When many moderns think of the plague, the uproarious "Bring Out Your Dead" skit by Monty Python probably comes to mind. But in medieval Europe, the Second Plague Pandemic was no laughing matter: it was a particularly deadly series of disease outbreaks that began in the 14th century with the Black Death.

The Black Death, which reached Europe in October of 1347, killed upwards of 30% of European populations and was responsible for far-reaching social, economic, and political consequences that likely changed the course of history. Now, in a truly fascinating multi-disciplinary study, a pan-European group of researchers has attempted to clarify several outstanding questions related to the spread of plague throughout Europe in the 14th century.

In their article recently published in PNAS entitled "Integrative approach using Yersinia pestis genomes to revisit the historical landscape of plague during the Medieval Period," Amine Namouchi and colleagues use ancient DNA (aDNA) evidence corroborated with historical, archaeological, and ecological data to shed light on the origins of and epidemiology surrounding the Second Plague Pandemic.

While plague reservoirs exist in the wild on all continents except Australia and Western Europe, it remains unclear whether or not plague reservoirs ever existed in Western Europe. If such a reservoirs did exist, their presence would support one existing hypothesis which supposes that plague, after its initial introduction at the outset of the Black Death, became endemic to the region for a time and was responsible for multiple later outbreaks in Europe, and possibly for the Third Pandemic which struck China centuries later.

Competing hypothesis

A competing hypothesis holds that plague originated in wildlife reservoirs in Eastern Europe or Central Asia and was spread through human movement and trade routes by which it was repeatedly introduced to various European population centers. This second hypothesis rules out a Western European plague reservoir.

It is important to note that the Second Plague, in relation to the a DNA samples discussed here, is generally sub-classified into two more specific historical periods: The Black Death, which represented the initial disease outbreak in Europe and lasted from approximately 1346-1353, and the pestis secunda, which represented a second wave of outbreaks from the early 1360s.

On the other hand, the Y. pestis strain recovered from Abbadia San Salvatore in Italy, with two single nucleotide polymorphisms (SNPs) in its sequence represents a more recent genetic derivate from the other Black Death samples.

Historical evidence 

Historical evidence suggests this strain reached Italy via Mediterranean shipping through Pisa where it subsequently devastated the Italian countryside for a period of months before reaching the Abbadia San Salvatore in Siena. The authors suggest here that the long chain of overland transmission, possibly following a pilgrimage route, may be responsible for the additional SNPs, rather than the presence of a local reservoir or the possibility that they existed prior to arrival at Pisa.

While the researchers don't entirely rule out the possibility that a stable plague reservoir existed in Western Europe—probably in the Alpine region—they find it more likely that the plague originated in areas around the Black Sea and further east. The final two Y. pestis strains gathered in their study from Bergen op Zoom in the Netherlands support this.

These two strains show four additional SNPs compared with the Black Death strains previously discussed, however they are genetically very close to another, later strain from London of the pestis secunda period and a strain from the city of Bolgar in Russia.

The similarity between these plague samples suggests, as does historical and archaeological evidence, that plague was spread between these geographically distinct areas via a fur trading route that linked Russian cities such as Bolgar, Novgorod and others to Western Europe as well as to ports on the Black Sea, with further contact with the Mediterranean.

Ultimately, the study authors find that the balance of evidence suggests multiple successive and repeated introductions of plague to Western Europe via trade routes rather than the existence of a Western European plague reservoir. They conclude, "All things considered, the hypothesis that Y. pestis reached Europe through multiple introductions during the Middle Ages through different routes, including the fur trade, appears very plausible, at least during the Second Plague Pandemic"