Gut bacteria linked to cataclysmic epidemic that wiped out 16th-century Mexico
In the wake of the Spanish conquest of the Aztec Empire in 1521, waves of epidemics slammed Mexico. By 1576, the population, which had been more than 20 million before the Spanish arrived, had crashed to two million. One brutal outbreak in 1545 was estimated to have killed between five and 15 million alone—or up to 80 percent of the population.
But, like the other epidemics, the disease behind the 1545 outbreak was a complete mystery—until now.
Genetic evidence pulled from the teeth of 10 victims suggests that the particularly nasty bacterium Salmonella enterica subsp. enterica serovar Paratyphi C contributed to the scourge of fever, bleeding, dysentery, and red rashes recorded at the time. The genetic data, published Monday in Nature Ecology and Evolution, offers the first molecular evidence to try to explain what’s “regarded as one of the most devastating epidemics in New World history,” the authors conclude.
For decades, researchers have speculated on the disease—or diseases—that caused the population collapse. Spanish invaders are thought to have unleashed a throng of pathogens and plagues from the Old World, including small pox and typhoid. In addition, some experts think that severe drought during the time may have awoken some dormant, native plagues. But it’s been a hard issue to settle with so few surviving clues and vague historical accounts. The series of epidemics, including the one in 1545, are simply referred to as cocoliztli, the generic Aztec word for pestilence.
To add some data to the discussion, researchers led by Kirsten Bos and Johannes Krause of the Max Planck Institute for the Science of Human History developed a new metagenomics analysis tool, called MALT. This allowed researchers to compare the known genetic codes of reference bacteria and viruses with the deciphered code of DNA in shabby shape after being buried for a few centuries.
In their analysis, the researchers extracted preserved DNA from the pulp chamber of teeth from indigenous victims excavated at the site of Teposcolula-Yucundaa, located in the highland Mixteca Alta region of Oaxaca, Mexico. Previous radiocarbon dating had pegged 10 of 29 skeletons to be victims of the 1545 epidemic.
By comparing the DNA sequences from the teeth to a database of 6,247 complete bacterial genomes, the researchers picked out bits from S. enterica. The sequences from three tooth samples were good enough to narrow the search further to S. Paratyphi C. The researchers also used samples from non-epidemic victims to rule out background microbes that may have just been floating around.
Paratyphi C causes a typhoid-like illness with fever, gastrointestinal symptoms, and rash, similar to the historical symptoms of the cocoliztli. Earlier research had found evidence that the pathogen existed in Europe since 1200, suggesting it’s an Old World plague. But researchers don’t have enough data to say if the European settlers brought the epidemic strain. Today, it mostly strikes in developing countries and infected about 27 million worldwide in 2000.
The new study isn’t conclusive, of course. Just because the bacteria were present in people’s teeth doesn’t mean they’re what killed the individuals sampled. And even if those individuals were sick with S. Paratyphi C, it still can’t be ruled out that another pathogen or a combination of pathogens caused the cocoliztli. Also, the study may have missed potential viral culprits. The authors mainly focused on searching for possible bacterial and viral DNA, because those are the types of genetic code that are relatively easy to extract from ancient samples. But the study didn’t assess the possible presence of RNA-based viruses, such as hantaviruses, which some have speculated may have contributed to the epidemics.
Still, the authors conclude: “Our findings contribute to the debate concerning the causative agent of this epidemic at Teposcolula-Yucundaa, where we propose that S.Paratyphi C be considered.”