A few years ago, I stumbled across a paper whose very title blew my mind. Using archeological evidence, the authors claimed that, “Rats cannot have been intermediate hosts for Yersinia pestis during medieval plague epidemics in Northern Europe.” (You can see more about their awesome work here.) That paper was the first one to really pique my interest in the medieval plague epidemics, but it wasn’t the last:
Most people only know about the most famous plague epidemic in Europe – the Black Death – but there were actually many European plague epidemics. We know that plague didn’t originate in Europe, though. The first introduction is thought to have come from Asia. After that initial introduction, the plague might have hung out in reservoir hosts in between outbreaks in Europe. That’s what people used to think, anyways. But the reservoir host that was typically implicated was the black rat (see my previous post), and recent evidence suggests that the black rat couldn’t be responsible for many of the epidemics that occurred. So, where the heck did the plague come from in all of those epidemics?
Schmid et al. (2015) combed thousands of records of plague outbreaks in medieval towns that were near areas for which climate proxies (e.g., tree ring records) exist. The vast majority of those outbreaks occurred shortly after a neighboring town had an outbreak, which might suggest that town-to-town transmission by people is a more reasonable cause for most outbreaks than a rodent reservoir. However, some out the outbreaks didn’t follow outbreaks in neighboring towns, and they occurred in maritime towns. In those cases, plague might have been introduced into towns via ships.
But where was the plague coming from? Well, there are really only two options. The ships could be bringing in the plague from other European towns, or they could be bringing the plague from someplace else, like Asia. And interestingly, Schmid et al. (2015) found a relationship between a climate proxy in Asia (tree ring growth in juniper trees in the Karakorum mountains) and plague outbreaks that occurred ~15 years later in Europe.
Here’s what Schmid et al. (2015) hypothesize was happening in a typical plague cycle: in years that were climatically favorable, rodent populations in Central Asia boomed. Gerbils were living fast and loose, making babies and spreading fleas and plague amongst themselves. The flea populations responded to the sudden spike in host density by making lots of flea babies. So, there were lots of gerbils and lots of fleas. Then the climate shifted to something less favorable, there wasn’t enough food to go around, and the giant gerbil population crashed. That was no good for the fleas, who suddenly found themselves homeless. The fleas then had to go find different hosts – such as camels or humans – and they took the plague with them to their new hosts. (By the way, a similar thing might happen with Lyme Disease: acorn mast years might cause large mouse and tick populations, and after subsequent declines in the mouse populations, hungry ticks go looking for other hosts, including humans.)
So, you’re thinking, “Uh, yeah, so a few years after Central Asia is the Land of Plenty for gerbils, a billion hungry, plague-y fleas and go looking for other hosts in Central Asia. How does the plague repeatedly travel 4000km from Central Asia to the Black Sea to hop on a boat to Europe?” Uhhhh… good question. One hypothesis is that the plague traveled the Silk Road along with the caravans via humans and camels for ~10-12 years, eventually reaching the Black Sea. This is an area of study that people are focusing on now.
So, when the mainstream media picked this story up, they had all these witty lines about how people are all prejudiced against black rats, when really it’s the innocent-looking gerbils who are to blame for the plague. (Have you ever seen a black rat being wrongly interrogated by the authorities? It’s terrifying.) I can’t think of anything wittier to say, but I made a cartoon of a gerbil wearing a blonde wig. You’re welcome.
Schmid, BV, U Buntgen, WR Easterday, C Ginzler, L Walloe, B Bramanti, and NC Stenseth. 2015. Climate-driven introduction of the Black Death and successive plague reintroductions into Europe. PNAS.