Body lice may have spread the plague more than previously thought, science suggests

Scientists have long wondered whether human body lice could have contributed to the rapid spread of the bacteria that caused the deadly plague in the Middle Ages, known as the Black Death.

It is clear that rat fleas played a major role, but some population studies suggest that the bites of these fleas may not have been enough to cause an epidemic that killed tens of millions of people in Europe, in Asia and other countries in the 14th century.

A study published Tuesday in PLOS Biology suggests that body lice may be more effective at transmitting the plague bacteria, Yersinia pestis, than previously thought and thus may have contributed to rising bubonic plague pandemic numbers .

Body lice are parasites that can spread disease and usually affect people living in crowded places. They are different from head lice, which are much more common in the United States and usually affect school-aged children. Both insects feed on human blood.

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

Studies looking at how quickly the plague spread in the Middle Ages have suggested that another blood-sucking insect may have played a role, Hinnebusch said.

The researchers first looked at the possibility that human fleas — there are thousands of species of fleas and some bite humans specifically — could have helped spread the disease. It turned out that human fleas weren’t good at spreading the bacteria.

Hinnebusch and his team then became interested in body lice. The researchers conducted a series of laboratory experiments in which body lice were allowed to feed – through specially designed artificial skin – on blood samples infected with Yersinia pestis at levels similar to those found in real cases of human plague. Sure enough, the lice caught the bacteria.

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

“You could see transmission from day one, but more bacteria was transmitted three to seven days after infection,” Hinnebusch said.

While cases of plague are mostly a thing of the past in the United States, each year between one and 17 cases are reported, according to the Centers for Disease Control and Prevention. Most occurred in the rural West. The plague bacteria is believed to have been transported to the United States around 1900 on rat-infested ships, according to the CDC.

The new study could help explain what happened in the Middle Ages, said Dr. Meghan Brett, associate professor of infectious diseases at the University of New Mexico in Albuquerque.

“About 30 to 50 percent of the population died during this pandemic,” she said. “One of the things that has been difficult to explain is how it was transmitted. It was suggested that rats and fleas were not enough. So this study is actually very interesting and could potentially provide an explanation.

Because the new study was done in a laboratory, it’s not possible to know what proportion of actual infections were transmitted by body lice, said Dr. William Schaffner, professor of infectious diseases at the University Medical Center. Vanderbilt University.

“We have known that rats and other rodents have played a role for ages,” Schaffner said. “Now we know that body lice can also transmit the bacteria.”

In the United States, people typically contract the plague by being bitten on the leg by a prairie dog flea while hunting or hiking near the rodents’ burrows, Schaffner said.

Once infected, the lymph nodes in the person’s groin swell. Prompt medical care and treatment with antibiotics can clear up the infection.

But the scenario is not always so positive.

If the person is in a dusty environment and breathes in the bacteria, they may develop pneumonia, Schaffner said. “It’s very bad,” he said. “Within a few days, they can become seriously ill. »