Drought and epidemic typhus, central Mexico, 1655-1918

Abstract

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 Historical documents describe how drought created large numbers of environmental refugees that fled the famine-stricken countryside for food relief in towns.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.

Keywords: Mexico; Pediculus humanus corporis; Rickettsia prowazekii; bacteria; body lice; drought; epidemics; tree rings; typhus.

Figure 1

A) Time series of summer Palmer Drought Severity Index (PDSI) averaged for 78 grid points in central Mexico, 1665–1918. Data were obtained from Cook et al. (12,13) and Therrell et al. (14). B) Time series of June–July PDSI reconstructed from the Cuahtemoc la Fragua tree-ring chronology in east-central Mexico by using an average of 22 grid locations from the monthly PDSI dataset of R.R. Heim, Jr. (National Climatic Data Center, Ashville, NC, USA). Circles indicate typhus epidemics. Red lines indicate high-frequency yearly variability of moisture reconstruction. Black lines indicate smoothed lower-frequency representation of this variability. Horizontal lines indicate average PDSI for period.

Figure 2

A) Superposed epoch analysis (20) of summer Palmer Drought Severity Index (PDSI) for central Mexico averaged for the 22 periods that had typhus epidemics (1655, 1710–1712, 1714, 1742, 1761–1762, 1785–1787, 1799–1802, 1805–1806, 1811–1812, 1821–1823, 1825–1828, 1835–1838, 1847–1848, 1861–1864, 1865–1868, 1870–1873, 1875–1877, 1884–1886, 1894–1895, 1902–1903, 1909–1911, and 1915–1918). Horizontal line indicates PDSI = 0. JJ, June–July; JJA, June–July–August. B) Superimposed epoch analysis of June–July PDSI for east-central Mexico also averaged for the 22 periods that had typhus epidemics. Superimposed epoch analyses were performed by using the Dendrochronology Program Library (21,22). Horizontal line indicates mean PDSI.

Figure 3

Tree-ring–reconstructed summer Palmer Drought Severity Index (PDSI) during A) 22 and B) 15 typhus epidemics in Mexico, 1665–1918. Drought conditions are indicated by negative values on the PDSI scale. Reconstructed summer PDSI values during the 15 typhus epidemic years with the most negative PDSI values for central Mexico are mapped in panel B. JJA, June–July–August.