Contribution of volcanism to the initiation of plague pandemics

The Yersinia pestis bacterium is responsible for the three major plague pandemics of the Common Era, cumulatively responsible for hundreds of millions of deaths and huge societal impacts. Factors influencing the initiation of plague pandemics are still controversial amongst scientists and historians. However recent advances in DNA sequencing and recovery, and ecological and epidemiological modelling, have provided new information regarding the evolution of Y. pestis and the potential influence of climate upon pandemic episodes respectively. Rapid bursts in genetic fixation rates and evolutionary divergences were identified preceding the initiation of the Black Death and Third Pandemic, and climatic factors are now accepted as a key influence upon plague prevalence over an annual to centennial timescale. By reassessing climatic perturbations prior to major pandemics and utilising recent satellite data this thesis suggests a mechanistic pathway linking explosive high sulphur and halogen eruptions within periods of sustained elevated global volcanic forcing, to evolutionary and epidemiological progression of Y. pestis. The periods of global volcanic forcing coincident with pandemic episodes are shown to do the following: (1) increase levels of a known mutagenic element in an area proposed as a source of diversity within the Y. pestis genome through a novel and previously unexplored mechanism and, (2) contribute to regional climatic shifts fully coincident with the timing and distribution of plague immediately preceding pandemic initiation. The volcanic influence suggested not only represents an alternate hypothesis for the genetic expansion and subsequent pandemic impact of plague but also illustrates a previously unexplored risk following large explosive volcanic eruptions.