Drought and Epidemic Typhus, Central Mexico, 1655–1918

Epidemic typhus is an infectious disease caused by the bacterium Rickettsia prowazekii and transmitted by body lice (Pediculus humanus corporis).This disease occurs where conditions are crowded and unsanitary.This disease accompanied war, famine, and poverty for centuries.Historical and proxy climate data indicate that drought was a major factor in the development of typhus epidemics in Mexico during 1655-1918.Evidence was found for 22 large typhus epidemics in central Mexico, and tree-ring chronologies were used to reconstruct moisture levels over central Mexico for the past 500 years.Below-average tree growth, reconstructed drought, and low crop yields occurred during 19 of these 22 typhus epidemics.Historical documents describe how drought created large numbers of environmental refugees that fled the famine-stricken countryside for food relief in towns.These refugees often ended up in improvised shelters in which crowding encouraged conditions necessary for spread of typhus.E pidemic typhus is a serious infectious disease caused by the obligate, intracellular, gram-negative bacterium Rickettsia prowazekii and transmitted by body lice (Pediculus humanus corporis).This disease is recognized for its high mortality rate throughout human history, particularly before modern sanitary practices and the availability of antimicrobial drugs (1).Typhus spreads where conditions are crowded and unsanitary.Historical epidemics often followed war, climate extremes, famine, and social upheaval.Zinsser (2) noted that throughout history, epidemic typhus might have claimed more human lives during war than combat.Epidemic typhus could reemerge as a serious infectious disease in areas of the world where social strife and underdeveloped public health programs persist (3).Typhus was first recorded in Mexico in 1655, and the most recent major epidemic began in 1915 during the Mexican revolution.Mexico's rich historical record of epidemic disease is documented in archives of demographic data that include census records, health records, death certificates, and accounts of physicians.Mexico City and the high, densely populated valleys of central Mexico were particularly susceptible to smallpox, cholera, and typhus epidemics because of crowding and poor sanitation (4).Numerous epidemics, some identified as typhus, occurred during the colonial and early modern eras.We have compiled a record of 22 typhus epidemics in Mexico during 1655-1918.We compared the timing of these typhus epidemics with tree-ring reconstructions of growing-season moisture conditions to assess the relationship between climate and typhus during this period.