Researchers examining the impacts of bubonic plague in London, since it first arrived from Asia in the mid 1300s, until the last major epidemic in 1666, have estimated the disease spread four times faster in the 17th century than it had in the 14th century.
After analysing thousands of documents covering a 300-year time span detailing the plague outbreaks in the capital, researchers from McMaster University in Canada have revealed how the spread of the disease accelerated during later epidemics.
Later known as the Black Death, the plague, caused by the Yersinia pestis bacteria and transmitted by fleas on black rats, arrived in England in June 1348. It reached London by the autumn, and covered the whole country by summer 1349.
That epidemic is estimated to have killed between 40-60 per cent of the population of England and more than one-third of the population of Europe. In England it died down by December 1349, but remained endemic over the next three centuries, occasionally flaring up.
The last major epidemic was the Great Plague of 1665 in London. In that outbreak an estimated 100,000 people, almost a quarter of London's population, died in 18 months.
The research team found that in the 14th century, the number of people infected during an outbreak doubled approximately every 43 days.
But by the 17th century, the number of infections was doubling every 11 days.
“It is an astounding difference in how fast plague epidemics grew,” said David Earn, a professor in the Department of Mathematics & Statistics at McMaster University and investigator with the Michael G DeGroote Institute for Infectious Disease Research, who is lead author on the study.
Professor Earn and a team including statisticians, biologists and evolutionary geneticists estimated death rates in London by analysing historical, demographic and epidemiological data from three sources: personal wills and testaments, parish registers, and the London Bills of Mortality.
It was not simply a matter of counting up the dead, since no published records of deaths are available for London prior to 1538.
Instead, the researchers scoured written records for information, even using individual wills and testaments to establish how the plague was spreading through the population.
“At that time, people typically wrote wills because they were dying or they feared they might die imminently, so we hypothesised that the dates of wills would be a good proxy for the spread of fear, and of death itself.
“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,” said Professor Earn.
“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.”
While previous genetic studies have identified Yersinia pestis as the pathogen which causes plague, little is known about how the disease was transmitted.
“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," says Hendrik Poinar, a professor in the Department of Anthropology at McMaster, who is also affiliated with the Michael G DeGroote Institute for Infectious Disease Research, and is a co-author on the study.
The estimated speed of these epidemics, along with other information about the biology of plague, suggest that during these centuries the plague bacterium did not spread primarily through human-to-human contact, known as pneumonic (relating to the lungs) transmission.
Growth rates for both the early and late epidemics are more consistent with bubonic plague, which is transmitted by the bites of infected fleas.
Researchers believe that population density, living conditions and cooler temperatures could potentially explain the acceleration, and that the transmission patterns of historical plague epidemics offer lessons for understanding Covid-19 and other modern pandemics.
The researchers said the new digitised archive developed over the course of the study provides a new way to analyse epidemiological patterns from the past and has the potential to lead to new discoveries about how infectious diseases, and the factors that drive their spread, have changed through time.
The research is published in the Proceedings of the National Academy of Sciences.