Scientists have never been able to pinpoint the exact disease that killed millions of native Aztecs in the middle 1500s, which in turn helped Europeans dominate the region during their conquest of the “New World.” In 1545, between 7 and 17 million people died during an outbreak in the Mexican highlands, and one group of scientists believe they may have figured out the specific pathogen that led to the death of so many.
Indigenous populations of the Americas experienced high mortality rates during the early contact period as a result of infectious diseases, many of which were introduced by Europeans. Most of the pathogenic agents that caused these outbreaks remain unknown. Through the introduction of a new metagenomic analysis tool called MALT, applied here to search for traces of ancient pathogen DNA, we were able to identify Salmonella enterica in individuals buried in an early contact era epidemic cemetery at Teposcolula-Yucundaa, Oaxaca in southern Mexico. This cemetery is linked, based on historical and archaeological evidence, to the 1545–1550 CE epidemic that affected large parts of Mexico. Locally, this epidemic was known as ‘cocoliztli’, the pathogenic cause of which has been debated for more than a century. Here, we present genome-wide data from ten individuals for Salmonella entericasubsp. enterica serovar Paratyphi C, a bacterial cause of enteric fever. We propose that S. Paratyphi C be considered a strong candidate for the epidemic population decline during the 1545 cocoliztli outbreak at Teposcolula-Yucundaa.
Salmonella was the likely culprit. Science Alert explains that it was during this specific outbreak that the overwhelming majority of the native population perished.
The worst of those outbreaks were known as cocoliztli, from the word "pestilence" in the Aztec language Nahuatl.
It's during one of these cocoliztli, between 1545 and 1550, that up to 80 percent of the native population is believed to have perished.
As a commenter points out below, that “80 percent” number above is potentially misleading. But what is not misleading or argued is that between 1545 and 1550, the cocoliztli killed the largest group of people living in the Mexico highlands in the hundred year span after contact between Europeans and native peoples in the region. The discovery is being touted as an achievement of analysis technology.
"It was an analytical technique that was really the game-changer for us," Bos explains. While scientists have been able to extract ancient DNA from bones and other tissue, until recently it was impossible to compare that extracted DNA to a wide variety of potential matches.
But a new computer program called MALT allowed them to do just that. "The major advancement was this algorithm," Bos says. "It offers a method of analyzing many, many, many small DNA fragments that we get, and actually identifying, by species name, the bacteria that are represented."
According to researchers, this study does not answer the how or why, but just what may have led to the death of so many indigenous people in the Mexican highlands.
But even if Europeans did not introduce the pathogen, they may still be responsible for its profound deadliness among indigenous people. "We know that Europeans very much changed the landscape once they entered the new world," Bos says. "They introduced new livestock, [and] there was lots of social disruption among the indigenous population which would have increased their susceptibility to infectious disease."
And while this is big and exciting news, it is still not set in stone.
But before we get too carried away, we need to keep in mind that, so far, this research hasn't been peer-reviewed - the results have been put up on pre-print server bioRxiv for now so that others in the field to get stuck into them and test them further.
It's also already been noted by some experts that the study technique wouldn't have been able to pick up the DNA of any viruses that might have been involved in the cocoliztli, so we could be missing some big pieces of the puzzle here.
The technology and at least one piece of that puzzle have been discovered, and the rest may be very close to being resolved. According to Nature, another study that was published last week suggests that a strain of Salmonella could have been brought over from Europe.
A team led by Mark Achtman, a microbiologist at the University of Warwick in Coventry, UK, 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, according to the study. (Both teams declined to comment on their research because their papers have been submitted to a peer-reviewed journal.)
“Really, what we’d like to do is look at both strains together,” says Hendrik Poinar, an evolutionary biologist at McMaster University in Hamilton, Canada. 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.
It will be interesting to see what comes next.
