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The medieval plagues: ecology, transmission modalities and routes of the infections

Final Report Summary - MEDPLAG (The medieval plagues: ecology, transmission modalities and routes of the infections.)

With MedPlag, we aimed to answer the open questions that remained after the first confirmation that Yersinia pestis, the causative agent of the Third Plague Pandemic, was also the cause of the two previous pandemics, the First (6th-7thcentury; Harbeck et al. 2013) and the Second Plague Pandemic (1346-19thcentury; Haensch et al. 2010). Our preeminent theory was that questions about the past plagues have arisen in the past few decades due to a lack of consideration of the ecological environment of Western Europe, which differs from that of Asia (in particular India and China), where most of the information about plague has been acquired at the beginning of the Third Plague Pandemic.
MedPlag explored the modalities of transmission of plague in medieval and early modern times along with the routes of dissemination of this infectious disease in Eurasia. By using high-throughput research methods for ancient DNA analyses, we investigated Y. pestis genetic variation and found signatures to trace the evolution of the bacterium on a phylogenetic tree. This phylogeny, interpreted on an historical and archaeological background, suggests that during the Second Pandemic, plague was reintroduced on multiple occasions into Western Europe from Central Asia (Namouchi et al. 2018, Guellil et al. 2, in prep.) and from a reservoir closer to Europe (Guellil et al.-1 in prep.). This finding seems to consistently support the theory proposed earlier (Bramanti et al. 2016) – also on the basis of a study on the Eurasian climate 14th-18thcentury (Schmid et al. 2015) – that no wild rodent reservoirs established in Europe during and after the Black Death. Focusing on Western Europe, we also reviewed scientific reports and narratives of the Third Pandemic. It was clear that the large number of cases and (small) outbreaks of this period, mainly found in port cities (Bramanti et al. under review), originated from outside Europe. This work provides also scenarios about how plague disappeared from Europe from the 1950s, due to improved hygiene and sanitation.
The role of human ectoparasites in plague transmission during the Second Pandemic was proven plausible by a modelling study (Dean et al. 2018), while the diffusion of human ectoparasites in Medieval times could be indirectly supported by the retrieval of Borrelia recurrentis, a louse-borne infectious disease, in a skeleton of the 15thcentury (Guellil et al. 2018). Human-to-human transmission has also been demonstrated to be the most plausible explanation for the outbreak occurred in Glasgow 1900 and accurately reported by the local sanitary controls (Dean et al. 2019). An additional study carried out in the frame of MedPlag deals with the genomic and anthropological investigation of a particular archaeological site, the lazaret of Imola, 1630 (Guellil et al. -3, in preparation). Based on a review of all anthropological studies carried out hitherto on plague burials (Bramanti et al. 2018), we could conclude that, apparently, plague did not exhibit a selective mortality towards victims on the basis of their sex, but of the age, and young adults were prevalent among the victims in the majority of the archaeological sites. Finally, a new study (Xu et al. under review) makes intensive use of newly developed modelling strategies to deeply revisit the history of the Third Pandemic worldwide. A work based on genetic engineering on a mutation possibly changing the virulence of Y. pestis is still ongoing (Derbise et al. planned).
An exhibition dedicated to the project MedPlag in 2018 (Ferrara, Italy), titled “(St) Roch and the plague: a natural history” has attracted the attention of a vast international audience.