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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