Were rats really responsible for the Black Death?

Rats have gotten a bad rap for spreading disease and destruction throughout history but they might not be to blame for one of the world's worst pandemics according to research published at the end of 2022.

The Bubonic Plague, better known today as the Black Death, was responsible for killing an estimated 25 million people in the mid-fourteenth century according to National Geographic.

It was always believed that rats were responsible for spreading the Black Death but it turns out we might have been blaming the wrong mammal and that could have an important impact on how we fight future deadly pandemics. 

That’s right. Rats never really caused the Black Death. The real culprit that killed half of Europe was a bacterium known as Yersinia pestis, an extremely dangerous microbe that can target your body’s immune system and destroy its defenses. 

"Yersinia pestis is the nastiest thing alive," Dr. Olaf Schneewind told the University of Chicago's The Conversation while discussing his 2005 study on how the bacterium disables the human immune system.

Essentially, Yersinia pestis works to undermine its host immune system by injecting a variety of toxins into its defense cells, destroying them so the body cannot defend itself against the bacteria’s reproduction process.  

"This is a very clever system for this particular kind of bacteria," Schneewind said. 

Well, the simplest answer is that rats acted as carriers for the bacteria’s favorite host, fleas. 

Yersinia pestis can only be spread in one of three ways according to the Centers for Disease Control and Prevention. 

In order to be infected someone must come into contact with contaminated fluid or tissue, inhale infectious droplets, or be bitten by a flea. 

Rats carry fleas, so they were always blamed for spreading the Black Death. But a study published in the Proceedings of the National Academy of Science in December 2022 showed that it was environmental conditions, not rats that spread Yersinia pestis to Europe.

“Our study offers two possibilities,” wrote authors and scientists Samuel Cohen and Philip Slavin in a breakdown of their new research. 

“One, the plague was being reintroduced from Asian reservoirs,” the professors continued, “Second, there could have been short- or medium-term temporary reservoirs in Europe.”

While the science may sound complicated, it essentially boils down to environmental conditions in Europe at the time that would have prevented Yersinia pestis from surviving in Europe’s rodent population except for in small and isolated pockets. 

“Our analyses strongly suggest that local environmental factors in Western and Central Europe, including the chemical composition of the soil, altitude, and climates, did not provide favorable conditions for persistent long-term plague reservoirs maintained by wild rodents and their ectoparasites,” the researchers wrote. 

Cohen and Philip estimated that at the time, only 0.6 percent of Europe was suitable for plague and questioned previous theories about Yersinia pestis’ spread in Europe. 

“We question the importance of wildlife rodents as the main hosts in Europe,” they added, concluding that the plague was probably continually reintroduced into the European population rather than carried throughout it on the backs of rats. 

This isn’t the first time modern researchers have questioned the theory that rats were responsible for spreading the Black Death in the fourteenth-century. 

In 2017, a similar study published in the Proceedings of the National Academy of Science found that it was humans, not rats, who were the main carriers of the fleas that infected Europe so quickly.