Scientists now suspect gerbils were the real villains in the Black Death

The Black Death was one of the worst pandemics in human history, killing tens of millions of people in Europe and Asia between 1347 and 1353. The plague then kept recurring in Europe for centuries thereafter.

And, for a long time, scientists blamed black rats for these repeated outbreaks. The idea was that Yersinia pestis, the bacteria that causes plague, came over to Europe from Asia via the Silk Road around 1347. The plague then stayed in Europe by festering among local rat populations, with new outbreaks occurring whenever fleas jumped from infected rats to humans. (The disease could then spread by air from person to person.) Or so the theory went.

But now a fascinating new study suggests that the story was far more complicated than that. As it turns out, there's little evidence that there were any such "rat reservoirs" in Europe. Instead, a key culprit may have been great gerbils in central Asia, whose population fluctuations helped push the plague into Europe over and over again across the centuries.

Climate data suggests gerbils drove new waves of plague

The study, published in the Proceedings of the National Academy Sciences, compared records of plague outbreaks in Europe with historical tree-ring data that provides clues as to the continent's past climate.

Earlier research has suggested that the optimal weather for rat-driven plague outbreaks in Europe would have been warm and somewhat dry summers. But that created a problem: the plague data and climate data don't line up, so it didn't seem likely that rats in Europe were the ones driving the repeated plague outbreaks. (As further evidence, past studies have found that rats were often absent from plague centers in northern Europe.)

So what was to blame? The researchers noticed another interesting pattern in the climate data over in central Asia. Every now and again, there were periods of warmer springs and wetter summers around Kazakhstan. These conditions are known to help boost populations of great gerbils (Rhombomys opimus), some of whom carry plague. That allows the disease to spread around the region.

Then, once the climate cools and gerbil populations shrink back down, there's an excess of fleas left over. So those fleas seek out an alternative host — namely humans and their domestic animals. This pattern would help spread plague further in central Asia.

The researchers found that every time these particular climate patterns in central Asia prevailed, plague would pop up in Europe about 15 years later. Given the frequent trade caravans traveling between Asia and Europe during this time, it seems far more likely that new cases of plague were being brought over repeatedly from Asia. And it was all thanks to booming gerbil populations. The map below shows what this pattern might have looked like:

(Schmid et al, 2015)

Now, this isn't absolute proof that great gerbils were to blame for Europe's outbreaks. The researchers told BBC's Rebecca Morelle that they next plan to analyze plague bacteria DNA from skeletons in Europe. If there's a lot of variation, that would suggest that plague was being reintroduced from Asia — rather than continually breaking out from a single source in Europe.

By the way, the plague still exists in a few places today — there about 1,000 to 3,000 cases every year in Africa, Asia, even the United States. People in rural areas occasionally get it from infected fleas associated with wild rodents (including squirrels). Fortunately, we now have antibiotics to treat it, so the plague is much less deadly than it was in the Middle Ages. Only 11 percent of US cases from 1990-2010 have been fatal.

Further reading: 7 adorable animals that are also murderous monsters

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