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Drought tied to multiple typhus outbreaks in Mexico
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Drought may have contributed to 22 typhus epidemics in Mexico between 1655 and 1918, according to a recent CDC report.
According to historical records, environmental refugees may have been the means through which drought influenced the spread of typhus.
The researchers evaluated historical records of disease in Mexico, identifying 22 typhus epidemics between 1655 and 1918. These epidemics were compared with tree-ring reconstructed soil moisture estimates for central Mexico by using the Palmer Drought Severity Index (PSDI). Years with mild, moderate, severe or extreme droughts were assigned PSDI values of 1, 2, 3 and 4, respectively. Correspondingly, conditions of moisture were given PSDI scores of +1, +2, +3, and +4.
The researchers utilized superimposed epoch analysis to determine the statistical significance of the relationship between drought and epidemic typhus. They sorted and averaged the PDSI values for years before, during and after the 22 typhus epidemics. The mean PDSI value was calculated for the typhus event periods and was subsequently tested against the mean of all other years in the reconstructed time series for the central and east-central regions of Mexico.
The reconstructed mean PDSI values for the 22 epidemic periods in central Mexico were significantly lower than the average of all remaining years (1.16; P<.01), with a particularly notable drought pattern in east-central Mexico during the events (1.68; P<.001).
The researchers discussed an established theory, which holds that drought-related famine may have led to an inundation of impoverished refugees from agricultural areas into city centers. This may have exacerbated the crowded and unsanitary conditions of urban life during that time.
“However, this theory needs further investigation,” the researchers wrote. “Verification of the role of environmental refugees in the transmission of typhus during these 22 epidemics is complicated by a lack of clarity in the historical sources and by the interaction between politics, geography, and climate.”
Disclosure: The researchers report no relevant financial disclosures.
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