Oct. 19 (UPI) -- From the 14th to the 19th centuries, Europe and Asia were struck by successive waves of the plague. New research -- published Monday in the journal PNAS -- suggests that over the course of the pandemic, from the Black Death of 1348 to the Great Plague of 1665, transmission rates increased four-fold.
In recent years, genomic analysis has offered scientists a variety of insights into the nature of bubonic plagues that decimated much of Europe and Asia during the Late Middle Ages, but details related to disease transmission mode and dynamics have been hard to come by.
Plague is caused by the bacterium Yersinia pestis, which can be transmitted via flea bites, the typical route for bubonic plague, or directly from human to human, called pneumonic plague.
"Both types of transmission can occur during a given epidemic, but which type of transmission seems most likely in various times and places, and why, is often debated," lead study author David Earn, a researcher in mathematical epidemiology at McMaster University in Canada, told UPI in an email.
By studying a variety of historical, demographic and epidemiological data, Earn and his colleagues were able to show that transmission steadily increased over the course of the epidemics that struck London between the 14th and 19th centuries.
The data also showed that pneumonic transmission could not have been the primary mode of transmission during the 14th century in London.
Scientists relied on three main sources of data: weekly all-cause mortality totals from parish registers, beginning in 1538, when deaths were first registered; weekly mortality from plague, recorded in the London Bills of Mortality; and last wills and testaments, dated to the day, running from 1340 to 1680.
Researchers used the mortality data to estimate transmission rates of each successive epidemic. The research team also ran models to estimate mortality totals had the early waves been driven primarily by human-to-human transmission.
"In this scenario of purely pneumonic transmission, we found that at most 15 recent of the population would have died from plague," Earn said. "But estimates of the mortality toll during the Black Death in 1348 are at least 30 percent of the population. This inconsistency makes purely pneumonic transmission in the 14th century implausible."
Earn and his colleagues suspect both climate and shifting population densities played a role in accelerating transmission rates during the later waves of the plague.
"Evidence has accumulated over the last 15 years or so that humidity affects influenza transmission, and it is reasonable to imagine that climate might affect plague transmission," Earn said.
In followup studies, researchers plan to investigate the effects of climate, population density, social structure and genetic changes on the transmissibility of plague and the primary transmission mode during the 16th and 17th centuries.