Two of the world’s deadliest pandemics – Justinian’s Plague and the Black Death – were caused by the same pathogen. These findings were revealed yesterday in an article published in the journal Lancet Infectious Diseases.
The researchers, who include scientists from McMaster University in Canada, the University of Sydney and Northern Arizona University, were able to gather minuscule plague DNA fragments from the 1,500-year-old teeth of two victims of Justinian’s plague, buried in Bavaria, Germany.
The team reconstructed the genome of the oldest strain of Yersinia pestis, the bacterium responsible for the bubonic plague, and compared it to a database of genomes of more than one hundred contemporary strains. It shows that the strain responsible for the Justinian outbreak was an evolutionary ‘dead-end’ and distinct from strains involved later in the Black Death and other plague pandemics that would follow.
“The research is both fascinating and perplexing, it generates new questions which need to be explored, for example why did this pandemic, which killed somewhere between 50 and 100 million people die out?” asks Hendrik Poinar, director of the McMaster Ancient DNA Centre.
The plague of Justinian struck in the sixth century and is estimated to have killed between 30 and 50 million people— virtually half the world’s population as it spread across Asia, North Africa, Arabia and Europe. The Black Death struck about 800 years later with similar force, killing 50 million Europeans between 1347 and 1351. These findings suggest a new strain of plague could emerge again in humans in the future.
Researchers now believe the Justinian Yersinia pestis strain originated in Asia, not in Africa as originally thought. But they could not establish a ‘molecular clock’ so its evolutionary time-scale remains elusive. This suggests that earlier epidemics, such as the Plague of Athens (430 BC) and the Antonine Plague (165 -180 AD), could also be separate, independent emergences of related Yersinia pestis strains into humans.
“The tick of the plague bacteria molecular clock is highly erratic. Determining why is an important goal for future research” says Edward Holmes, an NHMRC Australia Fellow at the University of Sydney. “This study raises intriguing questions about why a pathogen that was both so successful and so deadly died out. One testable possibility is that human populations evolved to become less susceptible.”
“Another possibility is that changes in the climate became less suitable for the plague bacterium to survive in the wild,” adds Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University.
While the Yersinia pestis plague is still active, and there have been isolated outbreaks reported throughout the world, the chances of another Justinian’s Plague or Black Death are unlikely because of antibiotics and better public health and hygiene. Wagner notes, “We don’t think we’re going to see new large-scale plague pandemics. Not because the organism has changed—it’s just as deadly as it always was—but humans have changed.”
The article, ‘Yersinia pestis and the Plague of Justinian 541—543 AD: a genomic analysis‘ is published in Lancet Infectious Diseases.
Sources: McMaster University, Northern Arizona University, University of Sydney