Using genomic sequencing, an international team of scientists have isolated DNA fragments from 1,500-year-old teeth and linked two of the world’s most devastating plagues. The plague of Justinian and the Bubonic Plague, each responsible for killing as many as half the people in Europe, were caused by sibling strains of the same pathogen, Yersinia pestis, and researchers believe a distinct strain of this pathogen could emerge again in the future. “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,” said Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University. “If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggests it could happen again.” The new research appears in The Lancet.

In 541, during the reign of Byzantine Emperor Justinian, a plague appeared in Egypt and then traveled swiftly, arriving first in the port of Alexandria and then spreading throughout Constantinople; from there it moved to the far reaches of Byzantium. Historians estimate the Plague of Justinian killed between 30 and 50 million people — virtually half the world’s population as it swept across Asia, North Africa, the Arab nations, and Europe. Despite histrionic aspects to the reporting of this plague in its time, what cannot be underestimated is the rapidity with which symptoms appeared and death followed. Roughly 800 years later, the Black Death would assault Europe with a similar force and swiftness, killing 50 million Europeans between just 1347 and 1351 alone. Despite a lack of evidence, many historians have for some time believed the two plagues arose from the same pathogen. Finally, contemporary scientific method has provided the evidence to prove their assumptions correct.

An international team of researchers from McMaster University, Northern Arizona University, and the University of Sydney, among others, isolated DNA fragments from the teeth of two victims of the Justinian plague buried in Bavaria 1,500 years ago. Scientists believe the two victims, buried in a small cemetery in the German town of Aschheim, died in the latter stages of the epidemic when it had reached southern Bavaria, likely sometime between 541 and 543. Using these miniscule fragments of genetic material, the team reconstructed the oldest pathogen genome obtained to date, and after analysis, determined it to be a strain of Y. pestis. Although related to other plagues, the strain responsible for the Justinian outbreak is also distinct from the strains involved in the Black Death and other pandemics that followed, and most importantly, it died off on its own. The scientists believe a third pandemic, which spread from Hong Kong across the globe, is also likely to be another descendant of the same strain though a much more successful one than that responsible for the Justinian Plague.

“This study raises intriguing questions about why a pathogen that was both so successful and so deadly died out,” said Edward Holmes, an NHMRC Australia Fellow at the University of Sydney. “One testable possibility is that human populations evolved to become less susceptible.” Wagner added. “Another possibility is that changes in the climate became less suitable for the plague bacterium to survive in the wild.”

The team of researchers believe the Justinian Y. pestis strain originated in Asia, not in Africa as originally thought, which leads them to believe earlier epidemics, such as the Plague of Athens (430 BC) and the Antonine Plague (165-180 AD), might also be separate though related strains of the gammaproteobacteria Y. pestis. After looking backward into history, the scientists next compared their findings to a database of genomes of more than a hundred contemporary strains. The response to infectious diseases of current times is simply a direct outcome of lessons learned from ancestral pandemics, say the researchers, and that's what makes their new findings important. “Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic,” Wagner stated in a press release.

 

Source: Gilbert MTP, Wagner D, Holmes E, et al. Yersinia pestis: one pandemic, two pandemics, three pandemics, more? The Lancet. 2014.