Two of history’s most devastating human plague outbreaks — the Justinian Plague of the 6th century and the Black Death of the 14th century — were caused by distinct strains of the same bacterium, Yersinia pestis, according to a new study by a team of international researchers.
The Y. pestis strain that caused the Justinian plague apparently experienced an “evolutionary dead-end” and has not reemerged, the study also found. But the one responsible for the Black Death did reemerge in Asia in the mid-19th century, causing what is known as the Third Pandemic.
The study was published Monday in The Lancet Infectious Diseases.
“The research is both fascinating and perplexing,” stated molecular evolutionary geneticist Hendrik Poinar, one of the authors of the paper and director of the McMaster Ancient DNA Centre at McMaster University in Ontario, Canada, in a press release. “It generates new questions which need to be explored. For example, why did [the Justinian] pandemic, which killed somewhere between 50 and 100 million people, die out?”
The study’s findings also suggest, write Poinar and his colleagues in their paper, “that several Y. pestis lineages, which are currently ecologically established in rodent foci worldwide, remain capable of emerging and igniting epidemics of plague in human beings, as they have repeatedly in the past.”
Transferred by fleas
The plague jumps from infected rodents to humans via fleas. The fleas become carriers of Y. pestis after biting an infected rodent. They can then infect humans with a similar bite. Once humans are infected, the disease, which invades the lungs, becomes highly contagious. Humans spread it by coughing up tiny droplets of phlegm that also carry the bacterium.
The Justinian Plague, which the new study suggests started in Asia rather than Africa as previously believed, was so deadly and destructive to existing cultures and societies, it is often cited as a factor in the ending of the Roman Empire and in the transition from the classical era to the medieval period.
Eight hundred years later, the Black Death, which also spread from Asia, killed an estimated 75 million to 200 million people, including half of Europe’s population at the time. It helped stoke religious fears and fanaticism, which led to attacks on minority communities, especially Jewish ones, which were ignorantly blamed for the pandemic.
The Third Pandemic also caused millions of deaths around the world until it died out in the 1950s. More than 12 million people succumbed to the disease in India alone in the late 1880s, causing the British colonial government to impose strict and controversial quarantines.
The World Health Organization still reports occasional outbreaks of new strains of the plague. These outbreaks are contained, however, largely due to the use of antibiotics.
For the new study, Poinar and his colleagues isolated DNA fragments from two teeth of victims of the Justinian plague who had been buried some 1,500 years ago in a grave in Bavaria, Germany. They screened the DNA for the presence of Y. pestis, and then reconstructed the bacterium’s genome. That genome was then compared to a database that contained the genomes of more than a hundred contemporary strains.
Scientists had already confirmed that the Y. pestis responsible for the Third Pandemic had likely descended from the Black Death strain. But they didn’t know if the bacterium that caused the Justinian Plague was a direct ancestor of those later strains.
The DNA analysis showed it wasn’t. The Justinian strain of Y. pestis was found to have no known descendants, which means, according to the researchers, that it is either extinct or health officials have not yet found it in the wild rodent populations they have sampled.
The authors of the new study seem to be leaning toward the extinct explanation. It’s possible, they say, that humans evolved to become less susceptible to that strain of the bacterium. Or climate changes may have made it impossible for the strain to survive in the wild.
Each of the plague pandemics has been linked to a broad change in climate. “All were preceded by periods of exceptional rainfall and ended during periods of climatic stability (around 700-1,000 AD in the case of the Plague of Justinian),” write the researchers.
A fourth pandemic?
Could another great pandemic of plague sweep around the world? It’s possible, say the researchers, but not likely.
“We know the bacterium of Y. pestis has jumped from rodents into humans throughout history and rodent reservoirs of plague still exist today in many part of the world,” states study co-author David Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University, in the press release. “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.”
In 2012, the U.S. Food and Drug Administration approved the antibiotic levofloxacin (Levaquin) for the prevention and treatment of plague. Government officials have cited concerns that Y. pestis might be used in a terrorist attack.
You’ll find an abstract of the study on The Lancet Infectious Diseases website.