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

The Black Death killed tens of millions of people in the mid-1300s, but scientists and historians are still trying to figure out how it spread. Wellcome Images via Wikimedia Commons under CC BY 4.0

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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Filed Under: Biology, Bugs, Death, Disease, Disease and Illnesses, European History, Health, Insects, Medicine, Medieval Ages, Microbes, Bacteria, Viruses, New Research, Pandemic

Body Lice May Be Pretty Good Spreading Bubonic Plague

Blood-sucking, parasitic human body lice may be more effective at transmitting the bacterium that causes bubonic plague than scientists originally thought. A laboratory study published May 21 in the journal PLOS Biology found that lice may have spread Yersinia pestis (Y. Pestis) efficiently and contributed to past pandemics the way that fleas do. 

Y. Pestis has been the bacterium responsible for numerous pandemics, most notably the Black Death. Bubonic plague killed an estimated 25 million people across Europe in the Middle Ages from 1347 to 1351. The Y. Pestis bacterium naturally cycles between rodents like rats and the fleas that bite the rodents. The fleas can then infect humans through bites, so fleas have been believed to be the primary driver behind plague pandemics. 

There have been debates among scientists and historians about the very nature of the spread of the Black Death, with some medical historians questioning if body lice may have also been driving up plague cases. These lice are different from head lice that frequently cause itchy scalps in elementary schools, but they both feed on human blood. Body lice can also carry Y. Pestis.

[Related: Plague DNA was just found in 4,000-year-old teeth.]

To clarify body lice's potential role in transmitting plague, the team on this study from the National Institute of Allergy and Infectious Diseases designed a series of experiments in a lab. The lice fed on blood samples containing Y. Pestis and used membrane feeders. These feeders simulate warm human skin, so the scientists could study transmission potential in a laboratory setting.

The body lice that became infected with Y. Pestis and were capable of routinely transmitting it after they ate blood containing levels of the pathogen that are similar to those found in actual human plague cases. The team also found that Y. Pestis can infect the lice's Pawlowsky glands. When infected, this pair of salivary glands transmitted the pathogen more consistently than the  lice who only carried the infection in their digestive tract. 

The team believes that Pawlowsky glands secrete some kind of lubricant onto the lice's mouthparts. In infected lice, these secretions may contaminate the bug's mouthparts with Y. Pestis, which may then spread to humans when bitten by the lice. This could mean that body lice may be more efficient spreaders of Y. Pestis and may have played a role in previous outbreaks.

[Related: You could get the plague (but probably won't).]

"You could see transmission as early as day one, but more bacteria were transmitted on days three to seven after infection," Joe Hinnebusch, a study co-author and retired senior investigator at the National Institute of Allergy and Infectious Diseases Laboratory of Bacteriology, told NBC News. According to the Centers for Disease Control and Prevention, about one to 17 cases are reported annually. Most cases are found in the rural western United States, where people usually contract the disease when hunting near prairie dog burrows or getting bitten by infected prairie dog fleas.

Human Body Lice Could Harbor The Plague And Spread It Through Biting

Rats and their fleas take the rap for spreading the plague, but lice that infest people may share the blame.

Human body lice can harbor plague-causing bacteria and can transmit the disease by biting, researchers report May 21 in PLOS Biology.

Some studies have suggested that Yersinia pestis, the bacterium that causes the plague, spread too quickly during the Black Death of 1346 to 1353 and other outbreaks to be the sole work of rats and fleas (SN: 6/15/22). Human body lice, which feed on human blood, might be involved, but previous research found that the lice aren't very efficient at spreading the disease.

Researchers from the U.S. National Institute of Allergy and Infectious Diseases in Hamilton, Mont., revisited whether body lice (Pediculus humanus humanus) can spread the plague. The team simulated human skin with membranes that lice can puncture with their mouth parts to get a blood meal. Lice fed blood with Y. Pestis in it developed infections that lasted about a week, the researchers found.

In some lice, the bacteria concentrated in a unique set of glands called Pawlowsky glands, which may produce a lubricant to help mouthparts extend and retract. Those lice could transmit the bacteria during a short three-hour feeding period and in large enough doses to cause disease, the researchers discovered. Lice that carried the plague in their guts could spread the bacteria if given 20 hours, but not as efficiently as the ones with the gland infections.

These findings suggest the plague could spread through lice bites — a new way for lice to spread disease. Body lice spread other bacterial diseases, including trench fever and typhus, to people when crushed lice or their bacteria-laden feces are ground into the skin.

And body lice could be infected with Y. Pestis strains that have genetic mutations that prevent the bacterium from infecting fleas, the team found. The findings suggest that body lice are better at transmitting plague than previously thought, the researchers note, and might be just the culprits on which to pin some plague outbreaks.

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Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. In molecular genetics from Washington University in St. Louis and a master's degree in science journalism from Boston University.

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