On January 18th, 2018 we learned about

Updating our spotty, rat-filled understanding of the 14th century plague epidemics

If there’s one thing we can learn from the Black Death in the 14th century, it’s the importance of record keeping in times of crisis. Granted, it was probably hard to focus on documenting what was going on when tens of millions of people were dropping dead for no obvious reason. However, piecing together exactly how the plague spread with the speed it did has been an ongoing question, even long after we’ve come to understand and successfully treat the Yersinia pestis bacteria that actually causes bubonic plague. While rats have long been thought to have carried fleas that carried the bacteria, new investigations are starting to cast doubt on what we thought we knew about these horrifying epidemics.

No rats required

To be clear, Y. pestis is still the cause of death that killed millions of Europeans on more than one occasion. The question is how big a role rats played in transmitting the bacteria to humans. Part of our evidence against the rodents is that they have often play a role in plague outbreaks today, which understandably makes a strong case for their guilt in the 14th century. However, there are some holes in the story of past epidemics, such as no reporting on dead rats turning up in large numbers (as the rodents can be killed by the plague just as we can.) Researchers have also questioned if the flow of infections that we do know about really required rats’ presence in the first place, so they ran some tests to find out.

These experiments obviously didn’t involve risking any human or rat lives. They were conducted as simulations in a computer, allowing changes in different variables to be run over and over, eventually revealing the likelihood of one scenario over another. Obviously, long-shots can still happen, but these simulations showed that fleas biting humans could be passed around quite efficiently with no help from furry friends. In fact, in seven out of nine cities’ virtual infections, the human-flea-human model was a better match for mortality records than scenarios that depended on the movement of rodents.

Looking at leprosy

While these simulations have tried to consider an array of data sources to build a more accurate picture of how the plague spread, some historical gaps have been filled erroneously. Many images that are now archived as contemporary depictions of plague victims are actually pictures of other diseases entirely, such as leprosy. This kind of mistake has become common enough that it’s likely reshaping people’s understanding of what symptoms the bubonic plague actually produces.

Medieval images of leprosy, later labeled as the plague, often include eye-catching lesions on the victims’ skin. It’s dramatic and easily understood as a sign of disease, making these mislabeled images all the more convince to audiences lucky enough to never encounter an actual bubo- the real calling card of the bubonic plague. While some victims could occasionally end up with dark red spots under their skin, most people would end up with a single swollen lymph node in the armpit or neck, depending on where the bacteria-carrying flea bit them. However, these buboes don’t turn up in any drawings or paintings from the 14th century outbreaks. Instead of showing the medical reality of the plague, the few contemporary images directly related to the epidemic focus on its effect on societies, such as a drawing of people burying coffins from 1349, or Jews being burned alive in the 1340s after they were blamed for the disease.

Seeing patterns in the symptoms

Even after the dramatic epidemic of the 14th century, the plague revisited Europe every few decades. Bit by bit, people started to put the pieces together, even making a point to record what an actual plague victim looked like. Images of swollen lymph nodes are directly connected to the plague in imagery from the 15th century, both in artwork and medical documents, some of which suggested lancing buboes to save infected patients.

It’s understandable that people didn’t know what to keep track of before they even knew what was making them sick. But it’s interesting to consider how much information about a curable disease is still hard to be sure of. As someone who was preemptively treated for bubonic plague once as a toddler, I guess I’m just grateful that someone around me knew what to look for at a time when it counted. For what it’s worth, in that case people blamed a flea-bitten cat.

Source: Maybe Rats Aren't to Blame for the Black Death by Michael Greshko, National Geographic