Seasonal variation in night blindness incidence among Union soldiers in the US Civil War

Background: During the US Civil War, medical officers typically attributed night blindness among soldiers to malingering. A dietary basis was not generally suspected or appreciated.

Design/methods: Incident cases of night blindness, scurvy, and diarrheal diseases, as well as mean troop strength among Union troops, were abstracted by month and race from tabulations of the US Surgeon General for the period from July 1861 through June 1866. Monthly incidence rates and annual incidence rates are presented as time series by race.

Results: Night blindness incidence was seasonal. Seasonal patterns of night blindness incidence were similar for white and black soldiers, although the peak incidence rates were approximately 2–3 times higher in black soldiers. The seasonal effect for white Union soldiers increased progressively to 1864. The seasonal pattern for night blindness roughly parallels that for scurvy and for diarrheal diseases. The peak season for night blindness incidence was summer, and the next highest season was spring. The mode of monthly incidence rates for diarrheal diseases slightly anticipated that for night blindness and scurvy. In addition, there was greater relative variation in monthly incidence for night blindness and scurvy than for diarrheal diseases.

Conclusions: Nutritional night blindness occurred in a seasonal pattern among soldiers forced to subsist on nutritionally inadequate diets. The seasonal pattern is consistent with seasonal variations in the availability of foodstuffs with high vitamin A or provitamin A content, superimposed on marginal vitamin A reserves, and possibly exacerbated by co-occurring seasonal patterns of diarrheal disease.