Researchers find just two plague strains wiped out 30% -60% of Europe, Ars Technica

        

The Black Death ravaged medieval Western Europe, wiping out roughly one-third of the population. Now researchers have traced the genetic history of the bacterium believed to be behind the plague ina recent paperpublished in Nature Communications. They found that one strain seemed to be the ancestor of all the strains that came after it, indicating that thepandemic spreadfrom a single entry point into Europe from the East — specifically, a Russian town called Laishevo.

Technically, we’re talking about the (second) plague pandemic. The first, known as the Justinian Plague, broke out about 541 CE and quickly spread across Asia, North Africa, the Middle East, and Europe. (The Eastern Roman Emperor Justinian I, for whom the pandemic is named, actually survived the disease.) There continued to be outbreaks of the plague over the next 300 years, although the disease gradually became less virulent and died out. Or so it seemed.

In the Middle Ages, the Black Death burst onto the scene, with the first historically documented outbreak occurring in 1346 in the Lower Volga and Black Sea regions. That was just the beginning of the second pandemic. During the 1630 s, fresh outbreaks of plague killed half the populations of affected cities. Another bout of the plague significantly culled the population of France during an outbreak between 1647 and 1649, followed by an epidemic in London in the summer of 1665. The latter was so virulent that, by October, one in 10 Londoners had succumbed to the disease — over 60, 000 people. Similar numbers perished in an outbreak in Holland in the 1660 s. The pandemic had run its course by the early 19 th century, but a third plague pandemic hit China and India in the 1890 s; there are still occasional outbreaks today.

“The second plague pandemic has arguably caused the highest levels of mortality of the three recorded plague pandemics,”the authors wrotein Nature Communications. “It serves as a classic historical example of rapid infectious disease emergence, long-term local persistence, and eventual extinction for reasons that are currently not understood.” And that makes studies of thegenetic historyof the bacterium behind the plague of great interest to epidemiologists, since genetic factors could influence the emergence and spread of such deadly pandemics.

Medieval doctors believed the disease spread via “bad air,” or “miasmas.” Thereal culpritisa bacteriumcalledYersinia pestis. While visiting Hong Kong in 1894 to study a plague outbreak there, a French scientist namedAlexandre Yersin(who had studied under Louis Pasteur) extracted pus from a dead soldier’s swollen lymph node (bubo) and injected it into guinea pigs; all the guinea pigs died.

The bodies of a large number of dead rats around Hong Kong contained the same type of bacteria. Yersin concluded thatY. pestiswas the culprit for the spread of plague. (It wasdiscovered independentlyby a Japanese scientist named Shibasaburō Kitasato, but the microbe is named after Yersin.) Other experiments with plague-infected rats and fleas revealed that when an infected rat was introduced to a group of healthy rats, the healthy ones only became sick if fleas were present.Y. pestisproved to be so virulent that mice died after being infected with just three bacilli.

A variant strain ofY. pestiswas likely also theculprit behindthe Justinian plague, according to genetic analysis ofthe teethof two German victims and theremains of Justinian plague victimsin an ancient German burial site. It waslargely the samestrain as bacterial samplesgleaned fromLondon’s plague pits. Andrecent studiesof ancientY. pestisDNA reconstructed from plague victims in southern France, Barcelona, ​​London, and Oslo were identical.

      

  

For Maria Spyrou and her colleagues at the Max Planck Institute for the Science of Human History, this was clear evidence that a single strain spread throughout Europe during the Black Death. But where did it come from, and how specifically did it spread? Without genomic data from early outbreaks in Russia and only a limited number of published genomes forY . pestis, making a definitive conclusion is difficult.

So Spyrouet al. set out to expand the selection of genomes from various time periods and locations, the better to study the early stages of the pandemic, as well as the genetic diversity that appeared in Europe after the Black Death. They were able to reconstruct plague genomes from the teeth of 34 victims culled from a collection of 180 teeth from nine sites, including two from Laishevo, which is part of Russia’s Volga region. They also reanalyzed existing published data from the same time period.

A single strain did indeed prove to be the forerunner to all the strains from the second pandemic, although the team noted that earlier strains may yet be found in DNA samples from sites yet to be tested. Neither was there much genetic diversity between samples from victims during the Black Death, bolstering the view that Y. pestisfound its way to Europe via a single entry point.

OnceY. pestisfound a foothold in medieval Europe, it branched off intoclades. Analysis ofY. pestisgenomes from later in the second pandemic revealed two sister lineages. One appears to be responsible for the spread ofY. pestiseastward, since that lineage includes strains from 14 th-century Bergen op Zoom, London, and the city of Bolgar, as well as some strains from Africa. The second, post-Black Death lineage showed an unusual degree of genetic diversity within local pockets in Germany, Switzerland, England, and France. Since there appear to be no modern descendants of this lineage, the authors surmise that it went extinct.

DOI: Nature Communications, 2019.10. 1038 / s 41467 – 019 – 12154 – 0(About DOIs).