Plague bacteria may have persisted in Europe from the 14th to 17th century in an unknown reservoir, according to new genetic research on victims of the deadly disease.
The bacteria, Yersinia pestis, played a major role in two historical pandemics, which killed millions of people.
The first was the so-called Justinian plague, named after the Byzantinian emperor Justinian I, and occurred from the 6th to 8th century.
The second pandemic spread between the 14th and the 17th century. In Europe it peaked in the years 1346–53, a period known as the "Black Death," in which almost one third of the entire population was wiped out.
Although molecular DNA analysis of archaeological skeletal material have confirmed the role of Y. pestis in both pandemics, researchers still don't know how the pandemics could have continued for several hundred years.
According to one theory, the bacteria may have been continuously reintroduced from central Asia to Europe in several waves along major trade routes, such as the Silk Road. Another hypothesis assumes Y. pestis existed in Europe for centuries in a yet unidentified host.
A team of German researchers investigated both hypothesis by analyzing 30 victims of the second plague pandemic. The remains were excavated from two different burial sites in Germany, and covered a time span of roughly 300 years.
If the bacteria were reintroduced from central Asia in multiple waves, "various different genotypes, reflecting the natural genetic diversity of Asian Y. pestis strains, should be detectable among different plague victims," Lisa Seifert, from Ludwig Maximilian University of Munich, and colleagues argued in the open-access journal PLOS ONE.
On the contrary, if there were unknown and persistent reservoirs, identical or very similar genotypes should be present in plague victims from different time periods of both pandemics.
Among the 300–600 year-old human remains, eight people were positive for Y. pestis-specific nucleic acid.
The genetic material from victims testing positive was highly similar to previously investigated plague victims from other European countries and had the identical Y. pestis genotype.
"This is why we assume that a certain genotype may have persisted for some 300 years in Europe," Holger Scholz, at the Bundeswehr Institute of Microbiology in Munich, Germany, told Discovery News.
Besides the assumed, continuous reintroduction of Y.pestis from central Asia in multiple waves during the second pandemic, it's possible that the bacteria lasted such a long time in Europe in a yet unknown host, the researchers wrote.
"Rodents, such as rats and others may be an option. Maybe we should look for Y. pestis in Middle Age rodents. But it will not be an easy task," Scholz said.
According to Michel Drancourt, professor of microbiology at Marseille Medical School, several complete genome sequences from various graves and various periods of time might be needed. Drancourt, who back in 2004 first solved the etiology of the Justinian plague, was not involved in the study.
"The question of whether or not plague established persistent reservoirs in Europe is very interesting and in fact, absolutely still unresolved," Drancourt, told Discovery News. "This study suggests, but not prove, that there were indeed some reservoirs in Medieval Europe."
"We will have to understand in one further step what were these reservoirs and the factors which contributed to their disappearance as plague vanished from Europe for exactly one century, the last cases recorded around 1920," he added.