In a study that threatens to overturn the popular history of one of the world’s greatest health disasters, scientists suggest that the bubonic plague had little to do with pest-ridden rats lurking in European cities, but was instead imported from Asia time and again over the 400 years the pandemic lasted.
Researchers in Norway found that historical climate fluctuations in Asia fuelled periodic explosions in the populations of native rodents, including great gerbils and marmots, which harboured the plague bacteria, Yersinia pestis .
When the local climate changed and rodent numbers crashed, their fleas found themselves in need of new homes. Many hopped on to traders and their camels on the ancient Silk Road, and found their way through maritime routes to European ports, the scientists say.
The Black Death arrived in Mediterranean harbours in 1347 and killed more than 30 per cent of the European population in six years. Successive waves of disease came to an end only after the Great Plague of London in 1665.
The pandemic is generally considered to have begun with the one-off arrival of infected rodents from Asia, with the disease then becoming rife in local rats, which spread the disease through their fleas from then on. But if that was the case, the outbreaks in Europe should have risen and fallen with changes in the local climate, because better conditions meant more rodents to carry the disease.
Nils Stenseth and his colleagues at the University of Oslo analysed more than 7,700 records of plague outbreaks in Europe and compared them with tree ring-based climate records from Europe and Asia. The researchers found that plague outbreaks in Europe indeed tracked the climate, but only that in Asia.
Writing in the journal Proceedings of the National Academy of Sciences, the researchers describe how wet spells followed by drought would have caused rodent populations to rise and then plummet, forcing infected fleas to find new hosts. The fleas hitching a ride on traders working the Silk Road is one possibility. More certain is that roughly 15 years after each crash in Asian rodent populations, the plague often sprang up in Europe.
“We find plague to have been repeatedly reimported along the same route as the Black Death was imported, triggered by large-scale climate events in central Asia,” said Boris Schmid, the first author on the study. The Oslo team is collecting tissue from victims of the European plague outbreaks in the hope of reconstructing the genomes of the ancient plague bugs that killed them. Once several of the genomes have been read, scientists will create an evolutionary tree for the pathogen that charts its course through history.
“If the plague that arrived with the Black Death was the ancestor of all the strains of plague in Europe in the centuries afterwards, you will find a different pattern of relatedness than when plague in Europe was repeatedly re—imported from plague reservoirs in Asia,” Mr. Schmid told the Guardian .
For now, great gerbils, marmots and ground squirrels in north west China remain the leading culprits for driving the European plague pandemic.
If the scientists are right in exonerating black rats or other European rodents, it would slash the chances of an outbreak happening again. “There’s no reason to assume there will be a major plague outbreak in Europe, simply because we don’t have the right small rodent species there,” said Mr. Stenseth.
According to Mr. Schmid, the findings should refocus historians’ attention on the later outbreaks of the plague pandemic, and not just the Black Death, to see if those can be traced back to their roots.
“It will be easier to study the process by which plague was transported from Asia to Europe based on historical descriptions and archeology, if scholars are not limited to only the Black Death event, but have multiple reintroductions to study,” he said. — © Guardian Newspapers Limited, 2015
