Did Body Lice Spread Bubonic Plague? Research Suggests the Parasites Are Better Vectors Than Thought

The bubonic plague killed an estimated 25 million people in Europe—about one-third of the continent’s population—during a pandemic known as the Black Death in the mid-1300s. But exactly how and why the plague-causing bacteria, called Yersinia pestis, spread so quickly has remained an enduring mystery.

Scientists and historians believe rat fleas were largely responsible for transmitting the bacteria from person to person. But rat fleas alone can’t totally account for the speed of the spread.

Now, researchers have come up with another likely culprit: human body lice. These blood-sucking parasites may be more effective at spreading Y. pestis than previously thought, according to a new paper published Tuesday in the journal PLOS Biology.

The findings build upon a 2018 study that compared historical records of plague outbreaks in nine European cities to plague outbreak simulations under different conditions—some spreading via rats and some spreading via parasites that live on humans. That research also indicated that, while rats were likely involved, the plague was able to infect so many people because of human parasites, including body lice.

Body lice (Pediculus humanus humanus) are tiny, wingless insects that typically live in bedding and clothing and feed on human blood. They’re different from head lice (Pediculus humanus capitis), which tend to live on human scalps—though both species usually spread via direct contact with an infected person or contact with contaminated materials.

Today, body lice are usually found in situations involving reduced hygiene and a lack of access to clean clothing and bedding, such as after natural disasters or among people experiencing homelessness. But these opportunistic bugs have been feasting on humans and their ancestors for between five million and six million years—and in the past, they were just a fact of life.

Scientists have previously linked body lice with spreading other types of bacteria that cause human diseases—including trench fever, louse-borne relapsing fever and epidemic typhus—but not specifically with Y. pestis. So, researchers set out to explore the possible link between body lice and Y. pestis transmission.

They set up a laboratory experiment involving a device referred to as a “feeding membrane” that’s designed to mimic human skin. They created blood samples contaminated with Y. pestis at levels similar to actual human plague cases, then set the body lice loose to feed.

While drinking blood samples through the artificial skin, the body lice picked up the bacteria. Then, scientists transferred them to a new feeding membrane that contained uninfected blood. Later, when they tested the once-sterile blood sample, the scientists detected Y. pestis.

The researchers also discovered some possible mechanisms of transmission. Some of the body lice had Y. pestis in their digestive tracts and in their feces. And, because body lice feed frequently—usually five or six times a day, or once roughly every four hours—they produce feces frequently.

After being bitten by an infected louse, humans would likely scratch their skin. This scratching could create small entry points into the body for the lice’s infected feces; scratching might also crush the infected lice themselves and allow their contaminated fluids to enter the body.

In addition, the scientists found Y. pestis in some of the lice’s Pawlowsky glands, which secrete lubricating saliva into their mouthparts. If an infected louse bites a human, these secretions could also directly transmit the bacteria.

This particular finding could be a jumping off point for future investigations.

“Research on the salivary molecules produced by body lice is an understudied topic that is worthy of further characterization,” says study co-author Joe Hinnebusch, a now-retired senior investigator at the National Institute of Allergy and Infectious Diseases Laboratory of Bacteriology, to Interesting Engineering’s Maria Bolevich. “Very little is known about the Pawlowsky glands.”

Still, because the study was conducted in a lab, it might not be a perfect comparison for how body lice may transmit Y. pestis in the real world, among real humans. But, even so, the findings suggest this previously overlooked parasite could have contributed to the Black Death’s spread nearly 700 years ago.

“About 30 to 50 percent of the population died during that pandemic,” says Meghan Brett, an infectious disease expert at the University of New Mexico who was not involved in the research, to NBC News’ Linda Carroll. “One of the things that’s been hard to explain is how it was transmitted. It’s been suggested that rats and fleas were not sufficient. So, this study is actually quite interesting and has potentially come up with the explanation.”

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.