In the summer of 1812, the legendary French general Napoleon Bonaparte led an army about half a million strong to invade Russia.

The Russians retreated but burned the countryside as they withdrew, using scorched-earth tactics that eventually left Napoleon’s troops occupying a ruined Moscow.

By autumn, his ill-fated troops began to leave for encampments along the Russian border, where an estimated 300,000 of them were eventually mowed down not by military might, but by illness coupled with extreme cold, starvation, and exhaustion.

Historical accounts of Napoleon’s campaign to invade Russia have focused the brunt of the blame on a louse-borne disease, typhus.

A new study of bacterial DNA preserved in the teeth of 13 soldiers adds two new diseases - paratyphoid fever and a relapsing fever - to the ailments that plagued one of the most disastrous episodes in military history.

Embedded in the teeth of long-dead soldiers, scientists found fragments of microbial DNA from Salmonella enterica, which is transmitted through contaminated food and water, and Borrelia recurrentis, carried by lice, according to research published at the weekend in the journal Current Biology.

Modern gene-sequencing methods have already rewritten our understanding of prehistoric migrations, human ancestry and major events in more recent history.

The new study from a team led by scientists at Institut Pasteur shows how studying the ancient DNA of microbes that shape individual human lives and the destiny of entire nations can add to historical narratives.

“There’s something perversely romantic about seeing typhus as the force that almost singlehandedly destroyed Napoleon’s army, as if it was a kind of judgment on his arrogance in attacking Russia, a natural enemy that even his brilliance couldn’t overcome,” Stephan Talty, author of The Illustrious Dead: The Terrifying Story of How Typhus Killed Napoleon’s Greatest Army wrote in an email.

“Though the sample size is small, the new evidence makes a strong case that there were other infections at work. It’s not really surprising that such a large army, covering so much ground in such terrible conditions, would suffer from other illnesses,” Talty added.

A close-up of a section of the pit during excavation. Photo / M. Signoli, Aix-Marseille University, EFS via The Washington Post

A mass grave

In 2001, construction workers at a former barracks for the Soviet Army in Vilnius, Lithuania, unearthed a mass grave filled with the remains of more than 3000 of Napoleon’s troops.

An excavation revealed the grim conditions of their hasty burial.

Horse skeletons were discovered alongside human bodies. Corpses appeared to have been thrown into the trench from the sides, with evidence that bodies in the middle had rolled down over others.

The positions of the skeletons suggested “the intense cold had frozen victims in the position of their death”, the research team reported in 2004. Bodies were buried with their boots on.

Historians have long reported that typhus outbreaks killed Napoleon’s troops.

Two decades ago, a team of scientists decided to see if they could find evidence for Rickettsia prowazekii, the bacteria that cause typhus, using technology that was then state-of-the-art.

When people are ill with a systemic infection that gets into their blood, remnants of DNA from microbes can be detected in the dental pulp, fed by a network of blood vessels, inside their teeth.

That team looked for and found DNA fragments from the typhus-causing bacteria, as well as Bartonella quintana, which causes trench fever.

Remi Barbieri, a postdoctoral researcher at Institut Pasteur, had the idea of returning to these old samples to sequence the whole genome of the typhus-causing bacteria using modern techniques. With Nicolas Rascovan, head of the microbial paleo genomics unit, he set out to study the evolution of a pathogen.

“We started like this, but we found other stuff than expected,” said Barbieri, who is now a postdoctoral researcher at the University of Tartu in Estonia.

View of the pit during excavation. Photo / M. Signoli/Aix-Marseille University, EFS via The Washington Post

Pathogens hiding in ancient teeth

Two decades ago, scientists would hunt for specific pathogens with a suspicion about what they expected to find, then search for fragments of that germ’s DNA.

With modern techniques, the researchers were able to sequence fragments of all the DNA in the teeth and match it to a database of all known microbes - an unbiased approach.

“It does definitely give you a sense of how much the field has changed,” said Anne Stone, an anthropological geneticist at Arizona State University not involved in the study.

“History buffs will be super interested. Technically, it’s very well done - with very challenging samples because the preservation is terrible.”

In four of the samples, they found matches toa version of Salmonella enterica that causes paratyphoid fever, an illnesses that causes rash, fever and a slew of gastrointestinal symptoms.

In one - and perhaps a second - they found evidence of Borrelia recurrentis, the pathogen that causes a lice-borne relapsing fever.

In a modern context, these diseases are not typically life-threatening, but Napoleon’s army was in dire straits.

“We are talking about an army that was in such a very fragile situation - a pathogen like this can really kill someone,” Rascovan said.

Barbieri and Rascovan say their study does not suggest typhus wasn’t present; it just shows that in the 13 individuals - out of more than 3000 in this one mass grave - there were other illnesses, too.

It adds to the portrait of misery that unfolded that winter.

And it makes sense, given some of the historical descriptions of gastrointestinal symptoms that could have been caused by contamination of food.

Their paper quotes an 1812 report from a physician in the army, J.R.L. de Kirckhoff, who described diarrhoea that he attributed to “large barrels of salted beets (buraki kwaszone), which we ate and drank the juice of when we were thirsty, greatly upsetting us and strongly irritating the intestinal tract”.

The broader field of microbial genetics has given scientists new insight into how diseases evolve.

In this case, the results are more of a genomic vignette that deepens understanding of an event that has long fascinated historians.

“This new study reinforces how impossible the whole enterprise was; in a time before railroads and antibiotics, the invasion was doomed before it even began,” Talty said.