Medical practice

New strains of the Black Death could emerge

"Plague that killed millions is able to rise from the dead,” warns The Independent after a new study looked at the genetic history of the Yersinia pestis bacteria.

Plague is a devastating disease which causes rapid death if left untreated. There have been three plague pandemics in recorded history. The most famous was the second – the “Black Death” of the 14th to 17th centuries in Europe.

There was a smaller pandemic in the 19th and 20th centuries. However, little is currently known about the genetics of the first pandemic in recorded history – the “Plague of Justinian” of 6th–8th centuries AD.

This was reported to have killed around 100 million people, and is seen by many historians as contributing to the fall of the Roman Empire and the onset of the Dark Ages.

Using DNA extracted from the teeth of two people who died at the time of the Plague of Justinian, researchers found it was a caused by strain unrelated to the Black Death.

The fact the first two pandemics were caused by two independent strains of Y. pestis demonstrates how fresh strains could be passed into human populations today.

Before you run for the hills, plague can be now be effectively treated with antibiotics.

It is important never to be complacent. Understanding the different strains of bacteria that have caused pandemics in the past is important for planning for possible future pandemics and antibiotic development.

Where did the story come from?

The study was carried out by a large collaboration of researchers from a number of different international research institutions and was funded by McMaster University, Northern Arizona University, Social Sciences and Humanities Research Council of Canada, Canada Research Chairs Program, US Department of Homeland Security, US National Institutes of Health, Australian National Health and Medical Research Council.

One of the study participants had a financial conflict of interest involved in the work – he had financial interests in one of the companies that produces genetic scanning equipment used during the study. However, it is difficult to see how this could lead to any bias in reporting or analysis.

The study was published as an open access article in the peer-reviewed medical journal, The Lancet Infectious Diseases. This means it is freely available online for all to read or download.

You won’t be surprised that UK headline writers went to town with this story, with dire warnings of the “Black Death striking again”. In fact, the Black Death has never gone away – there are still occasional outbreaks in the developing world such as the type that occurred in Madagascar at the end of 2013.

The actual reporting in the media did acknowledge that we have antibiotics to treat plague, and this may lessen the spread and deaths compared with the past – but you can be sure that the bleak yet inaccurate headlines will have sold more newspapers.

What kind of research was this?

The research was largely a laboratory and computer-based exercise looking to understand the strain of the plague bacteria Y. pestis that caused the Plague of Justinian. Researchers then wanted to know how it related to the strains of the bacteria that caused two other major plague pandemics, and to modern day strains of Y. pestis.

Y. pestis is a bacteria carried on the fleas of rodents, including rats. There are many varieties of Y. pestis, only some of which manage to transfer into humans, and only some of which cause disease or pandemics. The fact that rodents continually carry these bacteria-containing fleas is often referred to as a “disease reservoir”, in the acknowledgement that a strain harmful to humans may emerge.

Plague infections still occur in humans today, predominately in African and Asian countries. Plague can be treated with modern antibiotics, but these must be given as soon as possible to prevent serious illness or possible death.

By analysing the plague bacteria’s DNA, and noting differences and similarities, scientists can tell if the same or similar bacteria strain was involved in different plague pandemics at different periods in history. This is akin to a kind of genetic family tree of germs.

You can work out whether the same strain keeps re-emerging over the centuries, or whether new strains are created each time. This is important for developing treatments and strategies to reduce the impact of any future possible outbreaks (known as “pandemic preparedness”).

What did the research involve?

The researchers’ extracted DNA from the plague-causing bacteria Yersinia pestis from the teeth of two German people who died at the time of the Plague of Justinian. They analysed the bacteria DNA in the lab and compared it using a large computer database to the bacterial DNA known to have caused the two other pandemics so see how similar they were.

Radiocarbon dating of the two individuals placed them in the timeframe of the first pandemic (533AD and 504AD).

What were the basic results?

The researchers’ main finding was that the strain of Y. pestis from the Plague of Justinian period had a distinct history from all known modern day strains they compared it against. This meant it was either completely distinct and had died out, or it still existed somewhere but no one had yet recorded it in modern times.

As well as having a distinct history to modern day Y. pestis strains, the ancient bacteria was also distinct from the Y. pestis responsible for the two subsequent Black Death pandemics. This showed there was a lack of shared ancestry between the two pandemic strains, indicating the three pandemics weren’t the re-emergence of the same strain at different times.

How did the researchers interpret the results?

The authors’ interpretation was that “Y. pestis lineages that caused the Plague of Justinian and the Black Death 800 years later were independent emergences from rodents into human beings. These results show that rodent species worldwide represent important reservoirs for the repeated emergence of diverse lineages of Y. pestis into human populations.”


This study improves the understanding of the family tree of the plague-causing bacteria Yersinia pestis. It indicated the first plague pandemic was caused by a strain of Y. pestis distinct from the histories of all modern strains of the bacteria, and of the bacteria that caused two subsequent plague pandemics. This type of genetic evidence is persuasive so the conclusions are likely to be reliable.

There are two main interpretations of the results. First, the bacteria that caused the Plague of Justinian came into existence then died out. Second, the bacteria strain that caused the Plague of Justinian remains in existence, but scientists just don’t know about it, so it didn’t show up in their comparisons. The first option is probably more likely but is debateable.

The researchers mention that it is not clear why the Y. pestis lineage associated with the Plague of Justinian eventually died out.

As the researchers suggest, the fact that the causes of the first and second pandemics were two independent strains passed from rodents to humans demonstrates how rodents can act as reservoirs for diverse strains of the plague bacteria. And theoretically, these new strains could be passed into human populations today.

Because of its relative absence in developed nations, there is a false belief that plague has been eradicated, but this is not the case. Plague infections do still occur in humans today, predominately in African and Asian countries. Despite being potentially fatal, plague can now be effectively treated with prompt antibiotics. Understanding the different strains of bacteria that have caused pandemics in the past is important for planning for possible future pandemics and antibiotic development.

This study does reinforce the importance of combatting the increasing problem of antibiotic resistance. If we use antibiotics incorrectly now, such as not completing a full course of antibiotics as prescribed, or using them for conditions that would have got better anyway without the need for treatment, we could end up powerless if a new dangerous and deadly strain of the plague did emerge.

Read more about the NHS Antibiotic Resistance Campaign

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