Human body lice may be just as responsible for spreading the plague as rats and fleas, a new study suggests.
The findings challenge widespread views of how the Black Death wreaked havoc in the Middle Ages, say scientists.
And are pretty gross, to be honest.
The bubonic plague pandemic – one of the most deadly in history – spread across Britain and Europe from 1346 to 1353, wiping out as many as 50 million people, up to half the continent’s population.
The plague still exists today, including in the US.
Now a new lab study suggests that human body lice are more efficient at transmitting Yersinia pestis, the bacterium that causes plague, than previously thought.
The findings, published in the journal PLoS Biology, support the possibility that the parasites may have contributed to previous pandemics.
Dr David Bland conducted the research with colleagues at the United States National Institute of Allergy and Infectious Diseases based in Maryland.
‘Y. pestis has been the culprit behind numerous pandemics, including the Black Death of the Middle Ages that killed millions of people in Europe,’ he said.
‘It naturally cycles between rodents and fleas, and fleas sometimes infect humans through bites – thus, fleas and rats are thought to be the primary drivers of plague pandemics.
‘Body lice – which feed on human blood – can also carry Y. pestis, but are widely considered to be too inefficient at spreading it to contribute substantially to outbreaks.
‘However, the few studies that have addressed lice transmission efficiency have disagreed considerably.’
Dr Bland and his team conducted a series of lab experiments in which body lice fed on blood samples containing Y. pestis.
The experiments involved the use of membrane feeders, which simulate warm human skin, enabling the team to study transmission potential in a lab setting.
They discovered that the body lice became infected with Y. pestis and were capable of routinely transmitting it after feeding on blood containing levels of the pathogen similar to those found in actual human plague cases.
The team also found that Y. pestis can infect a pair of salivary glands found in body lice known as the Pawlowsky glands.
And lice with infected Pawlowsky glands transmitted the pathogen more consistently than lice whose infection was limited to their digestive tract.
It is believed that Pawlowsky glands secrete lubricant onto the lice’s mouthparts.
That led the research team to hypothesise that, in infected lice, such secretions may contaminate mouthparts with Y. pestis, which may in turn spread to humans when bitten.
Dr Bland said: ‘These findings suggest that body lice may be more efficient spreaders of Y. pestis than previously thought, and they could have played a role in past plague outbreaks.’
