Gerbils to blame for Black Death?

University of Oslo research suggests gerbils, not rats, were main cause of plague

Photo: Yuriy75 / Wikimedia Commons The Giant Gerbil of Kazakhstan is the species now believed to be the plague-carrier.

Photo: Yuriy75 / Wikimedia Commons
The Giant Gerbil of Kazakhstan is the species now believed to be the plague-carrier.

M. Michael Brady
Asker, Norway

The Black Death plague that broke out in Europe in 1347 and decimated the population has long been believed to have arrived from Asia via the Silk Road trading route, carried by black rats on merchant ships. That bit of conventional scientific wisdom seems straightforward, but fails to explain the epidemics that in the 400 years thereafter repeatedly broke out and killed millions. Research biologists at the University of Oslo believe that they now have an explanation. It wasn’t the rats after all. Their cousins the gerbils are to blame for the Black Death and the subsequent plagues.

The Black Death is a plague caused by the Yersinia pestis bacterium carried by fleas that affect rodents. The conventional rat theory holds that the rats that arrived in 1347 established reservoirs of Yersinia pestis infested rats that established the plague in Europe. Through the years, the rat reservoirs varied, so the waves of plague outbreaks were thought to have been triggered by peaks in the populations of the rat reservoirs.

The researchers found that assumed connection to be flawed. They knew that the peaks of the populations of rat reservoirs most likely would have been triggered by dry, warm summers. The warmer the weather, the more active the fleas and the more numerous their host rodents. In cooler weather, rodent populations decline, and the fleas migrate to new hosts. So the researchers looked at tree-ring records and other European climatic indicators to see if there was a relationship between rodent-favoring cycles of the weather and the known years of plague outbreaks. They found no relationship between occurrences of the plague and weather.

But they did find such a relationship in Asia, where the variations of weather conditions would have caused peaks in the populations of giant gerbils. Moreover, 61 of those climate-driven peaks occurred just a few years before plague outbreaks that started in 17 European port cities, including Hamburg and London. In the same period, trade between East and West along the Silk Road was at its peak. In short, whenever the weather in central Asia favored gerbils and their fleas, a few years later the plague bacteria showed up in ports in Europe and then spread across the continent.

The scenario of the research is like that of a detective story, in which a master sleuth deduces that the initial suspect is innocent while a previously unsuspected one actually committed the crime. In the real-life scientific story, there actually was a master sleuth on the case, Nils Christian Stenseth, a University of Oslo professor of population ecology and zoology. He also is the President of the Norwegian Academy of Sciences and Letters, and a member of the French Academy of Sciences, and of the Academia Europaea, an international non-governmental scientific association acting as an Academy.

Further reading: “Climate-driven introduction of the Black death and successive plague reintroductions into Europe,” Proceedings of the National Academy of Sciences, Vol. 112, No. 10, March 10, 2015, online at www.pnas.org/content/112/10/3020; the article also may be accessed via the Digital Object Identifier (DOI) system at www.doi.org using the DOI name: 10.1073/pnas.1412887112.

This article originally appeared in the April 3, 2015, issue of the Norwegian American Weekly. To subscribe, visit SUBSCRIBE or call us at (206) 784-4617.

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