What Killed The Aztecs? A Researcher Probes Role of 16th Century Megadrought

Public Health Perspectives is pleased to present the second of three guest posts by science journalist Linda Marsa, part of the ongoing PLOS BLOGS Network “Conversation on Climate Change.” The following is another preview of her book, “Fevered.”

The wild swings in weather that are expected to become commonplace as the planet gets warmer—more frequent and severe droughts, followed by drenching rains—change ecosystems in a way that awaken and expedite the transmission of once dormant diseases.  Intriguingly, this type of weather pattern may be what led to the fall of the once mighty Aztec Empire in the early 16th century–and not as is commonly held, by the invasion of European colonialists, who brought with them diseases like mumps, measles and smallpox for which the native populations lacked immunity.

Image: “Storming of the Teocalli by Cortez and His Troops,” 1848, painting by Emanuel Leutze

Image: “Storming of the Teocalli by Cortez and His Troops,” 1848, painting by Emanuel Leutze

When Hernando Cortes and his army conquered Mexico starting in 1519, there were roughly about 25 million people living in what is now Mexico.  A hundred years later, after a series of epidemics decimated the local population, perhaps as few as 1.2 million natives survived.  Records confirm there was a smallpox epidemic in 1519 and 1520, immediately after the Europeans arrived, killing between 5 and 8 million people.  But it was two cataclysmic epidemics that occurred in 1545 and 1576, 25 and 55 years after the Spanish conquest, which swept through the Mexican highlands and claimed as many as 17 million lives.

To Dr. Rodolfo Acuna-Soto, a Harvard-trained infectious disease specialist at the National Autonomous University of Mexico in Mexico City, it made no sense that a deadly outbreak of European origin could occur so long after the Spanish arrived, because the natives who survived previous plagues would have passed on their immunities.

megadrought To find answers, Acuna-Soto spent a dozen year pouring through ancient documents written by 16th century Spanish priests who worked with the Aztecs to preserve a record of their history, language and culture.   These texts also tracked key natural events—storms, droughts, frosts and illness. In particular, they detailed the plagues of cocoliztli (Nahuati for “pest”), a disease that seemed far more virulent than smallpox.  “Nobody had the health or strength to help the diseased or bury the dead,” one Franciscan friar wrote in 1577 about the devastation from cocolitzli. “In the cities and large towns, big ditches were dug, and from morning to sunset the priests did nothing else but carry the dead bodies and throw them into the ditches.”

Acuna-Soto also had access to exhaustive diaries kept by Francisco Hernandez, the surgeon general of New Spain who witnessed the second catastrophic epidemic in 1576.  He described a highly contagious and lethal scourge that killed within a few days, causing raging fevers, jaundice, tremors, dysentery, abdominal and chest pains, enormous thirst, delirium and seizures.  “Blood flowed from the ears,” the physician observed, “and in many cases blood truly gushed from the nose.”

“These symptoms didn’t sound like smallpox or any other known European disease that was in Mexico during the 16th century,” Acuna-Soto told me. “This sounded like a hemorrhagic fever.  So if the Spanish didn’t bring about the fever, what did?”  In his research, Acuna-Soto had noticed a pattern: the plague was preceded by years of severe drought but the epidemics occurred only during wet weather, and heavy rainfall. To confirm his observations, Acuna-Soto worked with a Mexican-American team of dendrochronologists—scientists who study tree rings to date changes in climate—and compared the 16th century historical accounts with tree-ring records from a forest of 450-year-old Douglas fir trees in a remote region of central Mexico near Durango.  The tree rings indicated that the most severe and sustained drought in North America in the past 500 years occurred in the mid-16th century.  But there were heavy downpours in the years around 1545 and 1576, which coincided with the cocolitzli outbreaks.   “The smoking gun was the tree ring data,” Acuna-Soto said.

Acuna-Soto is now convinced that the death knell for the Aztecs was an indigenous hemorrhagic fever virus spread by rodents, not the Spanish conquest.  The rat population was depleted during the drought, when food was scarce.  But once the rains returned, food and water were suddenly plentiful and the number of infected rats exploded, spreading the deadly scourge to humans.  As weather swings become more erratic and the Southwest bakes under increasingly prolonged droughts, epidemics like cocolitzli will doubtless return.  “Sooner or later, a new virus will emerge from the desert for which we don’t have any vaccine and we can’t treat with drugs,” said Acuna-Soto.  “That’s guaranteed.  That’s the big fear of science.  The only thing we don’t know is when.”

Come back next week when Linda Marsa will return to Public Health Perspectives to post on a paper in the PLOS Ecological Impact of Climate Change Collection, launching on August 5.

FeveredFor more information on Linda Marsa’s new book please go to her website here.

1. Rodolfo Acuna-Soto, David W. Stahle, Malcolm K. Cleaveland, Matthew D. Therrell (2002). Megadrought and Megadeath in 16th Century Mexico. Emerging Infectious Diseases. 2002 April; 8(4): 360–362.

2. Acuna-Soto, R., Romero, L., & Maguire, J. (2000). Large Epidemics of Hemorrhagic Fevers in Mexico 1545-1815. The American Society of Tropical Medicine and Hygiene, 62 (6), 733-739.


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11 Responses to What Killed The Aztecs? A Researcher Probes Role of 16th Century Megadrought

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  3. Andrew Bell says:

    I saw this in an episode of Naked Science “What Killed the Aztecs” season 5 episode 2 a few years ago but never saw it on TV and it seems it was never issued on DVD. Really wish I could get a copy of it somehow…

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  5. Rubinstein Mordechai says:

    A local virus would not have had such an impact, since the Aztecs and other pre-Columbian tribes would have had prior exposure and thus a certain degree of immunity. Also the Spanish and African new comers were not similarly affected, which doesnt make sense.

    This hypothesis doesnt hold water.

  6. hp says:

    Too brutal? Poor dears..

  7. hp says:

    Plagues and everything that they are. What else is there? Drought?
    C’mon man practically everyone isn’t going to die.
    Real plague, not some hyped up today’s Hanta , Ebola, etc., rather the real thing. Like what happened in early east coast N. America.
    Just because people stifle it to an epic level so it’s not common knowledge, doesn’t mean it isn’t history. It’s recorded and noted, just not talked about to the points of ridicule and scorn.

    A person can get a superior education from reading “Cracked Magazine” rather the muck which passes for learning at (haha) Princeton, Yale, etc., etc., etc..


  8. BAL22 says:

    So global warming killed the Aztecs?

  9. Markybel says:

    Nonsense, this is NOT what I was taught in school so I don’t believe it. My progressive professors said indigenous humans throughout the Americas were robust, healthy, peace loving people, living in social communes sharing the bounty of nature. In this garden of Eden they lived in harmony with the land and their neighbors, and they whistled while they worked. Gold and precious stones were only valued as offerings to benevolent Gods. It took a village to raise a child. Then the Europeans arrived and they brought pestilence, disease, greed, hatred, racism, climate change, extermination and capitalism. It’s gonna take more than an article to convince us otherwise.

  10. kc wildmoon says:

    ok, but why weren’t the Spanish affected?

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