Gerbils and the Black Death
Few pandemics in the history of humanity inspire as much interest and research as the so-called ‘Black Death’. This outbreak of bubonic plague (caused by the bacterium Yersinia pestis) killed an estimated 50 million people in Europe between 1346-53, and it widely thought to have been spread by fleas feasting on black rats (Rattus rattus). Traditional descriptions of the outbreak focus on a single introduction of bubonic plague to Europe around 1346 but was this really the case? A new paper has recently cast doubt on this explanation and suggests multiple reintroduction of plague to Europe over the following four centuries.
Today bubonic plague is still endemic in many parts of the world. In North America it is carried by prairie dogs and in Asia several rodent species can occasionally infect humans with plague. These animals are described as ‘reservoirs’ for the bacteria and are essential to explaining how the disease survives outside of its human hosts. In modern Europe however, there is no existing reservoir for plague. This is strange given that after the initial outbreak of the Black Death there were periodic recurrences over the next 400 years, suggesting an endemic disease. In this scenario, outbreaks are caused when the population of the reservoir species increases significantly and brings them into greater contact with humans. However, evidence now suggests that it wasn’t a European reservoir which caused these outbreaks, but an Asian one instead.
Evidence for this alternative explanation comes mainly from climate data. By examining a database of 7,711 historic outbreaks of plague and comparing them to 12 annually resolved tree ring records from Europe and Asia, it was possible to determine whether any outbreaks were linked to changes in the climate. What the researchers found was that although there were peaks of plague associated with fluctuations in climate the correlation wasn’t what you would expect. Instead of being associated with conditions which would benefit rat populations in Europe they discovered that it was conditions facilitating an increase in rodent populations in Asia which had the greatest effect, albeit with a fifteen year lag. They concluded that this supported a scenario whereby booms of rodent populations in Asia triggered fresh outbreaks in Europe by first infecting traders and their animals along the Silk Road routes. These traders then carried the disease to Europe where it spread to the local rats and from there to their human neighbours.
Reference: Schmid, B.V., et al. 2015. Climate-driven introduction of the Black Death and successive plague reintroductions into Europe. Proceedings of the National Academy of Sciences.
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