Body Lice and Their Potential Role in the Spread of the Black Death

Scientists have long debated whether human body lice might have played a significant role in driving the rapid spread of the bacteria responsible for the deadly plague in the Middle Ages, known as the Black Death. While it is clear that rat fleas played a major role, some population studies have suggested that bites from those fleas might not have been sufficient to drive a plague that killed tens of millions across Europe, Asia, and other regions in the 14th century.

A study published Tuesday in PLOS Biology suggests that body lice may be more efficient at transmitting the plague bacteria, Yersinia pestis, than previously thought, potentially contributing to the high mortality rates during the bubonic plague pandemic.

Body Lice: A Hidden Culprit?

Body lice are parasites that can spread disease and typically affect people living in crowded conditions. They differ from head lice, which are more common in the U.S. and typically affect school-age children. Both insects feed on human blood.

“There’s a long-standing medical historical debate about the Black Death pandemic in Europe,” said senior author Joe Hinnebusch, who was a senior investigator at the National Institute of Allergy and Infectious Diseases Laboratory of Bacteriology in Hamilton, Montana, when the research was conducted. He is now retired.

The Research Process

Studies examining the speed at which the plague spread in the Middle Ages have suggested that another blood-sucking bug might have played a role, Hinnebusch said. Initially, the researchers considered the possibility that human fleas could have helped spread the disease. However, it turned out that human fleas were not effective at spreading the bacteria.

Next, Hinnebusch and his team focused on body lice. They conducted a series of laboratory experiments in which body lice were allowed to feed through a specially constructed artificial skin on blood samples infected with Yersinia pestis at levels similar to those found in actual human plague cases. The lice successfully picked up the bacteria.

Once infected, the lice were placed on another bit of artificial skin and allowed to suck sterile blood through it. When the researchers later tested the formerly sterile blood sample, they found it had indeed been infected by the interaction with the lice.

“You could see transmission as early as day one, but more bacteria were transmitted on days three to seven after infection,” Hinnebusch said.

Implications and Expert Opinions

Although plague cases are mostly a thing of the past in the U.S., each year anywhere from one to 17 cases are reported, primarily in the rural West, according to the Centers for Disease Control and Prevention (CDC). The plague bacteria is thought to have been carried to the U.S. around 1900 by rat-infested ships, the CDC reports.

The new study might help explain the high mortality rates during the Middle Ages, said Dr. Meghan Brett, an associate professor of infectious diseases at the University of New Mexico in Albuquerque. “About 30% to 50% of the population died during that pandemic,” she said. “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 with the explanation.”

Dr. William Schaffner, a professor of infectious diseases at the Vanderbilt University Medical Center, noted that although the new study was performed in the lab, it is not possible to determine what proportion of real-life infections were transmitted by body lice. "We’ve known that rats and other rodents played a role for ages," Schaffner said. "Now we know that body lice can transmit the bacteria too."

Current Context and Risks

In the U.S., people usually contract plague by being bitten on the leg by prairie dog fleas when hunting or hiking near the rodents’ burrows, Schaffner explained. Once infected, the lymph glands in the person’s groin swell. Quick medical attention and treatment with antibiotics can clear up the infection.

However, if a person inhales the bacteria in a dusty environment, they can develop pneumonia, which can be fatal if not treated promptly. “That is very bad,” Schaffner said. “Within a few days they can become mortally ill.”

Conclusion

This new understanding of the role of body lice in the transmission of Yersinia pestis adds a crucial piece to the puzzle of how the Black Death spread so rapidly and lethally. As scientists continue to unravel the complex history of this pandemic, such studies provide valuable insights into the dynamics of infectious diseases and their impact on human history.

The study’s findings suggest that public health strategies during pandemics should consider multiple vectors of transmission, especially in crowded living conditions. Understanding the role of body lice in past pandemics can help inform better prevention and control measures in future outbreaks.