Plague origins could have originated centuries before outbreaks, research suggests

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In the largest DNA analysis of its kind, scientists have found evidence suggesting that historic plague pandemics, such as the Black Death, were not caused by newly evolved strains of bacteria, but that could have arisen centuries ago their outbreaks.

The plague-causing bacterium Yersinia pestis is said to have first appeared in humans 5,000 years ago. Through animals and trade routes, Y. pestis has spread around the world several times over time, according to a study published Thursday in the journal Communications Biology.

It caused the first plague pandemic in the sixth to eighth centuries and the second in the 14th to 19th centuries. The latest pandemic is believed to have started with the medieval outbreak of the Black Death, which is estimated to have killed more than half of Europe’s population. The bacteria also caused the third plague pandemic between the 19th and 20th centuries.

By collecting 601 Y. pestis genome sequences, including modern and ancient strains, researchers from Canada and Australia were able to calculate when the bacterial strains were likely to pose a threat. They divided the different strains of the plague bacteria and analyzed each strain population separately.

The tribe responsible for the Black Death, which the study says began in 1346, is estimated to have diverged from an ancestral tribe between 1214 and 1315 — as much as 132 years earlier.

The strain of Y. pestis associated with the first plague The pandemic was previously recorded as first appearing during the Plague of Justinian, which began in 541. However, the researchers estimate that the strain was already present between 272 and 465 – nearly 270 years before the outbreak.

“It shows that any major plague pandemic probably originated many decades to centuries earlier than what the historical records suggest,” study co-author and evolutionary geneticist Hendrik Poinar, director of the Ancient DNA Center at McMaster University in Canada, told Thursday via email to CNN.

He added that the bacteria showed up, caused small epidemics, and then “for reasons we don’t fully understand,” such as famine or war, “takes off.”

The study authors estimate that individually assessed third plague pandemic bacterial strains differed from an ancestral strain between 1806 and 1901, with highly localized plague cases beginning to appear in southern China between 1772 and 1880 and later diverging into several strains that spread globally. from Hong Kong between 1894 and 1901.

The study also found evidence to support recent academic research suggesting that the third and second plague pandemics were not mutually exclusive, but that the third was in part the continuation or end of the second. Despite the pandemics having their own diverse genetic lines that evolved differently, the third descended directly from the 14th century strain that caused the second.

Poinar called this finding significant because “it is taken into account that most of the history of this bacterium has been a Eurocentric view, so although the plague supposedly disappeared from Europe in the 18th (century), it continued to rage in the Ottoman Empire and throughout the century. Middle East and probably North Africa.”

But even with so many sequences of the plague bacteria, researchers couldn’t pinpoint the path of the plague’s global spread.

Many of the genetic samples come from Europe. For example, the emergence of the bacterium in Africa has meant that 90% of all modern plague cases occur 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 makes the appearance of Y. pestis in Africa to date.

Evolutionary geneticist Hendrik Poinar and a team of researchers studied more than 600 genome sequences of Y. pestis to predict the origin of a pandemic.

There is also much less surviving historical evidence of the second plague pandemic to help estimate its geographic origins 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 estimate 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 individuals believed to have died in 1338 in what is now Kyrgyzstan in Central Asia. It provided evidence that the Black Death came from a species that originated in the area near Lake Issyk-Kul in Kyrgyzstan in the early 14th century.

The latest study concluded that more ancient DNA will be needed to refine current estimates of the early events of the second pandemic.

Via email, Poinar described the species from Kyrgyzstan as “really fascinating” but said it “still hasn’t rooted. So I think 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 the plague bacterial strains “is with well-dated sequences, such as those from skeletal remains at Lake Issyk-Kul.”

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