Cocoliztli epidemics

The Cocoliztli epidemics were a series of devastating outbreaks in 16th-century New Spain.

The Aztec people used the Nahuatl word "cocoliztli" to describe this pestilence. These outbreaks primarily targeted the Mexican highlands. They caused massive demographic collapses among Indigenous populations. Some estimates suggest between 5 and 15 million people died. This makes it one of the most severe epidemics in Mexican history.

The disease was defined by its extreme virulence. This means it was very strong and spread quickly. Victims experienced a rapid onset of symptoms without warning. They suffered from high fevers and severe headaches. Other symptoms included vertigo and dark urine. Many people also experienced profuse bleeding from the nose, eyes, and mouth. In some cases, patients suffered from gastrointestinal hemorrhaging and bloody diarrhea. Death often occurred within just three to seven days after the first symptoms appeared.

Researchers face challenges when studying this disease in the archaeological record. Because cocoliztli killed so quickly, it often left no lesions on bones. Most bone-marking diseases leave physical traces on the skeleton. Cocoliztli primarily damaged the respiratory and gastrointestinal systems. This makes it difficult for scientists to identify the pathogen through skeletal remains alone. However, modern technology is helping to solve these mysteries.

History records at least 12 different cocoliztli epidemics. The most massive outbreaks occurred in 1545, 1576, 1736, and 1813.

The 1545 outbreak was particularly catastrophic. During this era, the Spanish clergyman Bernardino de Sahagún was affected. He was the author of the Florentine Codex. Sahagún contracted the disease in 1546 and again in 1590. He eventually died from the illness.

Several environmental and social factors likely fueled these outbreaks. Following the Spanish conquest in 1519, Indigenous peoples were moved into “reducciones”. These were crowded settlements designed for easier governance and religious conversion. Such density increased the contact between people and animals. Many animals brought from the Old World acted as disease vectors. Furthermore, the region suffered through massive megadroughts. One drought stretched from Venezuela all the way to Canada. These droughts may have worsened sanitation and hygiene. Some scientists believe the rains following these droughts increased populations of rodents. These rats and mice might have carried arenaviruses. An arenavirus is a type of virus that can cause hemorrhagic fevers.

There is significant debate regarding the exact biological cause. For a long time, many believed it was a viral hemorrhagic fever. This theory suggests a New World virus was responsible. However, in 2018, new DNA evidence emerged from the Oaxaca region.

Scientists examined the teeth of 29 individuals from a 16th-century cemetery. They used a tool called MALT to match DNA fragments. They discovered the presence of Salmonella enterica serovar Paratyphi C. This bacterium causes enteric fevers, also known as paratyphoid. This specific strain is unique to humans.

Even with this discovery, the scientific community remains divided. Some researchers, like evolutionary geneticist María Ávila-Arcos, question the Salmonella theory. They note that the symptoms of Paratyphi C do not perfectly match the historical descriptions. For example, current infections do not typically cause the severe gastrointestinal hemorrhaging seen in the 1500s. Others believe that RNA viruses have not been sufficiently investigated. The complexity of the history suggests that different outbreaks may have had different causes. The study of cocoliztli continues to bridge the gap between history, biology, and climate science.