The Plague Likely Arrived in Europe During the Stone Age

Plague, caused by the bacterium Yersinia pestis, was responsible for three major pandemics: the Plague of Justinian (6th–8th century), the Black Death of the 14th century with recurrent outbreaks until the 18th century, and a 19th century outbreak that spread worldwide and became endemic in several regions, including Madagascar.

The early spread and persistence of plague has largely been a mystery that new research appears to have just solved. A paper published in the journal Current Biology reports that migrants entering Europe starting around 4,800 years ago likely brought plague with them from the Great Steppe, which is a vast strip of land stretching from the Ukraine to Mongolia.

The disease's origins may even be as old as dirt.

"Yersinia pestis is actually a specific mutant lineage of the bacterium Y. pseudotuberculosis, which is found in soil," co-author Alexander Herbig of the Max Planck Institute for the Science of Human History told Seeker. "Y. pseudotuberculosis can cause gastrointestinal infections that are only very rarely lethal."

Rodents are often blamed for the spread of Y. pestis, but it is a little-known fact that over 200 species can catch and harbor the disease. Fleas may become infected by feeding on such animals and can then transmit the bacteria to others when they feed again.

In its pneumonic form, such as what is now observed in Madagascar, the plague is only transmitted from human to human, however. "This form is aggressive," Herbig said, "and if untreated, it kills infected individuals very quickly."

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According to the WHO, the plague death toll for the present epidemic in Madagascar is now close to 200 and rising.

During the Black Death, plague killed an estimated 25 million people in Europe alone. Since the pandemic coincided with the migration out of the Great Steppe, Herbig and his colleagues investigated the remains of people from that region dating from the Late Neolithic to the Bronze Age (4,800–3,700 years ago).

A total of 563 tooth and bone samples dating from the period were genetically screened for Y. pestis. The samples came from individuals from Russia (122), Hungary and Croatia (139), Lithuania (220), Estonia (45), Latvia (10) and Germany (220).

Lead author Aida Andrades Valtueña, Herbig, and their team recovered full Y. pestis genomes from six of the studied individuals. They determined that all of the genomes were fairly closely related.

"This suggests that the plague either entered Europe multiple times during this period from the same reservoir or entered once in the Stone Age and remained there," Andrades Valtueña said.

Herbig added that when plague was introduced to Europe, it likely "established a local reservoir before moving back towards Central Eurasia." The recovered genomes show that changes were occurring during this period in genes related to plague virulence. It is possible that the pathogen was capable of causing large-scale epidemics before it developed these traits, however.

If so, the migrations might have occurred as people tried to escape areas where people were sick and dying from the disease. This could help to explain why early farmers seemed often to be on the move. Other recent genetic research shows these individuals frequently wound up settling in various regions within Europe, where they sometimes mated with locals.

"Living close to domestic animals might have facilitated the spread of new diseases during that time," Herbig said. "Furthermore, agriculture and food storage could have attracted wild living rodents, increasing the likelihood of zoonotic infections. The advent of agriculture and domestication very likely had a great influence on the human disease landscape."

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The genetic evidence reveals that certain factors necessary for highly efficient flea-based disease transmission were missing in the Y. pestis genomes. This opens up many possibilities.

One is that the migrants were traveling with livestock animals, dogs, or other species that harbored Y. pestis. Another scenario is the migrants themselves harbored the disease and unwittingly passed it to others whom they met on their travels. This could have been directly, through human-to-human contact, or via fleas that they themselves carried on their bodies.

"Future research will have to show if the flea-based transmission was simply less efficient, or if the disease was transmitted in a completely different way," Herbig said.

Understanding the past of this disease could help to inform the present. A plague vaccine is still not commercially available, according to the US Centers for Disease Control and Prevention, but several are in development now. Many classes of antibiotics are effective in treating most strains of the plague, but there is growing concern over strains that are antibiotic resistant.

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