The Black Death strain of bubonic plague that killed 50 million people in Europe during the 14th century didn't disappear after the pandemic ended. It kept on killing Europeans, on a smaller scale, for centuries after the end of the original pandemic, two new studies show.

Black Death wiped out about half the population of the Continent between 1347 and 1351.

DNA analysis previously confirmed that the pandemic was caused by a virulent strain of Yersinia pestis, the bacteria that cause bubonic plague.

Since then, a huge number of distinct strains of the plague have been identified in rodents in Central Asia. Those discoveries had led scientists to suggest that outbreaks of bubonic plague in Europe over the next few centuries were caused by new strains being introduced via trade routes such as the Silk Road.

The Great Plague of Marseilles in the 18th century had a mortality of about 20 per cent and killed tens of thousands. (Michel Serre/Musée des Beaux Arts Marseille)

But an analysis of bacterial DNA in teeth from victims of a plague outbreak in Marseilles, France, in 1722, found they were not infected with new strains of plague. Instead, they were killed by descendants of the Black Death strain, reports a group of Canadian, German, U.S., French and Australian researchers in a paper published late last week in the journal eLife.

"That was a big surprise," said Hendrik Poinar, an associate professor and director of the Ancient DNA Centre at McMaster University, who co-authored the study. It was led by Kirsten Bos at the University of Tubingen.

Another study released earlier this month analyzed plague DNA in the teeth of people who died of the plague in Germany in the 17th century and found it belonged to the same strain that killed people in a different part of Germany during the 14th century.

"They're all nearly identical or identical," said Holger Scholz, senior scientist at the Bundeswehr Institute of Microbiology in Munich, who co-authored the paper published in PLOS ONE.

Soldiers who died of the plague in Germany in the 17th century had the same strain as plague victims from a different part of Germany in the 14th century. (Seifert et al.)

New pandemics of other diseases, such as influenza, are often started by new strains introduced from animals and carried along travel routes.

And a previous bubonic plague pandemic, the Plague of Justinian during Roman times, was caused by a strain distinct from the Black Death. The new findings suggest that the Black Death was hiding somewhere in Europe for centuries, re-emerging from time to time, and scientists still don't know whether it was in the soil, in animals or elsewhere.

Mysterious disappearance

Poinar added, "That brings up the followup million-dollar question: Why did it disappear?"

The bubonic plague hasn't been seen in Europe since the end of the 18th century.

However, bacterial descendants of the Black Death have caused recent outbreaks in the U.S. and Madagascar, along with one that hit Hong Kong in the late 1800s.

Apparently, new Asian strains didn't come down the Silk Road into Europe as thought. Instead, after devastating Europe for centuries, the Black Death went back up the Silk Road.

'Black Death is really responsible for the bulk of plague epidemics over the past 700 years,' said McMaster University researcher Hendrik Poinar. (McMaster University)

"What we continue to see is Black Death is really responsible for the bulk of plague epidemics over the past 700 years," Poinar said.

The researchers in the two groups used two different techniques to identify the Black Death strain.

Poinar and his colleagues used Y. Pestis DNA sequences to fish out the entire genome found in the plague victims.

Scholz and his colleagues used a technique that looks for DNA mutations in 19 positions that vary depending on the strain of bacteria.

Poinar said that because the DNA fragments in that technique are smaller, it's a less reliable technique for positively identifying different strains.

Scholz disagrees, but is pleased that both studies show similar results.

"We now have two publications telling the same. That's good."

Poinar said he is now working to confirm whether the same strain was responsible for plague outbreaks in other parts of Europe.