Drought Aided the Spread of Typhus

By combining epidemiological data with climate data, researchers at the University of Arkansas were able to provide evidence that drought contributed to the spread of epidemic typhus in Mexico from 1655 to 1918.

Typhus, not to be confused with typhoid fever, can cause delirium, severe muscle pain, joint pain, rash, fever and headache. The mortality rate is estimated between 10 percent and 60 percent for the typhus strain studied, according to the US National Library of Medicine. The study, published in Emerging Infectious Diseases, is relevant to modern day areas of the world, such as impoverished areas of South America, Asia and Africa, where typhus is still a threat and could reemerge with dire consequences.

"Historical records show that typhus has traditionally accompanied war, famine and poverty," said David Stahle, a professor of geosciences at University of Arkansas. "Now, because of Mexico's rich historical record of epidemic disease, we can see that drought, as reconstructed by tree-ring chronologies, caused conditions that allowed typhus to flourish in central Mexico over a 250-year period."

Stahle and Jordan Burns, a graduate student in geography at the university, compared the historical records of 22 typhus epidemics in central Mexico with climate data gathered from studying the varying ring growth sizes in the trunks of trees in the area. Larger rings show years of relatively high moisture levels, while slim rings show seasons of drought. In addition, the researchers studied almanacs, diaries, personal accounts, medical records and death certificates. This data was then compared to the Palmer Drought Severity Index, which accounts for the effects of temperature and precipitation on soil moisture dating back to 1895.

The researchers observed a strong correlation between drought and famine in the agricultural regions studied. Of the 22 typhus epidemics studied, 19 of them occurred during years of drought and low tree growth, according to a press release announcing the findings.

"The observed relationship between drought and typhus epidemics in colonial and modern Mexico is curious because drought has not been specifically considered a risk factor for typhus," Burns said. "But drought, much like war and natural disasters, caused famine in poor, agricultural regions and forced impoverished refugees to move into already crowded urban areas where infrastructure and sanitary systems were insufficient."  

Typhus is spread between humans by body lice, which means the disease thrives in crowded and dirty conditions. The disease has a high mortality rate and can still be found among lice infested homeless people living in industrialized societies and some areas of Africa, South America and Asia.