New Study Says Human Parasites, Not Rats, Spread The Bubonic Plague

The Black Death plague that came to Europe in the 14th century erased 60 percent of the population. The pandemic, which was considered one of the worst in human history, took the lives of tens of millions of people during outbreaks that occurred for 500 years.

The classic explanation says it was caused by the bacteria called Yersinia pestis, which made rats, gerbils or other rodents their hosts. The bacteria thrived on rodents and fleas bit them. They then jumped unto humans and feasted on their victims.

Rats suffered a bad rap for spreading the Black Death that wiped out a third of the European population.

For a very long time, rats had a bad reputation for spreading the plague. However, it turns out that rats or rodents might not be blamed after all. Rather, the bacteria may have spread from person to person. Scientists suggest that the transmission was caused by human-feeding parasites like lice and fleas.

Katherine Dean, the lead author of the study, said:

“The plague really transformed human history, so it’s really important to understand how it was spreading and why it was spreading so fast.”

In the study published in the journal Proceedings of the National Academy of Sciences, the bacteria was carried by human parasites instead of fleas found on rodents.

The deadly bite

In the past, it was thought that rodents are responsible for the spread of the deadly bacteria. When infected fleas from rats bite humans, the bacteria can readily enter the bloodstream and infect the victim’s lymph nodes, which are found all over the body.

The mode of transmission originally explained may not have been the same for each outbreak of the plague. For instance, in earlier outbreaks, the human deaths are not accompanied by rat deaths, which is termed as “rat falls”. However, for later outbreaks such as the Third Pandemic, humans, and rats deaths were rampant. This hints that maybe rats were not that responsible for the earlier outbreaks.

The researchers simulated Black Death outbreaks

The researchers, to land at an answer to the longstanding query, simulated Black Death outbreaks in the cities of Europe to understand how the plaque was spread.

The University of Oslo scientists modeled the three routes of transmission for the illness – rats, human fleas and lice and airborne. The team utilized mortality data for all nine outbreaks that happened during the second pandemic.

Human ectoparasites like lice and fleas are most likely the cause of the vast spread of the disease.

Based on their findings, human ectoparasites like fleas and lice showed the death or mortality trends more accurately.

Professor Nils Stenseth, from the University of Oslo, said in a statement:

“The conclusion was very clear. 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.”

The study will open the doors to more research about the plagues that took the lives of the people back then, and until now. In 2017, the World Health Organization (WHO) reported that a total of 2,348 confirmed, probable and suspected cases of plague were recorded in Madagascar, with 202 deaths.