Eighteenth century Yersinia pestis genomes reveal the long-term persistence of an historical plague focus

A bacterium called Yersina pestis is responsible for numerous human outbreaksof plague throughout history. It is carried by rats and other rodents and can spread to humans causing what we conventionally refer to as plague. The most notorious of these plague outbreaks– the Black Death – claimed millions of lives in Europe in the mid-14th century. Several other plague outbreaksemerged in Europe over the next 400 years. Then, there was a large gap before the plague re-emerged as threat in the 19th century and it continues to infect humans today, though on a smaller scale. Scientists have extensively studied Y. pestis to understand its origin and how it evolved to become such a deadly threat. These studies led to the assumption that the plague outbreaksof the 14–18th centuries likely originated in rodents in Asia and spread along trade routes to other parts of the world. However, it is not clear why the plague persisted in Europe for 400 years after the Black Death. Could the bacteria have gained a foothold in local rodents instead of being reintroduced from Asia each time? If it did, why did it then disappear for such a long period from the end of the 18th century? To help answer these questions, Bos, Herbig et al. sequenced the DNA of Y. pestis samples collected from the teeth of five individuals who died of plague during the last major European outbreakof plague in 1722 in Marseille, France. The DNA sequences of these bacterial samples were then compared with the DNA sequences of modern day Y. pestis and other historical samples of the bacteria. The results showed the bacteria in the Marseille outbreaklikely evolved from the strain that caused the Black Death back in the 14th century. The comparisons showed that the strain isolated from the teeth is not found today, and may be extinct. This suggests that a historical reservoir for plague existed somewhere, perhaps in Asia, or perhaps in Europe itself, and was able to cause outbreaksup until the 18th century.Bos, Herbig et al.’s findings may help researchers trying to control the current outbreaksof the plague in Madagascar and other places.