No, Gerbils Didn't Cause the Black Death
This is another great example of massive misreporting of an otherwise interesting science news item. Headlines, such as this one from the Washington Post, proclaim: After 8 centuries, rats exonerated in spread of Black Death. Gerbils implicated.
No and no.
The study published in PNAS proposes an alternate hypothesis to what drove the bubonic plague that peaked in Europe from 1346–53, but which actually continued to have outbreaks until the mid 19th century.
The bubonic plague is caused by bacteria, Yersinia pestis, which is transmitted by fleas that live on rodents, including black rats. The dynamics of a multi-century plague are complex, but in simple terms there are two main components.
The bacteria must have a reservoir, a place where they live and survive from year to year. Occasionally the bacteria gets from the reservoir into the human population, causing an outbreak.
There is no controversy over the fact that black rats were an important, and perhaps even the primary, cause of outbreaks of the plague. They spread the disease and were transported around on ships from harbor to harbor. The new study does not even address this issue.
What the study does address is the long term reservoir of the disease between outbreaks. The current dominant theory is that once the plague was introduced into Europe, fleas on black rats acted as a reservoir, occasionally feeding outbreaks.
The new study, published in PNAS, proposes an alternate hypothsis: that the plague was actually introduced into Europe not once, but multiple times, from a reservoir in Asia.
The chain of logic here is interesting, and is explained in detail in this article by Alison Atkin. The researchers hypothesize that a wet spring followed by a warm summer could cause a boom and bust cycle in a rodent population. They thrive during the wet spring, then die off during the dry summer. Their fleas therefore explode in population during the boom, and then during the bust they look for new hosts, spreading to other species and taking the plague with them.
They sought to correlate climate patterns that would cause a rodent boom and bust to historical records of outbreaks in Europe that appeared to come from outside – in ports distant from any other outbreak. They found such a correlation in 11 of 16 outbreaks where they had data.
That is an interesting finding, but is hardly proof of multiple introductions of the plague to Europe, as opposed to only one initial introduction. Further evidence, such as DNA evidence from plague victims, will be needed to explore this idea further.
What about the gerbils? The study does not directly implicate gerbils, although they are one possible Asian reservoir for fleas carrying the plauge. Ground squirrels and Altai marmots are also mentioned, and there are other possibilities.
So – the new study, while interesting, in no way “exonerates” black rats in their role of propagating the plague. The study also does not directly implicate gerbils as the cause of the plague.
What the study does is propose an alternate hypothesis in which there were multiple introductions of the plague into Europe from Asian reservoirs, based upon a novel theory of weather patterns and small mammal populations.
How this became twisted in mainstream media reporting to – gerbils replace rats as the cause of the black death – is unclear.