BLACK rats might not be responsible for the plagues that killed millions of people across medieval Europe, research suggests.
Instead scientists believe that repeated outbreaks of the Black Death may in fact be traced to gerbils arriving from Asia.
A study found that epidemics of the plague were triggered by fluctuations in climate, with peaks in Europe coming after periods of warm weather in central Asia.
The Black Death originated in Asia and arrived in the ports of the Mediterranean in 1347, brought in by the land and sea trade routes of the ancient Silk Road.
This epidemic marked the start of the second plague pandemic, which lasted in Europe until the early 19th century, killing millions of people.
Scientists propose that instead of being introduced once to Europe and surviving, the plague was in fact brought in many times over a period of more than 400 years.
The study, published in the the Proceedings of the National Academy of Sciences, saw researchers analyse more than 7,700 documented outbreaks of plague in Europe using tree-ring records from here and Asia.
While climate conditions in Europe were not optimal for the development of plague, when it warmed in Asia, plague appeared in Europe around 15 years later.
The researchers said: “We provide evidence for repeated climate-driven reintroductions of the bacterium into European harbours from reservoirs in Asia.
“Our analysis finds no support for the existence of permanent plague reservoirs in medieval Europe.”
The bacterium that caused the plague, Yersinia pestis, is carried in fleas which in central Asia were prevalent on giant gerbils.
Warmer springs and wetter summers on that continent led to a boom in the gerbil population.
Fleas would then jump ship to find different hosts – including domestic animals and humans.
And with trade between East and West becoming ever-more prevalent, the disease came into Europe via the caravans of merchants and traders.
The study said: “It is commonly thought that after its initial introduction from Asia, the disease persisted in Europe in rodent reservoirs until it eventually disappeared.
“Climate-driven outbreaks of Yersinia pestis in Asian rodent plague reservoirs are significantly associated with new waves of plague arriving into Europe through its maritime trade network with Asia.
“This association strongly suggests that the bacterium was continuously reimported into Europe during the second plague pandemic, and offers an alternative explanation to putative European rodent reservoirs for how the disease could have persisted in Europe for so long.”
Professor Nils Christian Stenseth, from the University of Oslo, said that if the research was correct it would mean rewriting history.
He told the BBC: “We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbour cities in Europe and then spreads across the continent.”
Wet springs followed by warm summers were “good conditions” for gerbils, which meant “a high gerbil population across huge areas and that is good for the plague”.
He added: “We originally thought [waves of plagues] was due to rats and climatic changes in Europe, but now we know it goes back to Central Asia.”
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