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Study: Medieval plague outbreak exploded over 300 years

The East Smithfield cemetery, located in London, built in 1348. (Credit: Museum of London Archeology)

(CN) — A study released Monday has found that over the course of 300 years, plague outbreaks in London dramatically picked up speed and spread up to four times faster in the 17th century than at their emergence in the 14th century.

Details of the study are published in the journal Proceedings of the National Academy of Sciences, where researchers from McMaster University in Canada discuss this concerning series of events which could potentially hold important guidance about our future.

The bubonic plague first arrived in Europe in 1347, carried by sailors on a trading ship from Asia. Over the next five years, the disease had claimed over 20 million European lives — over one-third of the continent’s entire population at the time.

Although the Black Death had officially ended by the early 1350s, the plague never went away and continued to reappear for the next few centuries. The most notable recurrence of this was the Great Plague of 1665, where a horrific outbreak in London was responsible for about 1,000 deaths a week, reaching its peak when a tragic 7,165 people were lost in a single week.

The researchers combed through thousands of documents related to the medieval disease, including personal recountings, church records and the London Bills of Mortality.

From these sources, the team of statisticians, biologists, and evolutionary geneticists approximated the loss of life, and found a chilling statistic: The infection rate in the 14th century doubled every estimated 43 days, but later in the 17th century, that rate had increased to doubling every 11 days.

“It is an astounding difference in how fast plague epidemics grew,” said lead author David Earn, a professor in the department of mathematics & statistics at McMaster.

The authors note that it was an especially challenging task to estimate the death rate in London from the beginning of the outbreaks in the 1300s to the peak in 1665, as no official death records exist from before 1538. In an attempt to gain a more rounded picture of how rapidly the disease spread, they looked to examine individuals’ wills and testaments.

“At that time, people typically wrote wills because they were dying or they feared they might die imminently, so we hypothesized that the dates of wills would be a good proxy for the spread of fear, and of death itself,” Earn said. “For the 17th century, when both wills and mortality were recorded, we compared what we can infer from each source, and we found the same growth rates.”

He added, “No one living in London in the 14th or 17th century could have imagined how these records might be used hundreds of years later to understand the spread of disease.”

The plague has been found to be associated with exposure to a bacteria called Yersina pestis, which travels through the air and through the bites of carriers like fleas and rats. Although these pests are said to be the cause of the rapid spread onboard ships, which carried the illness from port to port across Europe, scientists are still unclear about exactly how the disease spread.

A photo of the one of the London Bills of Mortality, for the week beginning Sept. 26, 1665. (Photo by Clair Lees)

“From genetic evidence, we have good reason to believe that the strains of bacterium responsible for plague changed very little over this time period, so this is a fascinating result,” said co-author Hendrik Poinar, an anthropology professor at McMaster.

Based on what researchers know about the epidemics and the gathered information from this study, they do not think it is likely that the pathogen’s primary mode of transmission was from person-to-person spread. They suspect that, given the accelerated growth rates, infected fleas probably played a large role.

The authors also emphasize that this research holds important lessons for society today as we face the effects of Covid-19. Looking at the trends from past epidemics, they believe that population density, living conditions, and cooler temperatures are all factors in facilitating the spread of disease.

Some of these factors can already be seen taking effect throughout communities today, as Covid-19 disproportionally affects minority groups, highly populated regions, those with less access to health care, communities affected by climate change and more.

In hopes that their work can provide some insight moving forward, Earn and his team have developed a digital archive capable of analyzing infection patterns across outbreaks of the past, and identifying significant changes over time.