It wasn’t just rats. Body lice may have helped spread bubonic plague.

By Bernard Turk On Jun 1, 2024

The bubonic plague pandemic of the 14th century gained infamy as much for its death toll — 25 million in Europe alone — as for the horror of the disease itself. Scientists have long blamed rat-transmitted fleas for the plague’s swift spread. But recent research points the finger at an additional culprit: body lice.

A study in the journal PLOS Biology suggests that body lice are capable of transmitting Yersinia pestis, the bacterium that causes plague, more effectively than previously thought. Past research has focused on rodent-borne fleas as a plague vector. However, body lice’s role in plague transmission has remained murky, despite a growing body of evidence that rodents were only partly responsible for past pandemics.

Unlike head lice, which live and breed on human scalps, body lice live and multiply in the seams of human clothing and bedding, hopping onto their human hosts to feed on their blood several times a day.

Using an artificial, skinlike membrane, the researchers simulated how likely it was that plague-infected lice were able to transmit the disease to humans. In one scenario, lice were fed infected human blood, then put on the “skin” to feed, simulating an outbreak in which infected lice hopped from human to human. In another, lice were fed infected blood, then isolated for 18 hours before being allowed to feed again, similar to a scenario in which infected lice might jump from clothing or bedding onto a human.

Up to 60 percent of lice in both groups remained infected for a week, and all could transmit disease. Direct-transfer lice became infectious within just 24 hours of feeding on plague-infected blood.

Unexpectedly, even lice whose feces showed no evidence of plague proved infectious, too. This finding led to other experiments showing that lice with infected Pawlowsky glands — glands in their heads thought to play a role in saliva secretion — “routinely” transmit plague at levels sufficient to infect human beings.

Body lice consume more blood than fleas, the researchers note, and humans tend to scratch when infested. Given these factors and the tendency of body lice with infected Pawlowsky glands to transmit enough bacteria to infect humans, the researchers conclude, the tiny creatures are likely “better vectors of plague bacilli than previously appreciated.”

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