A Reversal of Seasons

Mortality rates by month in Chicago in the 1870s and 1922. Isaac Rawlings et al, The Rise and Fall of Disease in Illinois, State Department of Health, 1927.

In the last post I mentioned that the federal mortality census can inform us of the seasonality of disease in the mid-nineteenth century. This chart illustrates the seasonality of death in Chicago during the 1870s and in 1922. The solid line represents the average deaths per month during the 1870s, while the lower dotted line represents deaths in 1922.  As you can see, Chicago experienced a nearly complete reversal of seasonal mortality. In the 1870s, deaths peaked in July and slowly declined through the autumn. By 1922 the summer months are the safest months of the year with the lowest death rates. Seasonality largely exists because the primary cause of death in the 19th century was infectious disease, and the major diseases displayed a marked seasonality. So malaria and gastrointestinal diseases like typhoid fever are more common in the summer and autumn, while all  infectious respiratory diseases are more common in the winter.

From what we know of common nineteenth century diseases summer had almost certainly been the deadliest season for most if not all of the nineteenth century. Statewide, the 1870 Illinois federal mortality census records cholera infantum, pneumonia, scarlet fever, typhoid, meningitis, dysentery and diarrhea as the most common causes of death (p. 98).   Yet these causes of death amount to only one third of all deaths. In the same year, children under 5 years were 50.3% of all deaths with children under age 1 accounting for 27.3% of the 33,672 deaths reported that year. Looking at Chicago alone, children under age 5 accounted for 62.8% of all deaths in 1870, but it dropped to 17.8% in 1925.

Consumption  or tuberculosis is a dark horse lurking in data. Deaths by consumption are not listed in the summary of the 1870 federal mortality census, but somehow they came up a death rate of 145.6 per 100,000 people in 1870. In 1922, they record 5,620 deaths or 83.3 deaths per 100,000 people (p. 364). For the topic at hand though, data for tuberculosis cases and deaths shows no seasonality to either.

The state health department credited the reversal of seasons to “the disappearance of malaria, the near disappearance of typhoid and the great decrease in diarrhoeal diseases and other causes more or less related to the work of the health departments” (Rawlings et al, p. 90).  The winter peak began to appear in the 1870s and continued to grow until the 1920s. The state department credited this rise almost entirely to pneumonia and influenza (Rawlings et al, 1927). We have to remember that there were major pandemics of influenza in 1892 and 1918. In the final chapter of The Rise and Fall of Disease in Illinois the health department summarizes their success against all the major diseases. For influenza they list just one line: “There has been no success in combating influenza.” (p. 404)

Source:  Isaac Rawlings et al, The Rise and Fall of Disease in Illinois. The Health Department, 1927.