A second article in today’s Observer concerns the destruction of Mexico’s population following the arrival of the Conquistadores. The massive fall is usually blamed on the introduction of diseases to which the native population had no resistance. Epidemiologist Rodolfo Acuna-Soto suggests that much of the damage could have been done by a local haemorrhagic fever not unlike the Ebola virus. There has been a similar argument in Europe over the cause of the Black Death.
Estimates of the population of Mexico before the Spanish conquest range from 6 million to 25 million; by 1600, it was just 2 million. While there were indeed terrible smallpox and measles epidemics in the 1520s and 1530s, Acuna-Soto suggests that two later destructive epidemics in 1545 and 1576 do not appear to be accounted for by imported diseases. Acuna-Soto's conviction that these epidemics - called cocoliztli - were caused by a haemorrhagic fever is partly based on the observations of Philip II's physician. 'Blood flowed from the ears and in many cases blood truly gushed from the nose,' Francisco Hernandez wrote. 'The tongue was dry and black. Enormous thirst. Urine the colours of sea green, vegetal green, and black'.
A little digging around the internet and I find that this story is hardly hot news. An article in the February edition of Discover magazine sets out Acuna’s hypothesis in greater detail.
Acuña-Soto’s studies of ancient documents revealed that the Aztecs were familiar with smallpox, perhaps even before Cortés arrived. They called it zahuatl. Spanish colonists wrote at the time that outbreaks of zahuatl occurred in 1520 and 1531 and, as many as 8 million people died from those outbreaks. But the epidemic that appeared in 1545, followed by another in 1576, seemed to be another disease altogether. The Aztecs called those outbreaks by a separate name, cocolitzli. "For them, cocolitzli was something completely different and far more virulent," Acuña-Soto says. "Cocolitzli brought incomparable devastation that passed readily from one region to the next and killed quickly."
Medical historians insisted that the cause of this affliction could only have been a European disease. But Acuña-Soto felt that it made no sense, that the Aztecs had invented a new name for smallpox. He also noticed that previous researchers had to pick and choose among the disease reports to make them fit a diagnosis of smallpox or typhus. Finally he also could not understand why Old World diseases would cause massive deaths 20 years and then 55 years after the arrival of the Spanish. "By this time," he says, "those who survived the earlier epidemics would have had immunities or would have passed them on."
If cocolitzli had been caused by a hemorrhagic virus, Acuña-Soto realized, the Spanish could not have brought it with them. Such diseases do not readily pass from one person to another, so the virus must have been native.
This raised two serious questions – First: were people prepared to absolve the Spanish of (at least partial) responsibility for one of the great evils of the colonial era? Second: If the Spanish didn't bring about the cocolitzli, what did?
For the Aztecs, as for any agricultural society, rainfall was so important that it was well recorded in their surviving codices. The Valley of Mexico in which the Aztecs lived was not easy land to farm. The rains, only 30 to 40 inches a year, come between May and October. There are frequent late and early frosts that can kill maize crops. It is no surprise that the codices all bear witness through evocative pictographs of heavy rains, frosts, or—more telling—catastrophic droughts. Acuña-Soto saw that each of the cocolitzli epidemics appeared to be preceded by several years of drought. He also found that the epidemics didn't happen during the drought. They appeared only in the wet periods that followed. That was the crucial clue he had missed: It was raining when people got sick.
Evidence from Dendrochronology shows that during the 16th century central Mexico not only lacked rain but also suffered the most severe and sustained drought in 500 years, one that encompassed nearly the entire continent. Moreover the tree-ring records show wet interludes setting in around the years 1545 and 1576, the years of the cocolitzli. With the climate data in place, Acuña-Soto could piece together a plausible explanation of those epidemic years.
Cocolitzli had been caused by a hemorrhagic fever virus that had lain dormant in its animal hosts. Severe drought would have contained the host population, forcing them to hole up wherever they could find water. Initially, only a small percentage may have been infected, but when forced into close quarters the virus was transmitted to others. When the rains returned, the hosts quickly bred and spread the virus as they came into contact with humans. Once infected, humans transmitted the virus to one another through contact with blood, sweat, and saliva.
Hemorrhagic viruses affect human populations that are already stressed, Acuña-Soto says. "The natives were poor and probably near starvation and living in unsanitary conditions where the rats would congregate. They also worked in the fields, where they'd be exposed to the rat droppings.
Elsa Malvido, a historian from Mexico's National Institute of Anthropology and History, dismisses the idea. She believes the later epidemics were bubonic plague spread by black rats from Europe. 'They were more extreme because they were attacking people with no immunity,' she said.
There has been a debate in Europe over the cause of the Black Death in 14th Century Europe. In 1894, two scientists, Alexandre Yersin and Shibasaburo Kitasato, independently identified the rod-shaped bacterium responsible for an epidemic of bubonic plague sweeping out of China.
Yersin soon linked the Black Death to the bacterium, named Yersinia pestis. Since then historians and scientists have strengthened the argument that bubonic plague was responsible for Black Death and similar outbreaks in medieval Europe. But other experts have expressed doubts, largely on account of the difficulty of identifying a disease based on the few medieval descriptions of the Black Death that have survived.
Two researchers from the University of Liverpool presented a new theory in 2001 arguing that a hemorrhagic virus, like Ebola, probably caused the Black Death and most of the smaller epidemics that struck Europe for the next three centuries, not bubonic plague. They argued that Bubonic plague is a disease of rodents and Europe had no rodent species that could harbor the disease between outbreaks. In addition the rats that passed the plague through fleas to humans during epidemics all died so the plague would have perished with them.
Samuel K. Cohn, a professor of medieval history at the University of Glasgow, agrees that other diseases are better candidates for the Black Death than Yersinia pestis but feels that an Ebola-like virus would burn out too quickly to produce the widespread mortality seen.
Most epidemiologists, however, argue that the evidence points most strongly toward the bubonic plague pathogen as the cause of the Black Death. Bubonic plague can take three forms in people, and those forms can account for the descriptions of Black Death symptoms. In addition French researchers reported that they had found Yersinia pestis in the dental pulp of three people buried in Montpellier, France, in the 14th century. "Medieval Black Death was the plague," they declared.
Some scientists suggest that more concrete evidence - for example, finding Yersinia pestis in the remains of victims from northern Europe and England - is needed to settle the question of what caused the Black Death. Perhaps the overall position is best summed up by Dr Joshua Lederberg an emeritus professor of microbiology and a Nobel laureate in medicine at Rockefeller University: "Yersinia still seems to me the most reasonable assignment as the cause of the Black Death, but I say that with less than unshakable conviction."