When I was only six years old I received my first pet. It chewed cedar chips and slept in a bed it made out of cotton balls. It loved to roll around the floor in a clear plastic ball. I called it Barney.

It turns out that Barney’s ancestors may have been responsible for bringing the Black Death to Europe.

For years it’s been believed that rats carried fleas infected with Yersinia pestis bacterium from the plains of Central Asia, along the Silk Road and throughout Europe by the mid-14th century, mostly via merchant shipping routes. The Yersinia pestis bacterium caused the bubonic plague, wiping out 30 – 60 percent of the total population of Europe.

gerbils plague

The guilty party. Pic: benmckune (Flickr CC)

Historically, rats have caught a really bad rap because of the plague, but an team of researchers from the University of Oslo in Norway believes the true causes of the bacteria-carrying fleas moving westward were not rats, but cute little hopping gerbils like my Barney.

What’s more, it was fluctuations in Asia’s climate that caused sudden population explosions, followed by crashes among Central Asian rodents like gerbils, ground squirrels and marmots. The bacteria-carrying fleas would multiply as the numbers of their hosts would multiply, but instead of simply dying off when their hosts did, they’d try to find new ones. Humans and camels travelling westward were opportune targets.

From the Associated Press:

The researchers identified 16 possible instances between 1346 and 1837 in which plague might have arrived at a European port from Asia. These events were consistently preceded by climate fluctuations in Asia, as recorded by tree rings from Pakistan, with a lag of about 15 years.

Other scientists maintain that rats still had a hand in spreading the plague at different times, for example to America at the start of the 20th century, but more evidence is piling up suggesting that it was Asian rodents that caused it to spread from Asia to Europe.

black death gerbils

Ill Morbetto from 1515/16

According to the World Health Organization, the bubonic plague still affects humans today, with 1,000 – 3,000 cases each year, mostly in African countries as well as in the Middle East. Outbreaks in those regions have also been found to follow climate fluctuations in Central Asia.

Boris Schmid, the first author on the study, is quoted in the Guardian:

We find plague to have been repeatedly re-imported along the same route as the Black Death was imported, triggered by large-scale climate events in Central Asia. If the plague that arrived with the Black Death was the ancestor of all the strains of plague in Europe in the centuries afterwards, you will find a different pattern of relatedness than when plague in Europe was repeatedly re-imported from plague reservoirs in Asia.

Scientists are still trying to figure out just why the plague stopped spreading through Europe even when it continued to travel from Asia to the Mediterranean. As for future outbreaks, it may be that Europe doesn’t have the right small rodent species to support another major outbreak of plague.