After Spanish conquistadors arrived in Mexico in 1519, one of the worst epidemics in human history tore through the once-mighty Aztec civilization. Within a century of the Spaniards’ arrival, the Aztec population had been reduced from some 25 million to just 1 million. Though speculation has long swirled over what caused the devastating outbreaks of disease, no one knows for sure. Now, a team of scientists has presented the first DNA evidence of bacteria found in the bodies of victims killed in one of the worst outbreaks. They suggest the culprit may have been a species of Salmonella bacteria, specifically a now-rare strain known as Paratyphi C.
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.
In the decades following the arrival of Spanish conquistadors in 1519, the Aztecs suffered several devastating waves of disease. In their native language, Nahuatl, the Aztecs called such outbreaks cocoliztli, for the word “pestilence.” One major cocoliztli, between 1545 and 1550, is believed to have claimed the lives of more than 80 percent of the Aztec population. Another major outbreak hit in 1576, bringing the total death toll to between 7 million and 18 million.
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.
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.
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.
As ScienceAlert reported, the study is not yet peer-reviewed, and others in the field will presumably test the results and weigh in now that it has been published on the bioRxiv server. At least one scientist—María Ávila-Arcos, an evolutionary geneticist at UNAM who was not involved in the new study—remained skeptical that salmonella could cause so many deaths. She argued that a virus could be responsible for the outbreak, as the method the geneticists used wouldn’t have detected a virus.
Another, unrelated DNA study published on the same pre-print site last week could provide additional insight into the new study’s salmonella theory. That research, led by the microbiologist Mark Achtman from the University of Warwick in Coventry, United Kingdom, collected and sequenced a bacterial genome taken from the remains of a young woman buried in a cemetery in Trondheim, Norway in the year 1200. By providing the earliest known genetic evidence of Paratyphi C in Europe, more than 300 years before the conquest of Mexico, the study appears to support the possibility that Europeans could have brought the strain of salmonella bacteria with them to the New World, with devastating consequences for the Aztecs.