A new strain of the Black Death – which killed half the people in the world in the 14th Century – could emerge without warning, scientists say. And while modern medicine would help to combat such an outbreak, the ease with which people move around the globe could make it worse.
Previously, scientists thought that the plague bug, Yersinia pestis, had jumped from rodents to humans just once and was now under control (though it killed more than 60 people in Madagascar in 2012).
But an analysis of the bug found on the teeth of Bavarian victims of the 6th Century Plague of Justinian shows that different genetic strains have crossed the species barrier at least twice.
The Justinian strain is quite different from the variety that caused the Black Death and the Third Pandemic that broke out in Yunan, China, in 1855, according to a paper published in Lancet Infectious Diseases.
The earlier strain quickly became extinct, possibly because it killed off so many of its hosts.
“If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggests it could happen again,” said David Wagner of Northern Arizona University.
The research also raises the possibility that two earlier outbreaks, the Plague of Athens in 430BC and the Antonine Plague of AD165-180, might have been due to Y pestis, though proving it will be difficult, said Thomas Gilbert, a professor of paleogenomics at the Natural History Museum of Denmark, who wrote a commentary to the Lancet article.
“What this shows is that the plague jumped into humans on several different occasions and has gone on a rampage. That shows the jump is not that difficult to make and wasn’t a wild fluke,” Dr Gilbert said. “Humans are infringing on rodents’ territory, so it’s only a matter of time before we get more exposure to them.”
Hendrik Poinar, director of the Ancient DNA Centre at McMaster University in Ontario, said about 200 rodent species act as reservoirs for the bug. “Plague is something that will continue to happen but modern-day antibiotics should be able to stop it.”
However, if the disease reaches the lungs and becomes airborne, spread by coughing, it would be harder to beat, he said. That type of plague can kill within a day.
Although the disease is common in rodents, it is passed from one victim to another by Oriental rat fleas, Xenopsylla cheopis. The bacteria create a biofilm within the gut of infected fleas that prevents them from feeding. Instead of sucking blood when they bite, they regurgitate the bacteria into the new host, where it typically infects the lymph system, causing swelling.
The medieval belief that plague was carried by cats and dogs persuaded some authorities to have them killed, which may have aided the spread of the disease by boosting rat populations.
Y pestis is thought to have had far reaching effects on human history. The Justinian Plague may have weakened Byzantine forces as they tried to re-establish the Roman Empire in Italy and it has been suggested that it may have helped invading Saxons conquer large parts of Britain.
In the 14th Century, the Black Death reduced European populations so much that the price of land fell while wages rose, setting the stage for broad social changes in later centuries. And the Great Plague of London is famously said to have driven Isaac Newton to the countryside, where a, possibly apocryphal, falling apple inspired his discovery of gravity.