Human Fleas and Lice Spread Black Death

A new study suggests that the plague, which killed millions of people, was not transmitted by rats.

By Ashley Yeager | January 16, 2018

Researchers have long thought that fleas on rats spread the Black Death during medieval times, but a new study suggests it was, instead, fleas and lice on people that transmitted the plague.FLICKR, BAYER CROPSCIENCE UKRats may have gotten a bad rap when it comes to the plague. It was fleas and lice on people, not rats, that spread the disease between the 14th and 19th centuries in Europe, according to a study published today (January 16) in PNAS.

The rodents have long been blamed for spreading the plague, which is caused by the bacterium Yersinia pestis. Researchers thought the bacterium would infect fleas on rats, and when the rats died, the fleas would jump to humans, infecting them. Rats are also thought to be responsible for spreading more recent cases of plague, including the outbreak in Madagascar in 2017.

But the long-held theory that rats spread the plague during the Second Pandemic in the 14th to 19th centuries had a problem—the disease did not spread quickly enough. This discrepancy spurred Katharine Dean, an ecologist at the University of Oslo, and her colleagues to study the patterns of how disease moves through a population. “The plague really transformed human history, so it’s really important to understand how it was spreading and why it was spreading so fast,” Dean tells National Geographic.

The researchers simulated the outbreak of the plague in nine European cities between 1348 and 1813. They modeled the spread of the disease by human fleas and lice, rat fleas, or transmission through the air. In seven of the nine cases, human fleas and lice best matched the pattern of outbreak, compared to the other two modes of transmission.

“The conclusion was very clear. . . . The lice model fits best,” study coauthor Nils Stenseth, a biologist at the University of Oslo, tells the BBC. “It would be unlikely to spread as fast as it did if it was transmitted by rats.”

The takeaway, Stenseth says: personal hygiene. The disease doesn’t spread if we stay clean. Another tip, he tells the BBC, “If you're ill, you shouldn't come into contact with too many people. So if you're sick, stay at home.” 

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Avatar of: Justin Champion

Justin Champion

Posts: 1

January 17, 2018

This is very old news indeed. Ever since the work of Biraben on French plague, and the studies of Graham Twigg, plus epidemiological studies of the Great Plague in London of 1665, there has been a consensus that the rat-flea-human vector was implausible to explain the shape of these epidemics. French historiography decades ago suggested that the patterns of infection in France were driven by human lice. The last outbreaks in the UK were mainly an urban phenomena, and the relational data enables the mapping of social status and wealth, alongside, place and seaonality of death. Rats, as the work of Twigg, Champion and others has established could not have been the vectors.

Avatar of:

Posts: 4

January 19, 2018



isn't possible that particular variants of yersinia activate fleas (or whatelse) maybe not only human pushing them toward unusual behavior, affecting therefore both human and fleas? e.g. fleas become more restless and more incline to leave their hosts, falling then in the need of find a new target?

I remind I read in the past (I don not know if that theory is accepted anymore) that there is an ormonal reguylator for the number of peasts living on a particular host. Something in those vairants can increase temporarily those ormons pushing fleas to leave!

Just to say!


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