In 1545 disaster struck Mexico’s Aztec nation when people started coming down with high fevers, headaches, and bleeding from the eyes, mouth, and nose. Death generally followed in three or four days.
By 1550, 15 million people, 80 percent of the Aztec population, had been wiped out. For centuries, scientists have been struggling to understand just how such a deadly event could transpire, and how it could have arrived in Mexico.
The locals described the disease as “cocoliztli,” which in the Aztec Nahuatl language means pestilence. Using DNA evidence from the teeth of long-dead victims, scientists were able to instead conclude that the cause of the pestilence was likely a typhoid-like “enteric fever” caused by Salmonella enterica, specifically a subspecies known as Paratyphi C.
“The cause of this epidemic has been debated for over a century by historians and now we are able to provide direct evidence through the use of ancient DNA to contribute to a longstanding historical question.”
For 500 years, the cause of this epidemic has puzzled scientists.d to a second smallpox outbreak beginning in 1576.)
Seeking clues as to what exactly caused these outbreaks, a team led by Johannes Krause, an evolutionary geneticist at Germany’s Max Planck Institute for the Science of Human History, decided to look at the burial ground in the Oaxacan highlands of southern Mexico.
According to their findings, published on the pre-print site bioRxiv earlier this month, the researchers extracted and sequenced DNA fragments from the teeth of 29 bodies buried there, 24 of which were victims of the 1545-1550 outbreak.
“In the cities and large towns, big ditches were dug, and from morning to sunset, the priests did nothing else but carry the dead bodies and throw them into the ditches,” is how Franciscan historian Fray Juan de Torquemada is cited as chronicling the period.
Even at the time, physicians said the symptoms did not match those of better-known diseases such as measles and malaria.
Today, those of us who are unlucky enough to get salmonella (or technically salmonellosis) will probably get it from eating undercooked meat, poultry or eggs. It’ll make us sick for about a week, including stomach cramps, nausea, vomiting, diarrhea, chills, and fever. A nasty bug, yes—but not life-threatening.
But some strains of salmonella bacteria can cause serious illnesses, such as typhoid fever, and can even be deadly. One strain in particular, known as Paratyphi C, causes enteric fever, or fever in the intestines. When left untreated, the bug can kill up to 10 to 15 percent of those it infects.
Paratyphi C is now extremely rare, and mostly strikes people in developing countries, where sanitary conditions may be poor. According to new DNA research, however, an outbreak of this deadly form of salmonella may have contributed to the 16th-century downfall of the Aztecs.
When Europeans arrived in North America, they carried with them pathogens against which the continent’s native people had no immunity. And the effects could be devastating
The study is based on DNA analysis of teeth extracted from the remains of 24 Aztecs interred in a recently discovered cemetery in the Mixteca Alta region of Oaxaca, Mexico. The epidemic grave was found in the Grand Plaza of the Teposcolula-Yucundaa site.
The study’s search for known pathogens was extensive. Study co-author Alexander Herbig says, “One of the most important aspects is we didn’t need to make any assumptions.”
The team used a DNA-sequencing program called MALT to analyze the teeth. “We tested for all bacterial pathogens and DNA viruses for which genomic data is available,” says Herbig. Teeth from 10 of the bodies had traces of salmonella.
After separating bacterial DNA from human DNA, the scientists compared their results with more than 2,700 modern bacterial genomes. They found that the bacterial DNA recovered from several people matched the bacteria genus Salmonella, and were eventually able to reconstruct two genomes of the Paratyphi C strain of Salmonella enterica, one of two species of Salmonella.
Researchers suspect the Spanish brought the disease in tainted food or livestock because the teeth from five people who died prior to the Europeans’ arrival show no trace of it—this is not a huge sample, of course, so it’s difficult to be certain. Another team member, Kirsten Bos says, “We cannot say with certainty that S. enterica was the cause of the cocoliztli epidemic,” adding, “We do believe that it should be considered a strong candidate.”
Previous studies have suggested typhus, smallpox and measles as possible causes of the massive Aztec demise. In 2002, researchers at the National Autonomous University of Mexico (UNAM) in Mexico City proposed that a viral hemorrhagic fever, combined with drought, killed millions of Aztecs.
They compared its magnitude to the Black Death, which killed as many as 20 million people across 14th-century Europe. None of these prior hypotheses have been supported by DNA evidence, however, making the new study a particularly intriguing development.