Bubonic plague was a serial visitor in European Middle Ages

Black Death may have been a repeat guest in medieval Europe, not a resident. Outbreaks of the plague that killed millions of people were triggered by spurts of warm weather a continent away, researchers suggest online February 23 in the Proceedings of the National Academy of Sciences.

Researchers poring through records of plague outbreaks and climate fluctuations saw that rashes of the disease in Europe followed periods of warm weather in Central Asia. Instead of being imported to Europe once from Asia and sticking around, the researchers propose that the plague was introduced many times between the 14th and 19th centuries.

“The Black Death cannot primarily be understood by what happened in Europe. You have to understand what happened … in Central Asia,” says evolutionary biologist Nils Stenseth of the University of Oslo in Norway, a coauthor of the new report.

Stenseth and his colleagues studied juniper tree ring records from Europe and Central Asia to track how the climate changed over the years in each region. The team also examined more than 7,700 documented plague outbreaks in Europe. The European climate had no link to plague outbreaks. But when the climate warmed in Central Asia, the plague showed up in Europe about 15 years later.

The plague bacterium (Yersinia pestis) is carried in fleas, which in Central Asia are hosted by great gerbils (Rhombomys opimus). Warm, wet weather led to more greenery for the gerbils to eat, as well as more active fleas, says Stenseth. “An increase in temperature of 1 degree will double the prevalence of plague in the gerbil population.”

Then predators such as foxes and weasels feasted on the bounty of gerbils. That, along with the drier weather, caused the gerbil population to crash. The plague-carrying fleas jumped ship in search of other hosts. As a result, the disease eventually migrated to Europe via the Silk Road land and sea trade routes, researchers say.

There, plague-ridden fleas infested rats, which ferried the disease until they succumbed to it. Unlike gerbils, rats are easily conquered by Black Death, leaving the plague without a host. “There is no appropriate rodent reservoir here in Europe,” Stenseth says of why the plague appeared sporadically.

Eventually, says Stenseth, improved hygiene helped end the cycle. But before the Black Death ran its course, it killed vast numbers of people. In the 1347–1353 outbreak alone, between 30 and 50 percent of Europe’s population is estimated to have perished.

With a pandemic this size, there was probably not just one cause for the bursts of Black Death that beset Europe, says Hendrik Poinar, an evolutionary geneticist at McMaster University in Hamilton, Canada. “There are so many factors influencing the arrival and repeated outbreaks of plague,” he says. “We’re still missing a lot of the pieces of the puzzle.”

Eruptions of the disease may also have depended on human events such as immigration, wars, trade routes that boomed or dwindled, or whether quarantines improved or broke down, he says.

And any archival or historical study is constrained by the available data, Poinar notes. The junipers Stenseth and his team examined grow in high altitudes, and may not represent the climate in lower terrain where the plague was striking.

“It would be nice to have better specific climate data from the region of interest, rather than trying to make extrapolations from high elevation juniper trees,” says Poinar. “But given the information that’s available, I think they did a good job.”