Friday, January 13, 2006

Megadeath in Mexico

The February 2006 issue of Discover magazine has a fascinating article titled "Megadeath in Mexico."

Here's the opening paragraph:
When Hernando Cortes and his Spanish army of fewer than a thousand men stormed into Mexico in 1519, the native population numbered about 22 million. By the end of the century, following a series of devastating epidemics, only 2 million people remained. Even compared with the casualties of the Black Death, the mortality rate was extraordinarily high. Mexican epidemiologist Rodolfo Acuna-Soto refers to it as the time of "megadeath." The toll forever altered the culture of Mesoamerica and branded the Spanish as the worst kind of conquerors, those from foreign lands who kill with their microbes as well as their swords.

As it turns out, though, that might be wrong. Which makes for an incredibly interesting article.

Here's the thing. Smallpox has always been blamed for this population decimation. But Acuna-Soto found some anomalies in the historical data. For one, the Aztecs were already familiar with smallpox--they called it zahuatl. There were outbreaks of that disease in 1520 and 1531, and they killed an estimated eight million people. However, the epidemics in 1545 and 1576, which were even more severe, were from a disease that the Aztecs referred to as cocolitzli, and this disease was "far more virulent" than zahuatl.

Acuna-Soto also found a manuscript that had remained unpublished for over four centuries, written by Francisco Hernandez, who was the personal physician to Phillip II of Spain. Hernandez was sent to Mexico and performed autopsies on victims of the epidemic. His descriptions, after being finally translated from their original Latin, described not smallpox but a kind of hemorrhagic fever.

So Acuna-Soto kept digging and found--drought. The Aztecs kept detailed records relating to agriculture, and each of the cocolitz epidemics were preceded by years of severe drought. Acuna-Soto was able to verify the accuracy of the records by consulting dendrochronologists who were able to use the tree rings in Douglas fir trees to verify the periods of severe drought. In fact, they were the most severe droughts in Mexico for nearly five hundred years.

That led Acuna-Soto to what he feels is the real answer: a hemorrahagic virus (not unlike the hantavirus) that had been dormant in its hosts--most likely, rodents. The severe drought forced the rodent population to live in extreme densities and under extreme stress as they clustered near available water, which greatly magnified the spread of disease among the rodent population. Then, when the rain started, their subsequently more normal distribution pattern brought them into close contact with humans, and once the virus was transmitted to humans, it could then be passed from human to human (blood, saliva, sweat). And the contemporaneous accounts of the disease and its symptoms closely correspond to the symptoms of some kind of hemorrhagic virus.

It's a stunning piece of scientific detective work, and the entire article is just amazing. It's not available online yet, but the magazine's well worth picking up, and if it gets added to the website, I'll post the link.

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