Blast from the deadly past

Many people think of the plague as a killer disease that’s been safely consigned to the annals of history.

However, the truth is that the notorious rodent-borne infection, which killed half the population of Europe during the 14th century’s Black Death, still strikes thousands of people every year, and bacteria similar to that which has caused three plague pandemics over the last 1,500 years still lurks in rat populations today.

New research into the pathogen (disease-causing agent) responsible for the first, Justinian, plague pandemic from 541AD, has found that it was caused by a distinct strain of the same pathogen that was responsible for the Black Death.

The findings suggest that a new strain of plague could emerge again in humans in the future.

The disease is usually transmitted through the bites of infected fleas which live on rodents, often rats. “Humans are just an incidental host, but a very important one,” explains Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University, who formed part of an international team of researchers that conducted the new plague research.

They extracted DNA from the 1,500-year-old remains of German victims of the Justinian plague, which is estimated to have killed between 30 and 50 million people - virtually half the world’s population at that time.

From the DNA, the team reconstructed the genome (complete set of genetic material) of the plague bacterium Yersinia pestis, and compared it to a database of plague genomes.

“What we found was that the closest relatives to that ancient strain that are still alive today are found in rodent populations in China,” says Wagner.

“There was really nothing that drastically separated the first pandemic strain from the strains that are circulating today.”

A bite from a flea infected with one of the strains can lead to bubonic plague, which causes fever, headache, chills, weakness, and swollen, tender and painful lymph nodes called buboes, which progressively darken, giving rise to the name Black Death.

If left untreated, bubonic plague can kill from septic shock within about three to six days of the onset of symptoms, or sometimes turn into the rarer pneumonic plague, which can be spread from person to person through infected droplets, in a similar way to the spread of colds and other respiratory illnesses.

But there’s no need to panic.

As Wagner points out, these days, if the plague did make a comeback, it’s unlikely there’d be a pandemic, because hygiene has improved dramatically and rat populations are controlled.

But the main reason plague isn’t likely to decimate humans again is because simple antibiotics, like doxycycline or tetracycline, can stop it in its tracks, if given early enough. Late diagnosis can lead to death, even with antibiotic treatment.

“If it looked like a big plague outbreak was happening, the World Health Organisation and other health bodies would control it with antibiotics,” reassures Wagner.

“We don’t want to scare people and make them think there’s going to be another major pandemic - we don’t think there will be because of the use of antibiotics.

“It’s a cautionary tale that there are diseases in animal reservoirs around the world. We have to remain vigilant to them and respond accordingly.

“Humans were involved in the spread of plague in all three pandemics, and we have to take responsibility for that and take care that we’re not spreading diseases around the world.”

Historically, plague pandemics are thought to have killed at least 100 million people in total, with the first major recorded incidence being the Justinian plague, which helped bring an end to the Roman Empire when it rampaged through Europe for 200 years around the sixth century.

The Black Death struck around 800 years later with similar force, killing 50 million Europeans between 1347 and 1351.

“With the Black Death, there were huge rat populations in places like London, which was important,” Wagner points out.

“It wasn’t that long ago that humans, and certainly poor humans, had fleas of their own, so in the bedding there would’ve been fleas that fed on humans at night.

“It’s just as likely that during the great pandemics, the plague was being spread from human to human by fleas, as much as it was by pneumonic plague.”

A third pandemic, which began in China in the 1860s and appeared in Hong Kong by 1894, spread around the world via steam ships over the next 20 years, causing around 10 million deaths.

“As most plague strains are highly virulent to humans, what the similarity between the first pandemic strain and those circulating today tells us, is that plague emerged from rodents multiple times to cause the great pandemics, and because it’s established in rodents around the world today, it still remains a human health threat,” warns Wagner.

The last outbreak of indigenous plague in the UK was recorded in 1918, and since then there’s been no trace of the disease in this country.

Dr Tim Brooks, director of Public Health England’s Rare and Imported Pathogens Laboratory, agrees with Wagner’s assertion that another pandemic’s unlikely.

“No cases of the plague, either indigenous or imported, are known to have been reported in the UK since 1918,” he states.

“Epidemics of plague are associated with particular combinations of living conditions, general public health and lack of infection control measures that no longer pertain in Western Europe.

“Individual cases can be treated very effectively with antibiotics, provided the disease is identified in its early stages, and infection is easily controlled by modern public health systems.”

Exactly why the UK’s plague problem died down when it did is uncertain.

“It didn’t go away for the rest of the world, just the UK and Europe,” Wagner points out.

He says speculation suggests that the reason for this may have been due to the climate changing, as this can have an adverse effect on rat and flea populations.

Equally, it’s unclear why Britain’s worst taste of the plague, the Black Death, finally stopped killing people in the 14th century, although Wagner suggests: “There could have been some resistance developing in humans, although that’s not clear, or it could just be that because it killed so many people, there were less people around, so there was less contact between individuals and less ability to spread from human to human.”

In modern times, most human cases have occurred in Madagascar and other parts of sub-Saharan Africa, usually in small agricultural towns and villages, rather than the large ports and cities that were ravaged by the disease in the past.

Between 1,000 and 2,000 cases a year are now reported to the World Health Organisation.

“Plague is probably going to exist in rodent populations for a long time,” concludes Wagner.

“It’s going to be with us, but there are now relatively few cases of plague every year, and they’re treated with antibiotics.

“I wouldn’t fear plague,” he adds. “I’d be more concerned about things like MRSA and other pathogens that are much more common.”