Plague is a disease that conjures a mix of horror and revulsion at its mere mention because of the mark it has left on human history. Plague wiped out entire regions, triggered economic and political collapse, and threatened Western civilization on more than one occasion.
As much of an impact plague has had over the course of millennia, The bacterium that causes plague infections, Yersinia pestis, has been haunting humans even longer than once thought, report scientists in the journal Cell. In fact, Y. pestis was common as far back as the Bronze Age.
For their study, a team of researchers drawn from a network of European academic institutions sequenced the DNA from the teeth of Bronze Age 101 individuals from Europe and Asia. What they found was a common ancestor to Y. pestis dating back 5,783 years.
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"Our findings suggest that the virulent, flea-borne Y. pestis strain that caused the historic bubonic plague pandemics evolved from a less pathogenic Y. pestis lineage infecting human populations long before recorded evidence of plague outbreaks," the authors wrote in their journal article.
Prior to the evolution of later strains of Y. pestis, this ancestral strain lacked two key weapons in its genetic arsenal.
Bronze Age Y. pestis lacked a gene called Yersinia murine toxin (ymt). Ymt allows the bacteria to survive in the gut of a flea, a critical disease vector. Beginning between 3,700 and 3,000 years ago, this gene turns up in the DNA of Iron Age individuals, which suggests human transmission was possible in that era.
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A second mutation not seen in Bronze Age Y. pestis directly affects the ability of the bacteria to cause bubonic plague. The gene pla facilitates deep tissue invasion, which allows the plague to travel into the lymph nodes, weakening the host's immune response and permitting widespread tissue damage. One of the most apparent symptoms of bubonic plague infection is swollen and tender lymph nodes, known as buboes.
Just because Bronze Age individuals didn't contract bubonic plague doesn't mean they weren't deathly ill. Y. pestis also leads to pneumoic plague, a disease of the lungs passed on through human-to-human contact by inhaling droplets coughed by an infected person.
Pneumonic plague is the deadliest disease variant triggered by Y. pestis. At its worst, it leads to bloody mucus, respiratory failure and shock, with the infected individual eventually coughing violently to death. Even today, the mortality rate for this disease stands at nearly 100 percent if antibiotics aren't administered within 24 hours of infection. And even with the best of modern medicine, the person only has a 50-50 shot at survival.
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The Bronze Age also happened to be a period of mass migration and intermixing among disparate populations, previous studies have shown. "In light of our findings, it is plausible that plague outbreaks could have facilitated - or have been facilitated by - these highly dynamic demographic events," the authors write.
Understanding how Y. pestis became the potent such an effective killer provides insights into the evolution as disease as well as additional history on the plague itself.
Following the Bronze Age, a more evolved strain of Y. pestis left its mark on human events. In the sixth century, the Justinian plague was followed up by frequent bouts of bubonic plague for two centuries that claimed an estimated 25 million lives, notes the Centers for Disease Control and Prevention (CDC).
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Eight hundred years later, plague triggered what would be known as the Black Death, which killed off upwards of 60 percent of Europe's total population at the time in less than a decade. The last major plague outbreak originated in China in the 1850s and by the end of the century took 10 million lives.
Although often ignored as a disease, considered a disease only found in the history books, plague is still around today, active in countries around the world, including Peru, the Democratic Republic of the Congo, India, China and even the United States. Upward of 2,000 new cases of plague are identified annualy to the World Health Organization, but the number is likely underreported.