In addition to Russian troops, paratyphoid fever and relapsing fever were on the heels of Napoleon's Grande Armée during its retreat from Russia in 1812, according to a study by the Pasteur Institute.
To reach this conclusion, the researchers focused on the teeth of soldiers from the Grande Armée, found in a mass grave in Lithuania.
It is June 1812 when Napoleon sets out to conquer the Russian Empire, at the head of his Grande Armée, a monumental force of 600,000 men from all the provinces of the French Empire and the allied states.
Six months later, only a few tens of thousands of them reached Vilnius, Lithuania, at the end of an infamous retreat.
And it was in 2001, during an urban planning project in Vilnius, that a pit containing nearly 3,000 bodies, identified as soldiers of the Grande Armée, was unearthed during an excavation campaign led by Michel Signoli of Aix-Marseille University.
In 2006, the remains of these soldiers had already revealed some secrets through PCR tests, including the fact that typhus had affected some of them, as well as "trench fever", caused by the Bartonella Quintana bacterium.
"I knew there were 13 teeth left to analyze, each belonging to a different soldier," Rémi Barbieri, a postdoctoral researcher in the Microbial Paleogenomics Unit at the Pasteur Institute during the study, and first author of the study published Friday by the journal Current Biology, told AFP.
With the advancement of new genomic technologies, Rémi Barbieri had the idea of continuing the research, seeking to find out if other pathogens could have struck Napoleon's soldiers.
The PCR tests used in 2006 "only allow you to find what you are looking for. We were able to do exactly the opposite. We did not target anything at all, we used next-generation sequencing techniques applied to old DNA," he explains.
"When we analyze the dental pulp of a tooth, we are analyzing the equivalent of a drop of blood," he explains.
– “DNA Soup” –
"Each tooth provided approximately 20 million short fragments of DNA transcribed into a text file," Nicolás Rascovan, lead author of the study and head of the Microbial Paleogenomics Unit at the Pasteur Institute, told AFP.
These files contained both the DNA of the soldier in question, that of soil contamination, environmental bacteria, and possible pathogens: everything was there "like in a DNA soup", as Rémi Barbieri put it.
“Each short text was then compared to a database containing the genomes of all microbes sequenced to date,” explains Nicolás Rascovan.
However, the most delicate task was verifying the authenticity of these potentially disease-carrying DNA fragments. After multiple verification steps, they were able to retain only those fragments that could be unequivocally attributed to a pathogen.
And the definitive confirmation came from a phylogenetic placement analysis: "We constructed a phylogenetic tree, that is, a family tree of the species, and we tried to see, among all the branches, all the clades or lineages, where the genome of the strain that had infected the soldiers fell," explains Nicolás Rascovan. "In this way, we were even able to determine which type of strain had infected them."
Of the 13 soldiers analyzed, 4 were positive for the infectious agent Salmonella enterica Paratyphy C (responsible for paratyphoid fever) and 2 for the agent Borrelia recurrentis, responsible for relapsing fever.
This does not mean that these soldiers died solely from these pathologies.
“It’s a combination of cold, numerous infectious diseases, hunger, and fatigue that could explain their deaths,” Rémi Barbieri points out. “We also know, from historical sources, that the ranks of Napoleon’s army were already ravaged by epidemics even before the start of the Russian campaign. This is extremely interesting because it opens a small window into understanding this enormous health crisis,” the two researchers conclude.
English 
French