Evidence for the intensive-exposure and cross-sex transmission hypotheses in epidemic poliomyelitis mortality patterns in southern Ontario, 1910–1937

Abstract

Poliomyelitis was a major emerging epidemic disease in the early 20th century, and models of its epidemiology continue to be revised. Nielsen and colleagues have recently presented two new hypotheses: 1) that polio severity is related to intensity of exposure, creating a U-shaped age curve rather than a linear increase in severity with age, and 2) that polio severity increases when transmission occurs between opposite sexes, and therefore the sex ratio in severe polio cases will be more equal when family sizes are larger. Data for polio deaths in Ontario’s Wentworth and York Counties from 1900-1937 were gathered from a variety of archival sources, including birth, marriage, and death registrations and census records, and entered into an Excel database. Analysis of mortality patterns in this sample revealed two distinct stages within the study period, discussed in part here. Stage One (1910 to 1927) is characterized by an equal sex ratio and a median known family size of 4. Stage Two (1928 to 1937) is characterized by excess male deaths and a median known family size of 2. For 1910-1937 inclusive, the sex ratio for ages 0-19 was 2.6 in families of 1-2 children and 0.9 in families of ≥3 children. A U-shaped age curve was observed in the 1928-1937 period, with a dip at ages 7-8, but not in 1910-1927. These results support Nielsen and colleagues’ cross-sex transmission hypothesis and intensive-exposure model, tying polio mortality patterns to demographic shifts in the early 20th century and indicating further research is warranted. This research was supported by funding from SSHRC (Canada Graduate Scholarship), OGS (Ontario Graduate Scholarship), and McMaster University Department of Anthropology.