An international team of scientists has discovered that two of the world’s most devastating plagues – the plague of Justinian and the Black Death, each responsible for killing as many as half the people in Europe — were caused by distinct Yersinia pestis strains, one that faded out on its own, the other leading to worldwide spread and re-emergence in the late 1800s. These findings suggest a new strain of plague could emerge again in humans in the future.
Burial of plague victims at the early medieval cemetery Aschheim. Foto: © H-P. Volpert
Fig. 2: Plague victim analyzed in the lab.
Using sophisticated methods, researchers from various institutions including McMaster University, State Collection of Anthropology and Paleoanatomy, Munich, Bundeswehr Institute of Microbiology, Munich, Northern Arizona University and the University of Sydney, isolated miniscule DNA fragments from the 1500-year-old teeth of two victims of the Justinian plague, buried in Bavaria, Germany. These are the oldest pathogen genomes obtained to date.
Using these short fragments, they reconstructed the genome of the oldest Yersinia pestis, the bacterium responsible for the plague, and compared it to a database of genomes of more than a hundred contemporary strains.
The results are currently published in the online edition of Lancet Infectious Disease. They show 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 has this particular Y. pestis strain no genetic successors and died out?” questions Holger Scholz, head of the department of Bacteriology and Toxinology at the Bundeswehr Institute of Microbiology in Munich.The findings are dramatic because little has been known about the origins or cause of the Justinian Plague– which helped bring an end to the Eastern Roman Empire – and its relationship to the Black Death, some 800 years later.
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 would strike some 800 years later with similar force, killing 50 million Europeans between just 1347 and 1351 alone.
The third pandemic, which spread from Hong Kong across the globe is likely a descendant of the Black Death strain and thus much more successful than the one responsible for the Justinian Plague.
“We know the bacterium Y. pestis has jumped from rodents into humans throughout history and rodent reservoirs of plague still exist today in many parts of the world. If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggests it could happen again. Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic” says Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University. However, we should not underestimate the devastating potential of plague, as in recent years strains emerged which are resistant to antibiotics, routinely used in plague therapy, adds Holger Scholz.
The samples used in the latest research were taken from two victims of the Justinian plague, buried in a gravesite in a small cemetery in the German town of Aschheim. The skeletal remains of the early medieval cemetery of Aschheim are examined by researchers of the Munich State Collection of Anthropology and Paleoanatomy since several years”, says Michaela Harbeck, curator of this institution which keeps ten thousands of skeletons, each of them an unique historical and biological source.The skeletal remains yielded important clues and raised more questions.
Researchers now believe the Justinian Y. pestis strain originated in Asia, not in Africa as originally thought.
“This study raises intriguing questions about why a pathogen that was both so successful and so deadly today only infects about 3000 people each year. From our genome analyses we know that Yersinia pestis from both the Black Death and the Justinian plague was not more dangerous than present Y. pestis strains, says Holger Scholz. One testable possibility is that human populations evolved to become less susceptible,” says Holmes. “Another possibility is that changes in the climate became less suitable for the plague bacterium to survive in the wild,” says Julia Riehm of the Bundeswehr Institut of Microbiology.
Scientists hope their research could lead to a better understanding of the dynamics of modern infectious disease, including a form of the plague that still kills thousands every year.
The research was funded in part by the Social Sciences and Humanities Research Council of Canada, Canada Research Chairs Program, U.S. Department of Homeland Security, U.S. National Institutes of Health and the Australian National Health and Medical Research Council.
For more information please contact:PD Dr. Holger C. Scholz
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