An international team of scientists discovered that two of the world's most devastating plagues -- the plague of Justinian and the Black Death, each of which killed up to half the people in Europe -- were caused by distinct strains of the same pathogen, 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.
The research generates new questions, including: Why did this pandemic die out?
The findings are dramatic because little has been known about the origins of the Justinian Plague, which contributed to the end to the Roman Empire, and its relationship to the Black Death, some 800 years later. The Plague of Justinian struck in the sixth century and 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.
Researchers from several universities isolated miniscule DNA fragments from the 1,500-year-old teeth of two victims of the Justinian plague, buried in Bavaria, Germany. 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 100 contemporary strains. The results are published in the online edition of The 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 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.
This study raises questions about why a pathogen that was 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.
Fortunately, we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large-scale human pandemic.
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