A new study by a pan-European group of scientists sheds new light on how the various strains of the second plague pandemic spread across the continent in the 14th century.
The Second Plague Pandemic in Europe started with the arrival of Black Death in October 1347. This lasted until 1353, and was followed by a second wave of outbreaks, dubbed pestis secunda, which started in early 1360s and lasted for next several years.
The second pandemic alone killed more than 30 per cent of the population of Europe.
Plague is zoonotic disease, that is to say, a disease that can be transmitted between animals and humans. It is caused by a bacterial strain called Yersinia pestis (Y. pestis) and spreads to humans from fleas hosted by rabbits, hares, rodents, ferrets, goats, sheep, and camels.
In the current study, published in PNAS, the Proceedings of the of the National Academy of Sciences in the US, scientists analysed ancient DNA (aDNA) taking into account historical, ecological and archaeological data to determine the origin of the Second Plague Pandemic.
The team sequenced sections from five ancient plague genome samples dating from the 14th century, recovered from four different archaeological sites in Western Europe.
Researchers applied a combination of computational genomics techniques and bioinformatics tools to the data collected and created a phylogenetic tree, a branching diagram demonstrating the evolutionary relationships between related biological entities, which enabled them to generate a graphical representation of the evolutionary history of bacterium Y. pestis.
They also used gene sequencing data of adjacent historical aDNA samples recovered from Second Plague sites in London, Barcelona, and Russia.
The sequencing results, analysed in relation to the archaeological and historical evidence, suggested that two Y. pestis strains collected from Oslo and St. Laurent de la Cabrerisse sites were identical to two clones from Barcelona and London from the time of the Black Death, suggesting a common origin for these strains.
A Y. pestis strain recovered from Abbadia San Salvatore in Siena, Italy was found to have two single nucleotide polymorphisms in its sequence, suggesting a more recent genetic derivate from the other Black Death samples.
Based on historical evidence, scientists believe it is likely that this strain arrived in Italy via Mediterranean shipping through Pisa. After devastating the countryside for some months, it finally reached the Abbadia San Salvatore in Siena.
The results also indicate that Western Europe most likely received repeated introductions of plague via trade routes, rather than hosting a 'plague reservoir', an area that naturally hosts Y. pestis among the animal population.
The study concludes that "the hypothesis that Y. pestis reached Europe through multiple introductions during the Middle Ages through different routes, including the fur trade, appears very plausible, at least during the Second Plague Pandemic."
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