Secret weapon of the conquistadores

After the Spanish invaded Mexico in the 1500s, a series of epidemics gripped the native populace leading to the decline of Aztec society. Within 100 years of Spanish conquest, the population of natives had dropped form 25 to one million people. The Aztec referred to these epidemics as cocoliztli, meaning pestilence, the largest of which wiped out between 7 and 18 million.

The cause of these epidemics was unknown and had previously been attributed to measles, smallpox, typhus, and viral hemorrhagic fever. Researchers now speculate that a deadly strain of salmonella brought over by the Europeans is what actually killed the native Aztecs as evidenced by DNA recovered from stomach bacteria of individuals who died during the time of one of the epidemics.

Researchers led by Johannes Krause, at the Max Planck Institute for the Science of Human History in Germany, used DNA extracted from teeth of skeletons uncovered in the highlands of southern Mexico, where ancient Aztec society had been established. They dated 24 of the 29 individuals to the cocoliztli. Bacterial DNA from several of these individuals was compared to bacterial databases and matched with Salmonella.

By piecing together fragments of DNA, they were able to reconstruct the genome of Salmonella enterica, the Paratyphi C strain that causes a typhus-like illness with enteric fever. Today, this strain kills 10-15 percent of those infected. They hypothesized that this strain was brought over by the Spaniards, who had built up resistance to it and therefore were able to carry the bug without getting ill themselves. The Aztecs had never been exposed to the bacteria before, which is why it caused an epidemic.

This hypothesis, that Salmonella Paratyphi C spread from Europe, is supported by another study by Mark Achtman, a researcher at the University of Warwick, who found evidence of the Salmonella strain in the remains of a 13th century woman in Norway. This did not prove the Spanish brought Salmonella Paratyphi C with them to Mexico, but it does support the idea that they could have. Krause’s work provides the framework for identifying the pathogenic agents responsible for ancient epidemics, and he plans to investigate burial sites in the Caribbean to find further proof of what was behind the fall of one of the greatest Pre-Columbian empires.

(featured image courtesy of Pixabay, CC0)