strains of the bacterium.

They determined that all 3 victims died from infection with a highly similar strain of Yersinia pestis.

The oldest Yersinia pestis strain, from Spain, was the same as previously sequenced strains originating from London at around the same time. This suggests that the bacterium entered the European continent just once during the Black Death, rather than through multiple waves.

The Russian isolate was also highly similar to the London one, although it contained several small mutations. It was also similar to all modern isolates of Yersinia pestis. This was a key finding, the researchers say, because it provides a link between the Black Death and modern plague outbreaks.

In future studies, the researchers also hope to provide further insight into the entry and end points of the Black Death in Europe. “Our research goals are to understand how plague has changed over time, where it spread, and when,” study coauthors Kirsten Bos, PhD, and Alexander Herbig, PhD, told Contagion. However, “[w]hile our current results are of historical interest, we do hope that our descriptions of how plague, and other pathogens, have changed over time will be of interest to the medical community,” they concluded.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.