An international team of researchers has discovered that two of the deadliest pandemics in history, the Plague of Justinian and the Black Plague, were caused by strains of the same plague. They warn that mutated — or even bioengineered — versions of the bacteria could lead to future outbreaks.
Above: Pieter Bruegel's The Triumph of Death (c. 1562)
The culprit is Yersinia pestis, a bacterium that can infect humans and other animals. Scientists now suspect that separate and independent emergences of this bacterium have been responsible for some of history's worst pandemics. And it may not be done yet.
During the sixth century AD, this bacterium caused the Plague of Justinian, killing between 30 and 50 million people — virtually half of the world's population at the time. It spread across Asia, North Africa, Arabia, and Europe before mysteriously petering out.
But it would eventually return. Some 800 years later it re-emerged as the Black Death, a blight that killed 50 million Europeans over a four year period from 1347 to 1351 — an astounding figure, to be sure. It's a death rate that averages out to 34,245 mortalities per day.
Just as disturbing, a strain of Yersinia pestis appeared in the late 1800s, spreading from Hong Kong to across the globe. Today, thousands of cases of plague are still reported to the World Health Organization each year. But with proper treatment (i.e. antibiotics), prognosis is considerably better than it was in the past.
Scientists from several universities, including McMaster University, Northern Arizona University, and the University of Sydney, were able to isolate Yersinia pestis as the agent responsible in these pandemics by isolating miniscule fragments of DNA from the 1,500-year-old teeth of two victims of the Justinian plague who were found buried in Bavaria, Germany. These are the oldest pathogen genomes obtained to date. The researchers reconstructed the genome of the oldest Yersinia pestis and compared it to a database of genomes of more than a hundred current strains.
The scientists learned that the strain responsible for the Justinian outbreak was an evolutionary "dead-end," and distinct from the strains that would re-emerge later. The going theory is that the bacterium originated in Asia and not in Africa as previously thought. Though a definitive "molecular clock" could not be established, the research suggests that earlier epidemics, such as the Plague of Athens in 430 BC and the Antonine Plague from 165 to 180 AD, may be separate versions related to Yersinia pestis.
"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," noted Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University.
The study, which now appears in an online edition of The Lancet Infectious Diseases, raises awareness of the possibility that more re-emergences are still to come.
"If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggest it could happen again," says Wagner. "Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic."
The researchers say that scientists should heighten their surveillance of plague in rodent populations to try avert future human infections.
Disturbingly, Yersinia pestis is considered a possible biological warfare agent and the Center for Disease Control in the United States has classified it as a category A pathogen requiring preparation for a possible terrorist attack.
Read the entire study at The Lancet: "Yersinia pestis and the Plague of Justinian 541—543 AD: a genomic analysis."