Researchers of the bubonic plague, also known as the Black Death, have long debated why the disease broke out repeatedly in medieval Europe: Did traders from elsewhere bring the dreaded pestilence back with them from other infected areas, thus sparking new epidemics? Or was the Yersinia pestis bacterium, now known to cause the plague, present but for some reason inactive during the 5 to 12 years that typically passed between outbreaks?
Now a researcher has found a pattern of recurrence by looking at ancient records of the Black Death and climatic conditions in what today is an eastern region of England called East Anglia. “It looks like, after a run of 1, 2, or 3 years of relatively average or slightly cool summers, once there is a jump to warm and dry weather, there is a large likelihood” of a plague epidemic, said Kathleen Pribyl of the University of East Anglia in Norwich, U.K., last month during the General Assembly of the European Geosciences Union in Vienna, Austria. Her findings suggest that the bacterium lingered silently in rodents during periods between plague flare-ups, only to threaten humans again when the rodent population exploded.
Rediscovering Past Climate
A climate historian, Pribyl made a reconstruction of the English climate between 1256 and 1431 using indirect evidence, for example, the date of the start of the grain harvest. She wondered if she might discover in these data the influences that triggered plague epidemics or quelled them. She realized that she needed to extend her climate record to the end of the 15th century, when public health measures such as quarantining were introduced, which confined outbreaks and lowered their impact.
Pribyl found the data she needed in the work of a Dutch independent climate historian, Jan Buisman, who has been compiling reports on weather phenomena and climate in the Low Countries, the coastal region nowadays mostly occupied by the Netherlands and Belgium. This area, lying only 200 kilometers to the east of East Anglia across a stretch of the North Sea, was close enough for its weather records to be suitable to supplement the English data, Pribyl told Eos. Buisman’s records date from as early as 764 CE.
With a sufficiently long temperature record thus established and with data on rainfall readily available via tree ring sequences, Pribyl could characterize each year: Was the winter cold or not? Was the summer cool, warm, dry, wet? At first, just like researchers before her, she saw no particular circumstances that would coincide with a plague outbreak. But that changed when she grouped summers not by their temperature, but by the difference in average temperature from the preceding summer. If a warm summer followed a normal one, then a plague outbreak was much more likely, she found.
This pattern, according to Pribyl, shows that it was not repeated arrivals from outside England that made the plague return again and again, but the waxing and waning of the population of some host animal. Knowing the preference of Yersinia pestis, it was likely a rodent, she explained.
In England, the field vole and the common vole are likely candidates to have played the role of plague reservoirs, and these species have naturally fluctuating population sizes. A normal year, or, better yet, a few normal years, followed by a warm year, said Pribyl, is just what sets up the population structure, the food situation, and the number of predators for a population explosion. This increase will, in turn, bring these rodents, and the fleas they carry, into closer contact with humans than in normal times, “especially after the population collapses again, which always happens at some point,” she said. “Because then the fleas will try to move to other hosts, and humans are as good as anything else. And that’s when transference of the disease becomes quite likely.”
Other signs from climate data also point to rodents as the reservoir. A slightly cold winter seems to have helped the plague: More snow would have insulated and hid the voles’ burrows. But outbreaks almost never occurred after a very cold winter, which would have killed a lot of voles, or a very mild one, when abundant rain would have flooded many burrows.
In an earlier study, Boris Schmid, an evolutionary biologist of the University of Oslo, Norway, and his colleagues analyzed a data set of 7711 plague outbreaks all over Europe and compared them to precipitation records from tree ring samples from Europe and Asia. Weather conditions that might have caused rodent numbers to spike, they reported, generally did not coincide with outbreaks. They concluded that most outbreaks of the plague must have been caused by renewed introduction of Yersinia pestis from Asia, brought into Europe by rats on merchant ships.
Pribyl, however, questions that research team’s analysis. “They looked at climate fluctuations for the whole of Europe and expected to find one kind of pattern. But that’s actually quite unlikely. With my data, I could look at just one region,” she noted.
Hendrik Poinar, an evolutionary biologist at McMaster University in Hamilton, Ontario, Canada, who did not participate in Pribyl’s study, told Eos that he found Pribyl’s conclusion that the plague did not enter Europe again and again but resided in a rodent reservoir “convincing and interesting.” Poinar took part in an investigation of the genome of a strain of Yersinia pestis that was found in Marseille, France, in the teeth of buried victims of one of the last outbreaks in Europe, in 1722. Last year, his group reported that this strain had descended from bacteria identified in 14th century victims in London. So genetics, too, points at the plague emerging repeatedly from a reservoir in or near Europe, Poinar said.
Could the source of the repeated English outbreaks have been in England itself, and could the field voles that Pribyl suspects indeed have been the reservoir? It’s quite possible, Poinar said. “All these small, human-associated rodents could be important. All these should be tested. If she has samples, we’d be happy to test them!” he declared.
—Bas den Hond (email: [email protected]), Freelance Science Journalist