The Black Death first came to Europe in 1347, engulfing the continent in sickness and turmoil. Transported through fleas (and, by extension, rats stowed away on seafaring ships) the Black Death killed off as much as 50 percent of the population in less than a decade, and outbreaks of the plague continued to sicken Europeans until the disease all but disappeared in the early 19th century. Or so the story goes.
“I always was of the belief that there must have been a rodent reservoir [in Europe],” says Nils Christian Stenseth, a professor at the University of Oslo and principle author of a new study published today in Proceedings of the National Academy of Sciences. “But this work of ours has made me change my mind.” Stenseth's new research shows that plague-harboring rodents may not be to blame for the disease outbreaks that plagued Europe for centuries. Instead, climate-induced outbreaks in Asia may have spilled over into Europe multiple times.
Climate fluctuations in Asia are known to trigger plague outbreaks, as climate variation is closely tied to rodent population densities. When climate fluctuations cause rodent populations to crash, fleas—which can act as a nice home for Yersinia pestis, the bacteria that causes the plague—seek out new hosts (including humans) to escape their over-crowded rodent homes.
The researchers sought to discover if European outbreaks during the plague pandemic that began in the mid-14th century were related to climate changes. To do that, they compared a historical record of more than 7,700 plague outbreaks with tree-ring data—which can give scientists a sense of the climate conditions for vegetation in any given year—from Europe trade routes stemming from Central Asia, including long-living Juniper trees from the mountains of Pakistan.
Just 24 of the outbreaks could have been associated with rodent reservoirs, according to the authors; eight of those were traced back to the arrival of plague-carrying ships, and there was no link between the remaining 16 outbreaks and climate fluctuations in Europe. Instead, climate variation in Asia appeared to influence plague outbreaks in Europe.
The authors found that plague outbreaks in Asia were consistently followed by flare-ups in Europe roughly 15 years later—just enough time for Asia’s rodent reservoirs to travel trade routes into Europe. In other words, European rats aren't to blame for the Black Death; it's their furry Asian counterparts.
“[It’s] quite interesting—and surprising—that the effect of climate variation in Asia can be seen so clearly in Europe more than 10 years later,” Stenseth says.
No natural plague reservoir is good news for Europe, according to Stenseth. “There is no reason whatsoever to believe that there ever will be a plague pandemic in Europe as we had during the Medieval time," he says. "Not even the small, or large-ish, outbreaks we see in Asia [today].”