Gerbils may have carried the plague from Asia to Europe along the Silk Road trade route. Image credit Jim McMahon.
In the mid-1300s, a deadly disease called the bubonic plague swept through Asia, Africa, and Europe, killing an estimated 50 million people. It was one of the deadliest outbreaks in human history, earning the nickname the Black Death.
For hundreds of years, scientists have blamed black rats for spreading the plague. They believed that the plague-causing bacteria, Yersinia pestis, infected local populations of rats in Europe. The rats carried infected fleas that transmitted the disease to humans by biting them.
But a recent study strongly suggests that a different animal may be to blame for the Black Death: gerbils.
Researchers used tree-ring records to look at climate conditions in Europe during outbreaks of the plague dating back to the 14th century. The data suggested that Europe experienced wet springs and warm summers at the times of the outbreaks. This type of weather would have hindered the rat population, as rats flourish in “warm summers with not too much precipitation—dry, but not too dry,” Nils Christian Stenseth, a professor at Norway’s University of Oslo and an author of the study, told reporters.
But while the weather would have been rotten for rats, those same conditions would have been ideal for another flea-carrying rodent: gerbils. Weather patterns showed that Europe tended to experience plague outbreaks after central Asia had a wet spring followed by a warm summer—the exact weather that might cause an explosion in the gerbil population.
Stenseth and his team believe that Asian plague-infested gerbils traveled to Europe on merchant caravans along the Silk Road, a network of trading pathways that stretched from near the city of Xi’an in China through the Middle East and into Europe. Fleas would then have transmitted the disease from the gerbils to humans.
TESTING THEIR THEORY
The scientists plan to test their gerbil theory by studying plague-bacteria DNA from skeletons of European plague victims. Wide variation in the DNA would suggest that the various plague outbreaks were caused by new waves of the disease from outside Europe rather than a resurgence of the disease from local rat populations.
“If we’re right,” Stenseth says, “we’ll have to rewrite that part of history.”