Plague bac­te­ria found in grave

An­cient ances­tor of the deadly dis­ease is dis­cov­ered in hu­man un­earthed in Swe­den.

Los Angeles Times - - CALIFORNIA - DEB­O­RAH NETBURN deb­o­rah.netburn @la­times.com

The an­cient ances­tor of a dis­ease that has killed mil­lions is found in a hu­man un­earthed in Swe­den.

In an an­cient grave in Swe­den, sci­en­tists have un­earthed the old­est known strain of deadly bac­te­ria that have killed mil­lions of peo­ple over thou­sands of years.

They call it Yersinia pestis. You may know it as the plague.

The new dis­cov­ery sug­gests that the mi­cro­scopic bac­te­ria have been wip­ing out great swaths of the hu­man pop­u­la­tion for more than 5,000 years — de­stroy­ing em­pires, spurring po­lit­i­cal up­ris­ings and leav­ing a per­ma­nent mark on re­gional gene pools.

“What we found in the Swedish grave site is not only the old­est sam­ple of the Y. pestis genome but also the old­est ver­sion of the genome,” said Si­mon Ras­mussen, a metage­nomics re­searcher at the Tech­ni­cal Univer­sity of Den­mark, who led the work. “Think of it as the root of the tree.”

The old­est recorded plague pan­demic, known as Jus­tinian’s Plague, dates to 541 AD. Over the course of 200 years, it killed more than 25 mil­lion peo­ple across the Byzan­tine Em­pire, hit­ting the cap­i­tal city of Con­stantino­ple es­pe­cially hard.

The next ma­jor plague pan­demic, known as the Black Death or the Great Plague, started in China in 1334 and spread along trade routes to Con­stantino­ple be­fore reach­ing Europe in the 1340s. It also claimed the lives of an es­ti­mated 25 mil­lion peo­ple, in­clud­ing about half the pop­u­la­tion of Europe, ac­cord­ing to re­searchers at the Cen­ters for Dis­ease Control and Preven­tion. Some towns weren’t left with enough sur­vivors to bury the dead.

The third ma­jor out­break, known as the Mod­ern Plague, took root in China in the 1860s. It popped up again in Hong Kong in 1894 and spread to port cities around the world over the next 20 years, car­ried by stow­away rats on steamships. It was dur­ing this pan­demic that sci­en­tists learned the bac­te­rial source of the dis­ease and that it is spread by fleas that pick it up from rats and pass it on to hu­mans. Even with that knowl­edge, how­ever, plague still man­aged to cause 10 mil­lion deaths.

Rat-as­so­ci­ated plague can still be found in pop­u­la­tions of ground squir­rels and other small mam­mals in the Amer­i­cas, Africa and Asia. It is now un­der control in most ur­ban ar­eas across the globe, and if it’s caught early enough, it can be treated with an­tibi­otics. From 2010 to 2015, there were 3,248 hu­man cases of plague re­ported world­wide and 584 deaths from the dis­ease, ac­cord­ing to the World Health Or­ga­ni­za­tion.

The new study, pub­lished this week in the jour­nal Cell, re­veals that the re­la­tion­ship be­tween hu­mans and plague goes back even fur­ther than sci­en­tists had real­ized. The bac­te­ria iden­ti­fied by Ras­mussen and his col­leagues may rep­re­sent a pre­vi­ously un­known out­break of plague that struck Europe as much as 5,700 years ago.

The re­searchers al­ready knew that the pop­u­la­tion of Europe plum­meted 5,000 to 6,000 years ago af­ter grow­ing for thou­sands of years. This sud­den plunge is known as the Ne­olithic de­cline, and its cause is still up for de­bate.

Ras­mussen and his col­leagues won­dered whether a plague pan­demic could have been re­spon­si­ble. “We were think­ing, ‘Where have we seen this drop be­fore?’ and that got us think­ing about the Black Plague,” he said.

The dis­ease was known to have ex­isted across Eura­sia at the dawn of the Bronze Age, which started around 4,500 years ago, but there was no ev­i­dence of its ex­is­tence ear­lier than that.

To see if the plague was in Europe at the end of the Ne­olithic pe­riod, the group turned to data­bases of DNA ex­tracted from an­cient hu­man re­mains — specif­i­cally, an­cient teeth.

Be­cause blood cir­cu­lates through the cen­ter of our teeth, Ras­mussen said, it is pos­si­ble to de­tect the DNA of pathogens that were present in a per­son’s blood­stream by ex­am­in­ing a tooth sam­ple.

“If you die from it and it’s in your blood,” he said, “then we can find it.”

Af­ter scan­ning for ge­netic se­quences re­sem­bling mod­ern-day Y. pestis, the group even­tu­ally found a match. It was in DNA ex­tracted from the tooth of a 20-year-old woman who died in west­ern Swe­den be­tween 5,040 and 4,867 years ago.

“This re­ally sur­prised us,” Ras­mussen said. “It was the old­est plague sam­ple ever found.”

Next, the authors com­pared the newly dis­cov­ered Y. pestis genome with 150 other plague sam­ples that spanned thou­sand of years, go­ing back all the way to the Bronze Age. This anal­y­sis re­vealed that the strain from the Swedish woman was closer to the ori­gin of Y. pestis than any other, and there­fore could in­form sci­en­tists about the first plague ances­tor from which all sub­se­quent strains evolved, Ras­mussen said.

How could this be? The Swedish woman lived in a small farm­ing com­mu­nity, far from the cen­ter of the Ne­olithic world. Plague thrives in en­vi­ron­ments where large groups of peo­ple live in close quar­ters, share space with an­i­mals and stored food, and con­tend with poor san­i­tary con­di­tions.

None of that ex­plains how this woman con­tracted the dis­ease.

So the re­searchers looked be­yond ge­net­ics and con­sid­ered the ar­chae­o­log­i­cal his­to­ries of hu­man pop­u­la­tions from the time pe­riod.

Al­though the young Swedish woman did not live in prime plague ter­ri­tory, there were other places in Europe where the dis­ease could have flour­ished in the Ne­olithic era, Ras­mussen said. These were the megaset­tle­ments of the Tryp­il­lia Cul­ture, built be­tween 6,100 and 5,400 years ago and lo­cated in present-day Ukraine, Ro­ma­nia and Moldova. The largest of these set­tle­ments was home to as many as 20,000 peo­ple.

Ar­chae­ol­o­gists have shown that these set­tle­ments were aban­doned and burned about once ev­ery 150 years. Usu­ally, sub­se­quent gen­er­a­tions re­built right on the ashes of the pre­vi­ously de­stroyed build­ings. Why the orig­i­nal struc­tures were burned has never been ex­plained.

Us­ing the new find­ings as a guide, the re­searchers pro­pose that the plague first evolved in these megaset­tle­ments, mor­ph­ing from a rel­a­tively be­nign stom­ach bug to a deadly killer about 5,700 years ago, around the time when the Swedish strain di­verged from all oth­ers then in ex­is­tence. This could ex­plain the pe­ri­odic burn­ing of the build­ings — per­haps they were set aflame to erad­i­cate the dis­ease.

The authors also sug­gest that the plague made its way from these set­tle­ments to the small Swedish farm­ing vil­lage thanks to a vast trade net­work that was made pos­si­ble by the re­cent ex­pan­sion of an­i­mal-pulled wag­ons. As the dis­ease spread along trade routes through­out the con­ti­nent, it could have caused the Ne­olithic de­cline.

AFP/Getty Im­ages

SCI­EN­TISTS have un­earthed the old­est known strain of Yersinia pestis, above, the bac­terium be­hind the plague. The find could point to a pre­vi­ously un­known out­break that struck Europe as long as 5,700 years ago.

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