Last updated January 30, 2018 at 1:24 pm
Humans themselves may have been the vehicle that spread the disease.
Rats, or more specifically the fleas carried by them, have been blamed for the Black Death – an outbreak of plague that swept through Europe and was responsible for the deaths of one third of the population within the space of five years between 1347-1353.
But new research suggests that it was the fleas and lice that made themselves at home on people, not rats, that were much more likely to have been responsible for the spread of the disease.
Plague is caused by the bacterium Yersinia pestis. Bubonic plague happens through skin infection, usually through the bite of an infected flea.
The bacteria then are transported to the lymph nodes which swell – the “buboes” that give bubonic plague its name.
There is also a form of pneumonic plague that can occur if the bacteria are spread through aerosols, like cough droplets from one infected person to another.
How did the Black Death spread?
Despite the enormous human toll associated with the prolonged outbreak of plague in Europe 14th to 19th centuries, starting with the Black Death, just how the disease was spread is up for debate. Previous studies have suggested that the climate of Europe was not suitable for the proliferation of rats that would have been needed to spread the disease during that time, which is supported by the lack of rats in the archaeological record.
Plague also spread at a high rate through households, suggesting that a more direct person-to-person transmission route might be likely.
To test this theory, Katharine Dean from the University of Oslo created a mathematical model. She and her team modelled three potential routes of transmission – plague spread by human lice or fleas, spread by rat fleas, or transmission through the air. The model was tested against the historical records from nine different outbreaks across Europe.
In seven of the nine outbreaks investigated, the transmission from human lice or fleas was the best predictor of the spread of disease.
It’s important to note that there still has not been documented transmission from body lice and human fleas to humans. But this study does provide a mathematical model that shows that the way the disease spread is more consistent with spread from human fleas and lice rather than rat fleas.
The plague is still with us
This is important because we did not leave the plague behind in the Middle Ages. The World Health Organization reported in November 2017 on a recent outbreak in Madagascar which infected over 2,000 people and resulted in 171 deaths.
While human fleas and lice are no longer a problem in most developed countries, they are still associated with areas of poverty and have been found in recent plague outbreaks in Tanzania, the Democratic Republic of Congo and Madagascar.
Understanding how plague has spread in the past could have important implications for how we manage and limit outbreaks today.
Research published in PNAS.