University of Tartu study finds recurring waves of plague in Estonia
A research group led by the Institute of Genomics at the University of Tartu in Estonia has found that plague repeatedly entered the country from the end of the 14th century and returned over several centuries. ERR News reports on the findings.
Scientists examined genetic material from the bacterium Yersinia pestis in the remains of people who lived between the 14th and 18th centuries. They reconstructed 26 genomes of the plague pathogen from samples found at 11 archaeological sites in Estonia, Russia, England, the Netherlands and Switzerland.
Genetic lineages of the pathogen
The analysis showed that plague came to Estonia not just once but repeatedly throughout nearly the entire second plague pandemic, which began with the Black Death of 1347–1353. Researchers also identified previously unknown genetic lineages of Yersinia pestis in urban and rural areas of Estonia.
Genetically nearly identical bacterial strains in different parts of Europe indicate the rapid spread of epidemics through contacts between regions. Around 1450–1500, the bacterium underwent genetic diversification and split into three major branches, which formed new natural reservoirs of infection among wild rodents.
The authors suggest that climate change may have influenced this process. They name the Great Renaissance Drought of 1540 as one possible factor. According to modern research, climate significantly affects the spread of plague among rodents, which are natural reservoirs of the pathogen.
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More precise dating of outbreaks
The team developed a method combining genetic and archaeological chronological information. Using the evolutionary tree of plague genomes, the researchers were able to determine the age of many samples more precisely and link them to specific historical outbreaks.
In addition to the new genomes, the scientists analyzed 64 previously published bacterial genomes. This was the first systematic attempt to compare most known genomes from the 14th–18th centuries with plague outbreaks recorded in written sources.
Wars and the spread of plague
The study includes new genomes linked to the Thirty Years' War of 1618–1648 and epidemics during the Great Northern War around 1700–1721. Genetic data indicate that plague often spread during wars and military campaigns, moving along the routes of troops, refugees and trade.
During the siege of Tallinn in 1710, plague claimed the lives of Swedish and Russian soldiers as well as civilians. The study's results were published in the Proceedings of the National Academy of Sciences.
