The speed of plague transmission in London increased four-fold between the Black Death of 1348 and the Great Plague of 1665, according to a new study.
In the 14th century the number of people infected during an outbreak doubled about every 43 days; 300 years later, it was every 11 days.
The researchers believe 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 modern pandemics.
“It is an astounding difference in how fast plague epidemics grew,” says David Earn from Canada’s McMaster University, who led a team that included statisticians, biologists and evolutionary geneticists.
As no published records of deaths are available for London prior to 1538, they estimated death rates by analysing historical, demographic and epidemiological data from three sources: personal wills and testaments, parish registers, and the London Bills of Mortality.
“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,” he says.
“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.”
While previous genetic studies have identified Yersinia pestis as the pathogen that causes plague, little has been 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 co-author of the team’s paper in the journal PNAS.
The estimated speed of these epidemics, along with other information about the biology of plague, suggest, the researchers say, that during these centuries the plague bacterium did not spread primarily through human-to-human contact, known as pneumonic 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.