Modeling plague transmission in Medieval European cities

The Black Death pandemic swept through Europe during the Middle Ages leading to high mortality from plague, caused by the bacterium Yersinia pestis. How the disease was transmitted in Europe is not fully elucidated, and prior to the identification of the bacterium in medieval tooth samples, the nature of the pandemic led to speculation that the Black Death was not the same disease as current-day plague. In the classical mode of transmission to humans, black rats act as an intermediate host and the disease spreads by infected rat fleas, but alternate modes have been proposed, either through pneumonic transmission of plague, or through an intermediate human ectoparasite vector, such as the human body louse. To understand the transmission dynamics within cities, we used a spatial metapopulation model with SIR-dynamics for three transmission scenarios and compared how the epidemic curve, epidemic duration, and total mortality differ between each mode and historical data. Here we show that 1) a model of louse-borne transmission of bubonic plague fits the pattern of plague transmission within cities during the Black Death with regards to epidemic duration and the distribution of deaths during an epidemic, and that 2) primary pneumonic plague can produce large scale epidemics, but only under conditions highly favorable for this mode of transmission. These results demonstrate that the louse-borne transmission of bubonic plague is a viable alternative to resolve the inconsistencies between plague during the Black Death and plague with rats. We anticipate that the models and parameters we have presented can be used in future work for more complex models that combine multiple transmission routes. For example, a model with both primary pneumonic and bubonic plague transmission during the same epidemic. Furthermore, the models can be adapted to explore the impact of immunity, public health measures, and seasonality on the disease dynamics.