New Study Finds Fleas from Humans, Not Rats, Spread the Black Death

Rats have gotten a bad reputation as the catalyst behind the Black Death, the infamous medieval pandemic that originated in China in 1334. It’s been popularly thought that rats infected with a bacterium called Yersinia pestis transmitted plague to humans through a bite, a horrific process that culminated in the death of 60 percent of the European population as it spread across continents.

However, while flea-invested rats do cause much of the plague that happens today, scientists now argue that it’s time to give rats a pass for the whole Black Death thing. In a paper released Monday by the Proceedings of the National Academy of Sciences, researchers from the University of Oslo argue that Yersinia pestis was more likely spread by body lice and human fleas.

“While it is commonly assumed that rats and their fleas spread plague during the Second Pandemic [the Black Death] there is little historical and archeological support for such a claim,” write the scientists. “Many studies have suggested that human ectoparasites, like human fleas and body life, were more likely than rats to have caused the rapidly spreading epidemics.”

Plague has caused numerous outbreaks and at least three pandemics during the course of history, and although the Black Death lasted from the 14th to the 19th century and killed between 75 million and 200 million people on the planet, it didn’t end there. In fact, it was until the third pandemic began in the 1860s, killing millions around the world over the course of 20 years, that scientists determined that plague was spread by rats accidentally hosting infectious flea bites.

But that doesn’t mean rats are to blame for every plague outbreak. The study authors write that it’s been difficult to assess how human ectoparasites have contributed to historical plagues because while they’ve been found present during modern plague outbreaks, “their role in these outbreaks are not clear.” Scientists have established that it’s possible that bubonic plague, which manifests when the Yersinia pestis bacteria enter the skin of the host, can be transmitted to humans from rats and ectoparasites, but, the study states, “the transmission from body lice and human fleas to humans has not yet been documented, and thus the importance of human ectoparasite transmission in current and historic settings remains an open question.”

Growing evidence casts doubt to the notion rats spread the Black Death: There’s a scarcity of rats in the archeological record; no masses of dead, infected rats found like there were discovered during the Third Pandemic; the climate of northern Europe could not have fostered their widespread distribution; and the high rate of household transmission of plague during this time suggests infection went from human to human.

In this new study, the scientists decided to test out these human ectoparasite theories with mathematical modeling. They created a “susceptible-infectious-recovered model” for plague transmission comparing the spread of plague by either a human ectoparasite vector, rat-flea transmission, or bacterial lung infection. The model was applied to outbreaks across nine cities that happened during the Second Pandemic, and they observed how the different spreaders of plague affected the pattern of simulated disease.

They found that in seven out of the nine cities, the pattern that most resembled what actually happened was instigated by the human ectoparasites. This signified to the scientists that the dominant transmission mode for plague during the Black Death was the human ectoparasites.

“The conclusion is very clear,” co-author Nils Stenseth, Ph.D., told BBC News. “The lice model fits best. It would be unlikely to spread as fast as it did if it was transmitted by rats.”

Stenseth also said the findings suggest the best way to prevent future diseases is hygiene and staying away from people when you’re sick — which is good advice, whether you’re infected with plague or even just the flu.