Plague transmission may have begun a century before the Black Death, study shows

Evidence from 13th-century chroniclers and physicians indicates plague may have been involved in epidemics a century before the Black Death, a new study shows.

Yersinia pestis, the bacterium that causes plague, was likely brought to the Middle East close to a hundred years before it caused the mass mortality events from 1346 to 1351 that we know as the Black Death.

Some experts have previously believed that plague focalised in western Eurasia only in the decades following the Black Death. Yersinia pestis quietly embedded itself into the local landscape of insects and rodents, and gradually — under the stress of climatic and environmental changes—erupted into the sequence of human outbreaks from 1346 onwards.

Series of epidemics

The new study, by historians of medicine Dr Monica H. Green, an independent scholar, and Professor Nahyan Fancy, from the University of Exeter, builds on earlier work which documented the likely role of plague in a series of epidemics in western Asia in the late 1250s. The researchers used over a dozen pre-1348 chronicles, religious and medical texts from the region.

In a previous study they showed how multiple contemporary observers were suggesting the new presence of a plague-like disease in western Asia in the second half of the thirteenth century, nearly a century before historians have hitherto assumed it arrived.

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In this new study they show how close examination of the symptoms described suggested that this was indeed the disease now known to be caused by the bacterium Yersinia pestis, whose evolutionary history is in the process of being newly outlined by geneticists. Some writers even used the specific Arabic term for plague, which has the distinctive characteristic of causing inflamed and tender swellings of the lymph nodes known as “buboes”.

Scenes of decay

Professor Fancy said: “In the thirteenth century, of course, there were no microscopes and no means to conceive of bacteria as the cause of disease. Rather, contemporary observers believed that epidemics were caused by “miasms,” airborne effluvia that could arise from scenes of decay, like the sites of great battles. Observers linked outbreaks of plague in Syria, Iraq and Egypt up through 1260 to the Mongol conquest of Baghdad in 1258, where epidemic disease was reported even among the Mongol troops.”

The new study gives further evidence for the arrival of plague in Western Iran, Iraq and Egypt in the 1250s, but also a framework for how medical historians can fruitfully engage with historical sources and modern genetics to reconstruct how the plague bacterium established itself in new environs for a century before the unprecedented crescendo of plague activity throughout the region and beyond in the 1340s.

Pestilence and fleas

Professor Fancy said: “It’s important to examine carefully how pathogenic organisms spread. Plague is primarily a rodent disease which is reported in historical (and often even contemporary) sources only when it spills over into humans. Moreover, in order to spillover, it needs vectors like fleas or lice. A prolonged and widespread human outbreak of plague, a pandemic like the Black Death and the hundreds of years of plague waves that followed it, can only have happened after an earlier period in which the bacterium had been able to find new suitable host animal species and insect vectors. This is what we are calling the ‘prodrome’ of the pandemic, in which the bacterium is present in the region but a full-blown pandemic has not started raging yet.”

Dr Green explained: “We know now that the strains of Yersinia pestis that reached Europe in the fourteenth century are most closely related to those still found in Central Asian marmots. That core biological narrative prompts us to look for an importation mechanism. The troops’ grain supplies in the 1250s could explain that. But what happened next?

“Because humans in the Middle Ages had no concept of microbes, we will never find bacteria mentioned in our written sources. But when a scribe reports ‘pestilence’ and an infestation of fleas in his region more than 25 years before the Black Death, we need to pay attention.”