Scientists decode genome of plague

It's not nearly as well-known as the Black Death, but the sixth-century Justinian plague was just as deadly, wiping out an estimated 30 million to 50 million people in only two years as it spread across Asia, North Africa, Arabia and Europe.

Now an international team of researchers, including Canadian disease detectives, have determined the two pandemics resulted from distinct strains of the bacterium that causes plague.

Using tiny fragments of DNA extracted from the 1,500-year-old teeth of two Justinian plague victims buried in Germany, the scientists were able to reconstruct the genome of the strain of Yersinia pestis that caused the AD 541-543 pandemic — making it the oldest pathogen genome decoded to date.

While the Justinian strain flared up periodically over the next hundred years to cause subsequent outbreaks, it eventually died out, said Hendrik Poinar, an evolutionary geneticist who directs the Ancient DNA Centre at McMaster University.

The strain that caused the Black Death, ravaging half the population of Europe eight centuries later, was a distinct form, which re-emerged in the late 1800s and spread worldwide, he said.

"The Justinian looks like it came, it flared up, it killed its people and then there were no more suitable hosts or it didn't survive underground and did not re-emerge in that form," Poinar said. "So the Black Death is actually a completely novel emergence of Yersinia pestis 800 years later."

The findings, published Monday in the journal Lancet Infectious Diseases, suggest a new strain of plague could emerge again in humans 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 of the Center for Microbial Genetics and Genomics at Northern Arizona University, one of the study's co-authors.

"If the Justinian plague could erupt in the human population, cause a massive pandemic and then die out, it suggests it could happen again," Wagner said in a release. "Fortunately, we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large-scale human pandemic."

The DNA samples used in the research were taken from two victims of the Plague of Justinian —named after the Roman emperor of the time — who were buried in gravesites in an early medieval cemetery in Aschheim, Germany, near Munich.

Based on carbon-dating and grave goods found with the skeletal remains, scientists believe the victims died in the latter stages of the epidemic when it had reached southern Bavaria, sometime between 541 and 543.



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