Humans, NOT rats, were to blame for spreading the Black Death across Europe, research suggests

Rats have long been synonymous with plague, blamed for carrying the parasites that spread disease and killed millions of people in Europe and Asia during the medieval period.

But the common rat (rattus rattus) might not be as culpable as first thought. A new study from Oslo and Ferrara universities points to human contact and body lice as the main culprits for the spread of plague bacteria between the 1300s and early 1800s.

One of the worst cases in this period was the Black Death, which claimed the lives of tens of millions of people and decimated Europe’s population in the 14th century. Humans were infected with the disease when bitten by fleas infected with the bacterium Yersinia pestis, and it has generally been believed that rats played a key role in spreading these fleas.

The researchers studied nine cities in Europe, using mortality data to model the plague’s disease dynamics. They created three models for each city, based on rats, airborne transmission and human-based fleas and lice. In seven of the nine cities, the latter model was a much better match for the pattern and speed of the outbreak as suggested by the mortality data.

“The conclusion was very clear,” professor Nils Stenseth, from the University of Oslo, 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. […] It would have to go through this extra loop of the rats, rather than being spread from person to person.”

So why do rats get such a bad name? Plague outbreaks since the 1800s are indeed connected to rodents, with fleas that drink the blood of infected rats spreading the disease as they move onto humans. But the speed of the Black Death’s spread doesn’t match with this pattern, and it may be the case that experts transplanted the mechanisms behind modern plague into medieval plague.

“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 note in their study.

Plague still exists; with the World Health Organisation charting 3.248 reported cases between 2010 and 2015, including 584 deaths.