Epidemic cycles and environmental pressure in colonial Quebec

Research on historical populations in Europe finds that infectious disease epidemics appear to induce predictable cycles in age-specific mortality. We know little, however, about whether such cycles also occurred in less dense founder populations of North America. We used high-quality data on the Quebecois population from 1680 to 1798 to examine the extent to which age-specific mortality showed predictable epidemic cycles. We further examined whether environmental pressures-temperature, lack of precipitation, or crop failure-may have set the stage for the emergence of epidemics.

We applied autoregressive, integrated, moving average time series methods to annual counts of period mortality for the following age groups: < 1 year, 1 to < 5 years, 5 to < 15 years, 15 to < 50 years, and 50 years and above. These methods controlled for other patterns (e.g., trend) before empirically identifying cycles.

Results indicate a strong seven-year cycle in mortality among infants and children under seven years of age. Warm temperatures (across Quebec overall) and relatively dry years (in Eastern Quebec) also predicted an increased risk of mortality in infancy and childhood, although these environmental variables appear to act independently of the epidemic cycle pattern.

Findings indicate a strong seven-year epidemic cycle in historical Quebec which afflicted naïve birth cohorts not previously exposed to the prior epidemic. We contend that smallpox epidemics likely contributed to this cycle. The seven-year cycle occurred only in the latter half of the test period (post 1740) with increasing size of the colony and population concentration in urban areas.