Are Rats Innocent of Spreading the Black Plague?

Human pests like fleas and lice may be responsible for spreading the pandemic that devastated Medieval Europe

rattus rattus
Francisco Martins via iStock

A new study suggests that human parasites—like fleas and lice—and not rats, may be responsible for spreading the Black Death that killed millions of people in Medieval Europe. 

Black death, which is also known as bubonic plague, is caused by the bacteria Yersinia pestis, which travels within the human bloodstream and builds up within lymph nodes, Michael Greshko reports for National Geographic. This causes the nodes to swell into “buboes” that give the bubonic plague its name. The disease spread across Europe in multiple outbreaks starting in the 500s, with the most intense episodes during the 1300s through early 1800s. The disease left tens of millions dead.

But researchers still don't fully understand the reasons behind this recurring disease. "There are so many questions that this pandemic raises and how it spread so quickly is one of them," Katharine R. Dean, researcher at the University of Norway ​who led the study, tells the CBC's Susan Noakes.

There have been many suggestions through the years of how the infamous Black Death spread, including one 2015 study that suggests warmer, wetter climate conditions influenced its transmission. But most of this finger pointing often turns back to the rodents. In more modern plagues, researchers have placed the blame on the rats—and more specifically on the fleas that drink their blood.

As Greshko reports, when the rats carry the disease, their blood-sucking fleas will also be infected, transmitting the plague to any humans they may later bite. But this mode of transmission may not have been the same for every outbreak of Black Death. During what's known as the Third Pandemic, which starting in 1855, human deaths were accompanied by widespread rat deaths, which were known as “rat falls,” Noakes reports. These rat falls are absent in records of earlier outbreaks of the plague, Greshko writes, hinting that perhaps rats held less responsibility in those earlier epidemics.

To delve into this more, the team decided to focus on the Second Pandemic — deaths in the 14th through 19th century. Not only were there higher-quality records during this period, the disease seemed to spread more rapidly during this time. And as Greshko reports, this may also hint at a different contagion pathway.

Dean and her team used mathematical models to study the movement and rate of spread. Their simulations were based on how the disease would move depending on if it was transmitted by rats and fleas versus human parasites. Then, the researchers turned to historical data, comparing the patterns in their model to nine outbreaks of the Black Death across Europe between 1348 and 1813.

As Noakes reports, for seven of the nine outbreaks studied, the observed deaths most closely matches the model of the disease spread by human parasites, not rat-flea interactions. They published their analysis this week in the journal Proceedings of the National Academy of Sciences.

As Dean acknowledges, it's important to note that such comparisons to mathematical models is not bulletproof evidence. And she anticipates the research will inspire controversy among plague scholars, Greshko reports. “In plague, there’s a lot of hot debate,” Dean tells Greshko. “We have no dogs in this fight.”

The research is not just for historical interest. Black Plague is still around today, including a 2017 outbreak in Madagascar, Greshko writes. "We're lucky today because there are not a lot of parasites because of higher standards of hygiene. That has helped to keep it down," Dean tells Noakes.

Dean acknowledges the research will benefit from more experiments—and this latest result continues to stoke the debate on what really caused the Black Death that shaped European history.

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