THE BLOG
07/31/2013 04:09 pm ET | Updated Sep 30, 2013

What Really Killed the Aztecs? Try a 16th Century Megadrought

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.

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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. To find answers, Acuna-Soto spent a dozen years 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.

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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."

2013-07-29-plos_logo.pngThis post, an excerpt from Linda Marsa's new book, Fevered (Rodale Aug/2013), is part of "The Science of Extinction and Survival, A PLOS BLOGS Conversation on Climate Change."