Black Death caused by the great gerbil, not black rats, scientists suggest

The Black Death, which killed 75 to 200 million people in Eurasia, was probably not caused by black rats, but by the great gerbil, European researchers suggest after comparing tree-ring records with 7,711 historical plague outbreaks.

If the researchers, from Norway and Switzerland, are right “we’ll have to rewrite that part of history,” said Professor Nils Christian Stenseth, from Oslo University’s Department of Biosciences.

The study findings have been published in the academic journal Proceedings of the National Academy of Sciences (citation below).

Nils Christian Stenseth

“If we’re right, we’ll have to rewrite that part of history,” says Prof. Stenseth. (Image: University of Oslo)

The Black Death, which came from Asia, hit Europe in 1347 and continued on and off for the next 400 years.

Historians have been certain that black rats were responsible for establishing the epidemics in Europe, with new epidemics occurring when fleas jumped from sick rats to humans.


Rat reservoir an unlikely culprit

In this new study, however, Prof. Stenseth and team do not believe the rat reservoir was the culprit.

The team set out to determine whether the weather conditions had been right for an outbreak caused by rats. They gathered and collected data from tree-ring records and compared them with 7,711 historical plague outbreaks.

For the ideal weather conditions, summers should have been warm, with not too much rainfall, i.e. dry summers but not too dry, the authors wrote.

Prof. Stenseth said:

“We have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather.”

The evidence they gathered pointed to another plague-carrying animal – the great gerbil, which would have thrived in the Asian weather conditions suggested by the tree-rings. The outbreaks would then make their way to Europe.

Great Gerbil

Great gerbils are found in Turkmenistan, Kazakhstan, Mongolia, China, Pakistan, Afghanistan, and Iran. They are 6 to 8 inches (15 to 20 cm) long. (Image: Wikipedia)

Prof. Stenseth said:

“We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbour cities in Europe and then spreads across the continent.”

Gerbil populations would have increased rapidly after a rainy spring and warm summer. High gerbil populations are good for the plague, the authors explained.

Their fleas, which also thrive under such conditions, would then have leapt to farm animals, domestic animals, and to humans.

This was all taking place during the peak period of trade between the West and East. Prof. Stenseth believes the plague made its way to Europe along the Silk Road (a historically important international trade route between China and the Mediterranean).

Why did plagues come in waves?

The findings surprised Prof. Stenseth and colleagues. However, it allowed them to explain why Europe was stricken with waves of plagues.

“We originally thought it was due to rats and climatic changes in Europe, but now we know it goes back to Central Asia,” he said.

The researchers say they plan to analyze plague bacteria DNA taken from skeletons across Europe and see if the genetic material shows a large amount of variation. If this is the case, it will back up their theory.

A strain originating from a rat reservoir would show smaller differences than several waves originating from Asia.

Citation: Climate-driven introduction of the Black Death and successive plague reintroductions into Europe,” Boris V. Schmida, Ulf Büntgen, W. Ryan Easterday, Christian Ginzler, Lars Walløe, Barbara Bramanti, and Nils Chr. Stenseth. Proceedings of the National Academy of Sciences. Feb 23rd, 2015.  DOI: 10.1073/pnas.141288711.

Video – Gerbils not Rats Caused the Black Death