Salmonella outbreak could be at fault for collapse of Aztec society



After analyzing the DNA of bacteria found inside buried ancient Aztecs, some scientists now believe an outbreak of Salmonella could actually be to blame for the death of 80% of their population, and in turn, the collapse of their society 500 years ago.

This is potentially the first genetic evidence of the pathogen that caused the massive decline in native populations after European colonization, Hannes Schroeder, an ancient-DNA researcher said in a recent article. “It’s a super-cool study.”

In 1519, when forces led by Spanish conquistador Hernando Cortés arrived in Mexico, the native population was estimated at about 25 million. A century later, after a Spanish victory and a series of epidemics, numbers had plunged to around one million.

The article explains that the largest of these disease outbreaks were known as cocoliztli (from the word for “pestilence” in Nahuatl, the Aztec language). Two major cocoliztli, beginning in 1545 and 1576, killed an estimated seven million to 18 million people living in Mexico’s highland regions.

There has been little consensus on the cause of cocoliztli — although measles, smallpox and typhus have all been proposed. But, in an attempt to settle the question, a team led by evolutionary geneticist Johannes Krause, extracted and sequenced DNA from the teeth of 29 people buried in the Oaxacan highlands of southern Mexico. All but five were linked to a cocoliztli that researchers think ran from 1545 to 1550.

Ancient bacterial DNA recovered from several of the people matched that of Salmonella, based on comparisons with a database of more than 2,700 modern bacterial genomes. Further sequencing of short, damaged DNA fragments from the remains allowed the team to reconstruct two genomes of a Salmonella enterica strain known as Paratyphi C.

It’s perfectly reasonable that the bacterium could have caused this epidemic, Schroeder said the article. “They make a really good case.”

María Ávila-Arcos, an evolutionary geneticist, isn’t convinced, however. She explains that a virus could have caused the cocoliztli, and that wouldn’t have been picked up by the team’s method.

But, thanks to another recent study showing that Salmonella Paratyphi C arrived in Mexico from Europe, the case for Salmonella being responsible for the collapse of the Aztec society gains additional strength.

In this study, a team of researchers collected and sequenced the genome of the bacterial strain from the remains of a young woman buried around 1200 in a cemetery in Trondheim, Norway. It is the earliest evidence for the now-rare Salmonella strain, and proof that it was circulating in Europe.

“Really, what we’d like to do is look at both strains together,” said Hendrik Poinar, an evolutionary biologist at McMaster University. And if more ancient genomes can be collected from Europe and the Americas, it should be possible to find out more conclusively whether deadly pathogens such as Salmonella arrived in the New World from Europe.

The existence of Salmonella Paratyphi C in Norway 300 years before it appeared in Mexico doesn’t prove that Europeans spread enteric fever to native Mexicans, Schroeder notes in the article, but that hypothesis is reasonable, he said. A small percentage of people infected with Salmonella Paratyphi C carry the bacterium without falling ill, so healthy Spaniards could have infected Mexicans who lacked natural resistance.

Furthermore, Paratyphi C is transmitted through fecal material, and a collapse of social order during the Spanish conquest might have led to the poor sanitary conditions that are ripe for Salmonella spread, the research team wrote in their study.

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