Epidemic cycles and environmental pressure in colonial Quebec Tim - PDF document

Epidemic cycles and environmental pressure in colonial Quebec Tim Bruckner 1 Samantha Gailey 1 Stacey Hallman 2 Marilyn Gentil 3 Lisa Dillon 3 Alain Gagnon 3 1 University California Irvine, Program in Public Health and School of Social Ecology 2 Statistics Canada, Ottawa, Canada 3 Département de démographie, Université de Montréal, Montréal, Québec, H3C 3J7 Canada Correspondence to: tim.bruckner@uci.edu or alain.gagnon.4@umontreal.ca 1

Abstract Objectives: Research on historical populations in Europe finds that infectious disease epidemics appear to induce predictable cycles in age-specific mortality. We know little, however, on whether such cycles also occurred in less dense founder populations of North America. We used high-quality data on the Quebecois population from 1680-1798 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. Methods: 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 between two- and ten-year lags. Results: Results indicate a strong seven-year cycle in mortality among infants and children under seven years of age, likely due to smallpox. 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. Discussion: Findings indicate a strong seven-year epidemic cycle in historical Quebec which afflicted naïve birth cohorts not previously exposed to the prior epidemic. The seven- year cycle, moreover, occurred only in the latter half of the test period (post 1740) with increasing size of the colony and population concentration in urban areas. 2

Introduction In pre-industrial Europe, infectious diseases in both rural and urban settings predominated as leading causes of death (Bengtsson, Campbell, & Lee, 2004). Infectious disease mortality included both endemic and epidemic events. Research using historical data in Europe before the 20 th century has characterized the mortality dynamics that arose from infectious disease epidemics. Although research indicates regional variation in the response to epidemics, the literature tends to converge on two findings. First, rural areas, small towns and large urban areas alike show epidemic “cycl es ” of high mortality ranging from a few to eight years (Bengtsson et al. 2004; Duncan, Scott, & Duncan, 1993; Mielke et al., 1984). Second, infants and children appear most susceptible to these epidemic cycles. Whether, and to what extent, epidemic cycles occurred in newly founded populations (e.g., English and French nascent colonies in North America) remains less clear. Patterns of mortality during epidemics may appear much less predictable, or “cyclical,” for frontier populations. Most individuals born in these isolated and sparsely populated areas did not have previous experience with the virulent diseases that affected their European counterparts. Colonists, for instance, could live many years without ever contacting smallpox or measles. Consequently, a larger proportion of frontier populations may have been susceptible to disease. Infectious diseases could surge following the arrival of a vessel or of a voyager along the fur trade routes and spread rapidly through the colonies, affecting all ages. Such a circumstance, for instance, occurred during the smallpox outbreak of 1702- 1703 in New France (Desjardins, 1996). Newly founded populations, such as the well-documented colonial Quebecois in the 17 th and 18 th centuries, may also have withstood the joint exposures of environmental pressure 3

and epidemic risk differently given their nature of subsistence (Charbonneau et al., 1993). In England and Sweden, periodic variations in grain price – a proxy for food supply – often coincided with epidemic cycles (Duncan et al., 1993, Bengtsson, 1999). One hypothesis, which enjoys empirical support (Bengtsson, 1999), posits that cycles of malnutrition during low subsistence years may have weakened the immune system and thereby increased risk of mortality due to infectious diseases (Duncan et al., 1993; Duncan, Duncan, & Scott, 2001). Unlike most of the populations studied in Europe, however, the frontier population in Quebec supplemented their diet through fishing and hunting. In addition, in their study of 18 th century Quebec mortality, Landry and Lessard (1996) find resilience to food shortages and extreme cold, and further report that epidemic diseases induced lower levels of mortality than in other places (e.g., Finland and Philadelphia). Short-run variations in food from arable land in Quebec, therefore, may not have impacted susceptibility to lethal infectious diseases. We contribute to the literature by investigating whether and to what extent mortality in the founder population of historical Quebec showed an epidemic “ cycle .” We use periodicity in age-specific mortality from 1680 to 1798 to approximate epidemic cycles. In addition, we analyze the extent to which environmental pressure (e.g., temperature, lack of precipitation, or crop scarcity) accounted for a portion of any discovered epidemic cycle and/or affected mortality. Lastly, we assess whether epidemic cycles appeared only after the establishment of urban centers (e.g., Quebec City circa 1740). 4

METHODS Variables and Data We retrieved age-specific counts of death, from 1680 to 1798, from the Registre de la population du Québec ancien, compiled by the Programme de recherche en démographie historique (PRDH) at the University of Montreal. The database comprises vital rates for the first centuries of the settlement of the French-Canadian population (Desjardins, 1998, 2012; Dillon et al., 2017). Researchers at the PRDH reconstituted individual and familial biographies by linking individuals to their baptismal, marriage, and burial certificates. Given the ubiquity of the Catholic Church in historical Quebec, the PRDH data include the entire population base of the historical French-Canadian territory, covering 153 parishes by 1800. Demographic estimates of complete follow-up of life histories range from 81 to 91 percent for married individuals, which to our knowledge remains among the best follow-up of any population over this time period (Gagnon & Mazan, 2009). This circumstance avoids loss to follow-up problems observed in other historical datasets that could arise due to inter-regional or inter-parish migration following extremely harsh years. Over 200 publications use the high-quality PRDH data (Dillon et al., 2017). The PRDH data do not include cause of death information systematically recorded at the individual level. Only 2.2 % of death certificates issued between 1625 and 1799 cited a cause of death or described the circumstances in which death took place (Landry & Lessard, 1996, p. 50). Only 6% of cited causes of death specifically identified an infectious or parasitic disease, most often smallpox. Collecting information about causes of death, if known at all, was not mandatory. We also know of no dataset of Quebec that documents all 5

infectious disease epidemics for the 17 th and 18 th centuries. As a surrogate, we examined counts of age-specific mortality, by calendar year of death, to identify mortality cycles. Consistent with the literature on historical populations, we (as described below) infer an infectious disease epidemic cycle from the existence of mortality periodicity during this time period. When possible, we also used historical accounts of smallpox epidemics such as the correspondence between the colonial authorities and the French administration (see Amorevieta-Gentil, 2009; Goulet & Paradis, 1992; Lessard, 2012). We initially categorized mortality into age groups that cohere with the literature as well as with distinct life course stages of mortality risk. The age groups also provide a sufficient number of deaths in each age category to produce an annual time series that does not have “0” counts in any year. These age groups include infancy (0 to <1 year), childhood (1 to <5 years), adolescence (5 to <15 years), adulthood (15 to <50 years), and older age (50 years or greater). High or low temperatures, drought, and crop scarcity all may affect immune function and pathogen resistance. We retrieved annual data on these variables from several sources. We used two North American temperature reconstructions and calculated the mean of these temperature series to define annual temperature (Mann, Bradley, Hughes, 1999; Rutherford et al., 2005). Both reconstructions include annual measures, from land and marine locations for the North American continent, from 0-90°N. Both reconstructions also incorporate tree- ring and ice-core measurements from locations across North America. We retrieved precipitation data from the North American Drought Atlas PDSI Reconstructions (Cook et al., 1999), made available through the NOAA/NESDIS North American Drought Variability section of the NOAA Climatic Data Center 6