Human ectoparasites and the spread of plague in Europe during the Second Pandemic

Area of Science:

• Epidemiology
• Historical disease transmission
• Mathematical modeling

Background:

• The Second Pandemic (14th-19th centuries) caused devastating plague epidemics in Europe, including the Black Death.
• The primary transmission routes of historical plague, particularly during major epidemics, remain debated.
• Current understanding often implicates rodent-borne fleas or pneumonic spread, but evidence for human ectoparasites is less clear.

Purpose of the Study:

• To investigate the role of human ectoparasites as plague vectors during Europe's Second Pandemic.
• To compare the fit of an ectoparasite transmission model against other plague spread models using historical mortality data.

Main Methods:

• Development of a compartmental model simulating plague transmission via human ectoparasites (fleas, lice).
• Application of Bayesian inference to fit the model to mortality data from nine distinct European plague outbreaks.
• Comparison of the ectoparasite model's performance against models representing pneumonic or rodent-borne transmission.

Main Results:

• The human ectoparasite transmission model provided a superior fit to historical plague mortality curves compared to pneumonic or rodent-borne models.
• The findings suggest that human fleas and body lice were significant vectors during major plague outbreaks.
• This provides quantitative support for the hypothesis of ectoparasite-driven plague epidemics.

Conclusions:

• Human ectoparasites, not primarily rats, were likely the main vectors responsible for the rapid spread and high mortality of plague during the Second Pandemic in Europe.
• The study revises the understanding of historical plague transmission dynamics, emphasizing the role of human-associated vectors.
• This research highlights the importance of considering human ectoparasites in the epidemiology of historical infectious diseases.