Black Death: Humans, And Not Rats, More Likely Spread The Plague, Claims ‘Provocative’ New Study

Rats didn’t spread the Black Death, humans did; a new study challenges what we thought we knew about the plague outbreaks in Medieval Europe.

A costume depicting the mask of a plague doctor in the early 17th century.
Dennis van de Hoef / Shutterstock

Rats didn’t spread the Black Death, humans did; a new study challenges what we thought we knew about the plague outbreaks in Medieval Europe.

It seems rats may have been wrongfully accused of spreading the parasites that caused the plague to run rampant in Medieval Europe. A recent study exonerates the rodents and points to human parasites as the more likely culprit for the swift spread of the Black Death, the death toll of which rose to tens of millions in the mid-1300s, wiping out a third of Europe’s population.

The study, described as “provocative” by National Geographic, argues that not rats, but humans were more likely responsible for the rapid transmission of the disease, due to human ectoparasites (or external parasites), such as fleas and body lice.

To prove their theory, the study authors analyzed plague transmission patterns during the Second Pandemic, a series of devastating outbreaks spanning hundreds of years and which includes the Black Death (1347–1353).

“While it is commonly assumed that rats and their fleas spread plague during the Second Pandemic, there is little historical and archaeological support for such a claim,” the researchers wrote in their study, published on January 16 in the journal Proceedings of the National Academy of Sciences.

Instead, the authors suggest that Yersinia pestis, the bacterium that caused the bubonic plague, may have spread from person to person, carried by human parasites.

Yersinia pestis was thus passed on to other people in close quarters, when human fleas and body lice changed hosts after biting infected individuals. The bacteria amassed in lymph nodes throughout the body, causing them to swell into “buboes,” from which the bubonic plague gets its name.

“The classic example is the rat-flea transmission,” lead study author Katharine Dean, a research fellow at the University of Oslo (UiO), in Norway, said in a statement.

Yet, unlike other plague outbreaks in human history, including the most recent Madagascar plague outbreak in 2017, the Second Pandemic (14th through 19th century) points to another mode of transmission.

According to medieval records, the plague seems to have spread more rapidly during the Second Pandemic compared with modern outbreaks (occurring after the late 1800s), in which rats and other rodents played a critical part in spreading the disease.

Moreover,the Smithsonian magazine notes that the Third Pandemic (starting in 1855) was preceded by massive deaths in the local rat populations, known as “rat falls,” whereas medieval records make no mention of such a thing.

These peculiarities lead Dean and her team to believe that the Black Death wasn’t spread by rats, but by something else altogether.

“The plague really transformed human history, so it’s really important to understand how it was spreading and why it was spreading so fast,” said Dean, who studies infectious disease at UiO’s Centre for Ecological and Evolutionary Synthesis.

Image of a plague doctor in protective clothing, dated circa 1656. The beak mask held spices thought to purify air, while the wand was used to avoid touching patients.
Image of a plague doctor in protective clothing, dated circa 1656. The beak mask held spices thought to purify air, while the wand was used to avoid touching patients. Hulton Archive / Getty Images

For the purpose of the study, the team devised a mathematical model comparing the rat-flea and human-parasite modes of transmission, in order to establish how the plague would spread in both instances.

Their model — in essence, a virtual simulation of a plague pandemic — explored three possible outbreak scenarios, in which the disease had three different vectors: rats (and their parasites), human fleas (Pulex irritans) and body lice (Pediculus humanus humanus), and airborne Yersinia pestis bacteria (spread by coughing humans in a version of the disease called the pneumonic plague).

Through a series of equations, the researchers replicated the rise and fall of a plague outbreak and calculated how the three vectors would behave and spread the disease.

The next step was to compare this mathematical model with mortality patterns from nine different plague outbreaks in Europe during the Second Pandemic (between 1348 and 1813), in order to see which of the three scenarios would best fit historic records.

“It’s basically bookkeeping — you see how people move [in the simulation],” says co-author Boris Valentijn Schmid, computational biologist at UiO and Dean’s Ph.D. adviser.

Statistic evaluations revealed that, in seven of the nine outbreaks, the historic mortality records were better reflected by the human-parasite mode of transmission, which fit the profile more closely than the rat-flea model. This suggests that in the seven cases the disease may have been spread by human parasites rather than by rats.

“Our results support that human ectoparasites were primary vectors for plague during the Second Pandemic, including the Black Death,” the authors conclude in their study.

Nevertheless, Schmid acknowledges that their model, while showing that plague was more likely spread by human parasites, doesn’t disregard other means of transmission.

Although it admittedly challenges “the assumption that plague in Europe was predominantly spread by rats,” as the authors themselves write in their paper, their theory was deemed “plausible” by Nukhet Varlik, a history professor at Rutgers University in New Jersey, the Washington Post reports.

However, Varlik, who has studied the plague in the Ottoman Empire, says Dean’s study does have one shortcoming, namely it focuses “exclusively on the European experience.”

“Plague spread across Afro-Eurasia during the Black Death and continued on and off for several centuries,” Varlik points out.