Black Death: Don't blame the rats, it was the gerbils

In the 14th century, Black Death spread across Europe and central Asia, killing nearly half of the western continent's population. Until now, black rats have shouldered much of the blame, but new research suggests giant gerbils of central Asia may have played a larger role in transporting the disease.
Researchers at Norway's University of Oslo recently looked at tree rings in Europe and Asia to gain a better understanding of climatic conditions during the middle of the 14th century. As detailed in a new study published in PNAS, scientists found that wind and weather conditions in Europe would not have been conducive to large rodent- and flea-spawned disease outbreaks.
Asia, on the other hand, would have been ripe for Yersinia pestis, the bacterial infection more commonly known as bubonic plague. The spice trade across the Silk Road would have provided an ideal highway for the delivery of new iterations of the plague. Researchers contend this explanation is more likely than the theory that successive outbreaks of the bacterial infection leapt from Europe's same population of flea-ridden rats.
"For this, you would need warm summers, with not too much precipitation," study author Nils Christian Stenseth, head of Oslo's Center for Ecological and Evolutionary Synthesis, told the BBC. "We have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather."
The weather that would encourage a larger gerbil-led outbreak is different, researchers say -- a wet spring, followed by a warm summer. Lab tests have shown that giant gerbils of Asia are exceptional carriers of the disease.
"There are great individual differences, but many individuals can handle an absurd amount of plague bacteria," explained Oslo researcher Pernille Nilsson. "Sometimes a single bacterium kills a mouse. Common rats can tolerate injection of 10,000 bacteria. Gerbils can tolerate 100 billion bacteria. That is ten million times as many bacteria."
The research not only shows that weather and gerbil genetics were ideal for the development and delivery of plague, but that the spread of the disease often began in European port cities -- suggesting trade by both land and sea facilitated the Black Death.
To confirm their theories, researchers are beginning to test for plague DNA among the remains of skeletons from the time period. If their results show large variations among the disease strains, it will lend further credence to their idea that the Black Death arrived in waves from central Asia.