Salmonella was possibly one of the major killers that wiped out 80-percent of Aztec population

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Human history had had its fair share of epidemics, but among them a 16th century pestilence is still considered as one of the worst cases with incredibly high rates of fatalities. This particular epidemic in question that possibly wiped out 80-percent of Mexico’s native population (circa 1540s) might have been caused by a deadly form of salmonella. Moreover recent studies have hypothesized how the salmonella was possibly transmitted from Europe, thus making the disease far more disastrous for the Aztecs that a foreign military intervention.

According to some experts (like Hannes Schroeder, an ancient-DNA researcher at the Natural History Museum of Denmark), the studies may just pertain to the first known genetic evidence of a pathogen that rather decimated the native population after the said region was colonized by the Europeans. The numbers surely match up with such a conjecture. For example, in early 16th century (circa 1519 AD), when Spanish conquistador Hernando Cortés arrived in Mexico, the native population was estimated to be somewhere around the 25 million mark. And after just a century, the population plummeted to barely a million people.

Now historically from the Aztec perspective, these baleful occurrences were termed as cocoliztli, which roughly translates to pestilence in the Nahuatl language. Two of these ominous cocoliztli (in 1545 and 1576) accounted for somewhere between 7 million to 18 million deaths, thus severely affecting the populace living on the highlands. Unfortunately, researchers didn’t have much insight into these epidemics, in spite of comparisons with the infamous Black Death and a flurry . A study conducted back in 2002 by the National Autonomous University of Mexico (UNAM) put forth a conjecture that these pestilences was possibly related a viral haemorrhagic fever.

But this time around the focus was more on the availability of genetic evidence, with the scientific project being led by evolutionary geneticist Johannes Krause at the Max Planck Institute for the Science of Human History. Consequently, the researchers were successfully able to extract and sequence DNA from the remains (teeth material) of 29 people interred in the Oaxacan highlands of southern Mexico – and 24 of these deaths were probably caused by the cocoliztli from 1545 to 1550 AD. And bacterial DNA salvaged from many of these remains were found to match up with Salmonella, based on the database of 2,700 modern bacterial genomes.

In fact, the research team went on to further sequence the damaged DNA fragments, which allowed them to basically reconstruct two genomes of a Salmonella enterica strain known as Paratyphi C. In our modern times, this particular bacterial strain is found to be affecting rural populations of developing countries, and as such the typhus-like illness can cause fatalities in the range of 10-15 percent if left untreated.

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Fall of Tenochtitlan.

Interestingly enough, in another separate study conducted by researchers at the University of Warwick, a genome collection and sequencing of the aforementioned bacterial strain provided the earliest known evidence of Salmonella – and it pointed to an early 13th century cemetery in Trondheim, Norway. Simply put, scientists have more-or-less uncovered evidence that Salmonella Paratyphi C existed in Europe around 300 years before its presence in Mexico.

Now of course that does not precisely prove that the deadly bacteria was carried forth and then transmitted by the Europeans after their arrival in the New World, though a conjecture could certainly be drawn. To that end, the hypothesis might just carry weight if we consider the biological and socio-political aspects during those time circa 16th century. Pertaining to first factor, only a few hosts are needed to carry the strain (without falling ill, because of natural immunity) and transmit it to the precariously poised Mexicans with no natural immunity. As for the second scope, the Spanish intervention derailed the political and social system of the Aztecs, which could have led to the collapse of economy and order, thus resulting in people living in unsanitary conditions. These segments of the squalor populace were ripe for the spread of Salmonella, especially since Paratyphi C is transmitted through fecal material.

However beyond just conjectures there is the entire science of epidemiology to consider. In that regard, the potential late-medieval ‘epidemic’ link between Europe and Mexico can only be established after further analysis of ancient genomes collected from Europe and the Americas. And moreover, like we fleetingly mentioned before, there are other conjectures that discard the Salmonella theory in favor of hypotheses involving viral fevers – as reiterated by María Ávila-Arcos, an evolutionary geneticist at UNAM.

The study was originally published on bioRxiv.

Source: Nature

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