Black Death May Have Spread Via Human Fleas & Lice, Not Rats

Plague arrived in Europe by sea via Genoese trading ships that docked at the Sicilian port of Messina after traveling through the Black Sea. Since then, the question of how it spread during the Black Death (1346-1353) and throughout the Second Pandemic (13th-19th CE) has remained a controversial subject.

Now, findings from a modeling study suggest that human fleas and body lice—not rodents—may have been the cause of Europe’s Second Pandemic.

Caused by Yersinia pestis, plague is a disease that affects humans and other mammals, and it can be spread throughout human populations via several pathways. One of the most commonly recognized routes of transmission to humans is through the bites of rodent fleas that are carrying the bacterium, or via infected animals, according to the Centers for Disease Control and Prevention (CDC); this is commonly referred to as bubonic plague. Plague can also spread from human-to-human via infectious droplets from a pneumonic infection, according to study authors; when this happens, it’s considered pneumonic plague.

“Modern plague transmission is well-documented for the Third Pandemic (beginning in the mid-19th CE),” the authors write. “However, there is controversy over how plague spread during the Black Death, and throughout the Second Pandemic, due to the relatively high mortality and rapid geographic spread in Europe as compared to India.”

Previous studies have suggested that the mode of transmission for the Second Pandemic was either via rodents or direct human-to-human transmission via ectoparasites, such as human fleas and body lice, according to a press release on the study.

In hopes of getting some answers, the investigators created 3 “susceptible-infected-recovered” (SIR) models to test different proposed transmission mechanisms: pneumonic plague, bubonic plague with a human ectoparasite vector, and bubonic plague with rats and fleas using data taken from historical records. They used the 3 models to compare epidemics with known pneumonic or rat transmission that occurred during the Third Pandemic with 6 epidemics in Europe with unknown modes of transmission that occurred during the Second Pandemic.

"We fitted the deterministic models in a Bayesian framework to the observed data by estimating the transmission parameters, and the initial number of susceptible and infected hosts, using Markov chain Monte Carlo (MCMC) in PyMC2," the authors write. "We assigned the best fitting model for each epidemic using the deviance information criterion and we estimated R0 using the next-generation matrix method."

Their findings? In 7 out of the 9 regions that the team studied, the bubonic plague with a human ectoparasite vector model “better reflected mortality patterns,” than the other 2 models. This means that contrary to previous suggestions, human fleas and body lice may have been the main mode of transmission during the Second Pandemic and not bubonic plague with rats and fleas as has been long speculated.

The authors note that there has been indirect evidence in the past indicating the role that body lice could have when it comes to plague spread. These types of evidence include various experimental studies, the co-detection of plague and a louse-borne disease in a skeleton from medieval France, and the collection of infected lice during epidemics, among others.

“Our results support the hypothesis of an alternate mode of plague transmission by a human ectoparasite vector in Europe during the Second Pandemic,” the authors write. “This work supports the growing body of evidence that plague was not spread primarily by rats in Europe, and therefore, has epidemiological characteristics different from modern plague epidemics.”