The Black Death May Not Have Been Caused by Rats

It may surprise some, but we are still dealing with the plague today. It’s not causing the mass deaths and grinding halt of society as it did in the Middle Ages, but the virus, Yersinia pestis, is still infecting people today. Once inside cities the disease spreads very quickly, making it difficult to eradicate. For centuries it has been presumed that rats and rodents have been major spreaders of the plague since the fleas they often come with are carriers of the virus.

Researchers call these animals long-term plague reservoirs, meaning they have the ability to store the virus over many years within their populations. Rodents have long been blamed as a result, with fables like the Pied Piper showing how desperate the people in Europe were to be rid of their rat problems. Now, new research has contributed to the theory that perhaps rats weren’t actually to blame for the Black Death.

In a joint effort between scientists and historians, the historical data surrounding the plagues of 7th century onwards have been studied for clues as to how the disease spread. The peer-reviewed findings were published in the journal Proceedings of the National Academy of Sciences (PNAS) in December of 2022.

Researchers from across the globe joined forces to study the conditions and ramifications of plague outbreaks across recorded history. They found that these long-term plague reservoirs may not have been present in Europe at the time in wild rat populations. There are certain temperatures, minerals and pH levels in soil, and rainfall conditions that Y. pestis needs in order to maintain a passive presence as described above and the data from the outbreaks in European history show that the soil there didn’t posses the correct properties to allow the virus to live in the soil- outside any host.

However, these conditions were present in Asia, leading researchers to ponder if the European plagues were imported from Asia each time, rather than homegrown from a population of unchecked European rats.

To further back up this theory, the study raises the idea that if there was a long-term reservoir in Europe that there might have been another plague outbreak in the past 100 years- which there was not been. However, there have been outbreaks in Asia and in the US in the past century- places where the soil conditions are much more favorable to the disease being able to survive outside a host before being picked up by local rodent populations.

Two of the researchers, writing for The Conversation, pointed out that the time it takes to spread the disease is much shorter than how long it takes rodents to travel across large tracks of land. This casts doubt on whether rats could have really spread the disease fast enough to decimate the cities of Europe with such speed. It’s more likely that traveling humans were the biggest contribution to how fast the disease spread.

This doesn’t mean that rats didn’t play a role in these outbreaks, but appears there is a chance that they might not have been exclusively responsible for the rapid transmission of the plague across Europe.