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In the largest DNA analysis of its kind, scientists have found evidence to suggest that historical plague pandemics, such as the Black Death, were not caused by newly evolved strains of bacteria, but those that might have appeared up to centuries before their outbreaks.
The bacteria responsible for plague, Yersinia pestis, first appeared in humans around 5000 years ago. According to a study published Thursday in the journal Communications Biology, Y. pestis has spread globally over time multiple times through animals and trade routes.
It caused the first plague pandemic from the 6th to the 8th century and the second from the 14th to the 19th century. This latest pandemic is believed to have started with the medieval outbreak of the Black Death, which is believed to have killed more than half of Europe’s population. The bacterium also caused the third plague pandemic between the 19th and 20th centuries.
By gathering 601 genomic sequences of Y. pestis, including modern and ancient strains, researchers from Canada and Australia were able to calculate when the bacterial strains likely emerged as a threat. They split the different strains of the plague bacteria and analyzed each strain population individually.
The strain responsible for the Black Death, which the study says began in 1346, was recently estimated to have diverged from an ancestral strain between 1214 and 1315 – up to 132 years earlier.
The strain of Y. pestis associated with the first plague The pandemic has already been recorded as first appearing during the Plague of Justinian, which began in 541. However, researchers have estimated that the strain was already present between 272 and 465 – up to almost 270 years before the epidemic.
“This shows that every major plague pandemic likely arose decades or even centuries earlier than historical records suggest,” study co-author and evolutionary geneticist Hendrik told CNN on Thursday via email. Poinar, director of the Center for Ancient DNA at McMaster University in Canada.
He added that the bacteria appeared, created small epidemics, then “for reasons that we don’t quite understand”, such as famine or war, “it takes off”.
The study authors estimated that the individually assessed bacterial strains of the third plague pandemic diverged from an ancestral strain between 1806 and 1901, with highly localized cases of plague beginning to appear in southern China between 1772. and 1880 and later diverging into various strains that spread globally. Hong Kong between 1894 and 1901.
The study also found evidence supporting recent academic research suggesting that the third and second plague pandemics were not mutually exclusive events, but that the third was partly a continuation or end of the second. Although the pandemics have their own diverse genetic lines that evolved differently, the third descends directly from the 14th century strain that caused the second.
Poinar called this discovery important because “it takes into account that most of the history of this bacterium has been a Eurocentric view, whereas the plague supposedly disappeared from Europe in the 18th (century), it continued to rage in the Ottoman Empire and throughout the Middle East and probably North Africa.
However, even with so many sequences of the plague bacteria, the researchers were unable to determine the path of the global spread of the plague.
Many genetic samples come from Europe. For example, the emergence of the bacterium in Africa led to 90% of all modern plague cases on the continent, but there are no ancient sequences from the region, which is represented by only 1.5% of all genome samples – making it difficult to date for Y. pestis to appear in Africa.
There is also much less historical evidence of the second plague pandemic to help estimate its geographic origins compared to the third, with the earliest textual evidence of the pandemic in Europe coming from the Black Death in 1346, the study authors said. The researchers believed that the second pandemic originated in Russia.
A study published in the journal Nature in June used DNA analysis to find the plague bacteria in three people who died in 1338 in what is now Kyrgyzstan in Central Asia. It provided evidence that the Black Death originated from a strain that originated in the region near Lake Issyk-Kul in Kyrgyzstan in the early 14th century.
The latest study concluded that older DNA will be needed to refine current estimates of the early events of the second pandemic.
Over email, Poinar described the Kyrgyzstan strain as “truly fascinating” but said it “still doesn’t sit at the root. So I guess we’re still looking for something 20-50 years earlier.
He and the other authors noted that the only way to accurately estimate the evolution of strains of the plague bacterium “is with well-dated sequences, such as those from skeletal remains from Lake Issyk-Kul.”