Some disease-ridden French soldiers have just received a new diagnosis -- although it comes about two centuries too late. While retreating from Moscow in 1812, French emperor Napoleon I lost most of his army to hunger, illness and the frigid Russian winter. Now, analysis of some of the fallen soldiers' DNA has uncovered two unsuspected diseases that plagued the troops.
The results reveal that the reasons for the army's demise might be more complex than was once thought, the authors report today in Current Biology. "This paper will be really interesting to history buffs," says Anne Stone, who researches anthropological genetics at Arizona State University in Tempe. "This study was done beautifully."
Historians had long debated the factors that led to the collapse of Napoleon's army. Eventually, in 2006, researchers studying DNA in some of the soldiers' remains identified two disease agents: Rickettsia prowazekii, which causes typhus, and Bartonella quintana, which causes trench fever. These two had already seemed likely culprits on the basis of symptoms described in historical records and the discovery of body lice, which spread typhus, on the soldiers' remains.
However, these identifications relied on the polymerase chain-reaction (PCR) technique, which creates millions of copies of a short DNA segment. That technique allows researchers to detect DNA sequences for specific pathogens, so it excelled at verifying the presence of microorganisms that historians had already thought were present in the remains. But it isn't as well suited for identifying unsuspected diseases.
After 20 years, it felt like the right time to return to the mystery, says Nicolás Rascovan, a co-author of the new study and a palaeogenomicist at the Pasteur Institute in Paris. Armed with new techniques that can screen ancient DNA for any known pathogen, the team set out to identify other microbes that might have afflicted the troops. The researchers extracted DNA from the teeth of 13 fallen soldiers whose remains had been excavated in Lithuania in 2002. They then used next-generation techniques to sequence millions of different DNA fragments from a sample. That allowed the authors to look for a wide array of pathogens all at once.
Of the 13 soldiers, four showed evidence of infection with Salmonella enterica, which causes paratyphoid fever -- symptoms of which include a loss of appetite and rose-coloured spots. And two soldiers hosted DNA from Borrelia recurrentis, a pathogen that is carried by lice and causes a disease called relapsing fever, which entails recurring episodes of a high fever.
The team found no traces of the diseases identified in 2006, possibly because those infections didn't spread to the small group of 13 soldiers. Or it could be that the previously reported pathogens sneaked by undetected, because the new sequencing techniques are less sensitive in some ways than the PCR method. Either way, the results show that the army's downfall wasn't brought about by "one single epidemic", says Rascovan. "These people died of a multiplicity of factors ... probably many more than the ones we've found."
When it comes to determining the rest of those factors, Rascovan thinks scientists would be wise to take another break from their investigations. Already, researchers have analyzed most of the dental samples from the Lithuania excavation site. Instead of using up any remaining samples now, "let's wait until we have more powerful techniques to do things that we cannot even imagine today", Rascovan says.