The Justinian plague and the Black Death are two of the deadliest plagues in human history – both responsible for killing almost half of the European population. Now, an international research team has discovered that both plagues were caused by different strains of the same bacterium.

This is according to a study recently published online in The Lancet Infectious Disease.

The Justinian plague occurred in the sixth century. It is estimated to have have killed between 30 and 50 million people as it spread across Asia, North Africa, Arabia and Europe.

The Black Death, also known as the Bubonic plague, occurred almost 800 years later. Between 1347 and 1351, this plague was responsible for the death of approximately 50 million Europeans.

The researchers say very little is known about what caused the Justinian plague and whether it was linked to the cause of the Black Death.

With a view to finding out, the investigators analyzed short DNA fragments from the teeth of two 1500-year-old victims of the Justinian plague who were buried in Bavaria, Germany.

The researchers say that to date, these DNA are the oldest pathogen genomes found.

The researchers reconstructed the genome of the oldest Yersinia pestis – the bacterium responsible for the Justinian plague – using the short DNA fragments.

The plague genome was them compared with over 100 contemporary strains of the bacterium.

From this analysis, the investigators discovered that the Y. pestis strain that was responsible for the Justinian plague was an “evolutionary dead-end” and different from the strains that contributed to the Black Death and plagues that followed.

This means the plague erupted, spread dramatically and killed millions of people before dying out.

The researchers believe that the Y. pestis strain involved in the third pandemic plague, which spread from Hong Kong in 1855, may be a product of the strain involved in the Black Death – explaining why it was significantly more successful than the strain that caused the Justinian plague.

Commenting on the findings, Dave Wagner, of the Center for Microbial Genetics and Genomics at Northern Arizona University and co-author of the study, says:

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.

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.”

The researchers say their findings raise questions as to why a bacterium strain that spread so successfully and was so deadly managed to die out.

“One testable possibility is that human populations evolved to become less susceptible,” says Edward Holmes of the University of Sydney in Australia and co-author of the study.

“Another possibility is that changes in the climate became less suitable for the plague bacterium to survive in the wild,” he adds.

Previous to this study, researchers believed that the Y. pestis strain that caused the Justinian plague originated from Africa. However, the investigators say these new findings have led them to believe that it originated in Asia.

But they say they were unable to create a “molecular clock,” so they do not know the time-scale in which the strain evolved.

They add that their findings also suggest that earlier outbreaks, such as the Antonine plague and the plague of Athens, may have been caused by other distinct strains of Y. pestis.

Last year, Medical News Today reported on a study suggesting that the Black Death could return in the form of antibiotic-resistant bacteria.