FAYETTEVILLE, Ark. - Tree ring reconstruction of rainfall dating back to the 1500s may provide insight into some of the epidemics that decimated the native population of Mexico shortly after the arrival of Europeans. The evidence points to hemorrhagic fevers caused by an indigenous virus instead of diseases introduced from the Old World.

"The evidence suggests that there was a serious, unidentified disease vector at large in the 16th century that the epidemiology community is now investigating," said University of Arkansas geosciences professor David Stahle.

Stahle, University of Arkansas geosciences professor Malcolm Cleaveland, research associate Matthew Therrell and lead author Rodolfo Acuna-Soto of the Universidad Nacional Autonoma de Mexico report their findings in the journal Emerging Infectious Diseases.

Shortly after Europeans arrived in Mexico, the native population became plagued by a series of epidemics that killed tens of millions of people. Researchers had long thought that these epidemics stemmed from non-native diseases introduced from Europe and Africa, such as smallpox and measles. Indeed, a smallpox epidemic destroyed 5 to 8 million people in 1519-1520.

But re-examinations of two massive epidemics in 1545 and 1576 suggest that the illness referred to as "cocolitzli," or Nahuatl for "pest," may have been caused by indigenous hemorrhagic fevers. Francisco Hernandez, the Proto-Medico (like a modern-day Surgeon General) of New Spain and former personal physician of King Phillip II, described the symptoms of the 1576 cocolitzli infections. The symptoms—including high fever, severe headache, vertigo, profuse bleeding from the eyes, nose and mouth, black tongue, dark urine, large nodules behind the ears and death in three to four days—seemed to present a disease previously unknown to Hernandez or his European contemporaries in Mexico.

Tree ring reconstructions of precipitation in Mexico that date back to 1386 indicate that climate may have played a role in the mass destruction of a native population. Both outbreaks occurred during a 16th century megadrought that gripped North America and parts of Mexico for nearly 40 years.

"It’s just one rung in a ladder of evidence that these epidemics were hemmorhagic fevers, leveraged by climate," Stahle said.

The most devastating cocolitzli epidemic—which from 1545-1548 killed up to 80 percent of the native population living in Mexico at that time—swept across the northern and central high valleys of Mexico, ending in Chiapas and Guatemala. But both the 1545 and 1576 epidemics spared the low-lying coastal plains on the Gulf of Mexico and the Pacific coast. Thus, the epidemiology of the disease did not follow geographic patterns that researchers might have expected to find with the introduction of a non-native virus to Mexico.

"We looked at some of the literature on outbreaks in the West," Cleaveland said. "It’s a good model for how the outbreaks occur."

The researchers find a climactic correlation to a modern-day hantavirus outbreak on the Colorado Plateau in 1993. In both cases, a prolonged period of drought preceded a short burst of moist weather.

In the years before the 1993 outbreak of pulmonary hantavirus, drought appears to have forced the deer mouse population to congregate in close quarters to seek food and water, which may have fanned the spread of the hantavirus among the animals. When the weather improved, the rodent population exploded, and the host animals spread out and came in contact with human populations. This scenario also may describe the ecological history of the cocolitzli virus, the researchers contend.

"Drought does not just stress rats. It stresses people as well," Stahle said.

The native population, malnourished, poorly clothed and overworked under Spanish rule, succumbed en masse to this epidemic—one of the worst recorded cases of devastation of a human population in history, approaching even the bubonic plague which killed about 25 million, or 50 percent of the regional population.

The disease vector that caused the cocolitzli epidemics has yet to be found. Researchers might find the vector in living rodent populations or possibly in genetic material from human remains affected by the epidemic. Acuna-Soto believes that the vector still exists today, but Stahle believes large epidemics like the cocolitzli remain unlikely.

"There are fundamental differences between the 16th century and the 21st century," Stahle said. The 1500s were a time of conquest, subjugation and social upheaval in the Americas, and that combined with the longest, most severe drought in the 600-year climatic record combined to create a catastrophe almost unheard of in human history.

The researchers are working to extend the climatic record from tree rings and retrieve information from historic documents in other parts of Meso-America to help round out in greater detail the history of human disease both before and after the Europeans arrived.

"There are a lot of unexplained mortality events in Mexico’s history," Stahle said. Detailed history of climate and an examination of Mexico’s historic records may provide more insights into these disease outbreaks.

"The questions are endless," Cleaveland said. "We’re doing the best we can without a time machine."

# # #