Feature pest control | PPC90 March 2018

A new study suggests that rats might not be responsible for spreading the Black Death and subsequent epidemics of bubonic plague that rampaged across Europe, Asia and Africa for over 600 years.

For many people, the first thoughts about pest control came from a primary school history lesson where you learned about the Black Death. Your teacher may have told you how an estimated third of Europe's population (25 million people) was wiped out between 1347 and 1351 because rats spread fleas, which in turn spread bubonic plague.

A particularly good teacher may have even told you that’s why we keep rats away from people – so we stop the spread of deadly diseases and learn the lessons history taught us. The general public is generally pretty ignorant about our sector – but most people will tell you: rats equal plague.

A new study in the Proceedings of the National Academy of Science suggests, while people commonly assume that rats and their fleas spread the plague, “human ectoparasites, like body lice and human fleas, might be more likely than rats to have caused the rapidly developing epidemics in pre-industrial Europe”. In other words, humans spread the plague. A win for the rattus rattus PR team if ever there was one.

In modern instances of plague, such as the outbreak in Madagascar in 2017, rats and other rodents helped spread the disease. If Y. pestis bacterium infects rats, they can pass it to their fleas as they drink the rodents’ blood. When a plague-infected rat dies, its parasites abandon the corpse and can go on to bite humans – often with the help of domestic animals.
This was thought to be how medieval plagues spread. However, the new study presents an alternative argument.

Prof Nils Stenseth, from the University of Oslo, told the BBC that, “we have good mortality data from outbreaks in nine cities in Europe, so we could construct models of the disease dynamics.”
Using sophisticated computer simulations, the study tested three models for spreading disease outbreaks in each of these cities. They were by rats; airborne transmission; and lice and fleas living on humans and their clothes.

Seven out of the nine cities simulated found the ‘human parasite model’ produced the best match for the pattern of the outbreak.

The study shows they believe that historical plagues spread far too quickly for rats to be the primary transmitter of the disease and instead human lice and fleas are to blame.

Before we give rats a free-pass on history’s most infamous epidemics, it’s worth saying that the study has plenty of room to improve its simulation model, and many scholars are still firmly pointing the finger at rodents.

However historical plagues spread, it’s a fact that from 2010 to 2015 there were 3,248 cases of bubonic plague reported worldwide, including 584 deaths, according to the World Health Organisation (WHO) – and rats and their fleas have been a significant transmitter. So, we’re not letting rats off the hook quite yet.

Plague facts:

• 3,248 cases of bubonic plague were reported worldwide in 2010-15.
• Without treatment, bubonic plague results in the death, in around ten days, of up to 90% of those infected.
• ‘Black Death’ is a relatively new term. During the event itself it was often called ‘the Pestilence’.
• In 2001, a US study tried to map the plague genome using a bacterium that had come from a dead vet. The vet died after a plague-infested cat sneezed on him as he tried to rescue it.

Source: PPC90