In the new study, researchers from the University of Oslo analyzed climate data going back to the 14th century, looking for information about the weather conditions that coincided with outbreaks of the bubonic plague, or Black Death. Specifically, the scientists analyzed tree-ring records from Europe and compared them with 7,711 historical plague outbreaks. Their findings, published in the Proceedings of the National Academy of Sciences, contradict the commonly held belief that rats hosted the disease-carrying fleas responsible for causing plague outbreaks.
Nils Christian Stenseth, one of the study’s authors, told BBC News that in order for the weather to be optimum for an outbreak caused by flea-infested rats, “[Y]ou would need warm summers, with not too much precipitation. Dry but not too dry.” This was not the case, Stenseth says; according to him, he and his colleagues “looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather.”
Instead, they say, outbreaks of the Black Death seem to correspond with weather patterns in Asia, not Europe itself. According to their analysis of the climate data, plague outbreaks in Europe can be linked to the years that central Asia experienced wet springs followed by warm summer. While such conditions would have been terrible for black rats, the scientists point out, they would have created ideal breeding conditions for another plague-bearing rodent: the gerbil.
Found across central Asia, great gerbils (also known as giant gerbils) can grow to adult sizes of about 15-20 centimeters (or 6-8 inches) in body length. Stenseth and his team believe that after such ideal weather conditions caused an explosion in the Asian gerbil population, gerbils bearing fleas infected with the plague bacteria (Yersinia pestis) made their way to Europe along bustling Silk Road trade routes. According to Stenseth: “We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbor cities in Europe and then spreads across the continent.”
Though it peaked in the mid-14th century, bubonic plague resurfaced in periodic outbreaks over the centuries to come, killing a total of some 100 million people by the 1800s. The threat continues even today; according to the World Health Organization, nearly 800 cases of plague were reported worldwide in 2013, including 126 deaths. According to the new study’s authors, the fact that the plague went away and recurred intermittently over the centuries rather than lingered also indicates that rats were not the culprits. If they were, the plague would have endured as long as rats were on hand to carry it.
Last year, in another similar challenge to long-held beliefs about the Black Death, scientists working with plague DNA extracted from 14th-century skeletons suggested the disease could have been airborne rather than passed through fleabites. Researchers from the new study say they will follow up on their work with similar DNA analysis using ancient skeletons. Significant genetic variation across time periods would indicate that outbreaks were due to newly arrived waves of disease rather than resurgences from Europe’s existing rat population–offering further support for the gerbil theory. As Stenseth puts it: “If we’re right, we’ll have to rewrite that part of history.”