A Spatial Analysis of Individual- and Neighborhood-Level Determinants of Malaria Incidence in Adults, Ontario, Canada

Rose Eckhardt

, Lea Berrang-Ford

, Nancy A. Ross, Dylan R. Pillai¹, and David L. Buckeridge

Author affiliations: McGill University, Montreal, Quebec, Canada (R. Eckhardt, L. Berrang-Ford, N.A. Ross, D.L. Buckeridge); University of Toronto, Toronto, Ontario, Canada (D.R. Pillai); Ontario Agency for Health Protection and Promotion, Toronto, Ontario, Canada (D.R. Pillai); Agence de la Santé et des Services Sociaux de Montréal, Montreal (D.L. Buckeridge)

Abstract

Malaria, once endemic in Canada, is now restricted to imported cases. Imported malaria in Canada has not been examined recently in the context of increased international mobility, which may influence incidence of imported and autochthonous cases. Surveillance of imported cases can highlight high-risk populations and help target prevention and control measures. To identify geographic and individual determinants of malaria incidence in Ontario, Canada, we conducted a descriptive spatial analysis. We then compared characteristics of case-patients and controls. Case-patients were significantly more likely to be male and live in low-income neighborhoods that had a higher proportion of residents who had emigrated from malaria-endemic regions. This method’s usefulness in clarifying the local patterns of imported malaria in Ontario shows its potential to help identify areas and populations at highest risk for imported and emerging infectious disease.

Malaria is a parasitic, vector-borne disease that causes ≈1 million deaths each year and substantial global public health costs (1,2). The disease was previously endemic in North America, with transmission in most of the United States and parts of southern Canada (3). The malaria parasite, Plasmodium spp., was introduced into North America during the 16th–17th centuries through the arrival of European colonists and African slaves (3). Malaria was eliminated in North America by the 1950s through several different interventions, including vector control by changes in vector habitat, introduction of new antimalarial medications (such as quinine) for improved treatment, and decreased contact between humans and mosquitoes, in part because of changes in housing conditions (3).
Cases of locally acquired, mosquito-transmitted (autochthonous) malaria still occur in the United States, particularly in the Northeast. Most recently, reported autochthonous cases in the United States have occurred in suburban or urban areas (3). Although no confirmed cases of autochthonous malaria have been recorded in Canada in recent years, ≈400 cases of imported malaria are identified in Canada each year, a significantly higher prevalence than in the United States (4,5). Increased international travel and immigration have the potential to change the probability of autochthonous transmission in Canada and the United States, where competent vectors and suitable regional climates already exist (3,6).
Malaria incidence is affected by socioeconomic inequality and human movement (7). Malaria transmission in Canada was associated historically with socioeconomic inequality and migration; in particular, malaria transmission surged among migrant workers on the Rideau Canal in Ontario during 1826–1832 (8). Regional and international travel and migration patterns have also been implicated in malaria incidence across the globe, including reports of so-called airport malaria, refugee outbreaks, and cases in migrant populations (9–12). Rates of transmission in highly traveled areas have been found to affect rates of imported malaria cases (13). Length and type of travel and travel behavior are associated with risk for malaria transmission (14–16). At particularly high risk of acquiring malaria and importing it to their country of residence are travelers who visit friends and relatives (VFRs) in countries where malaria is endemic (16–21). Research has recently shown that VFRs are also at risk for multidrug-resistant malaria because of the inappropriate use of antimalarial drugs available over the counter in malaria-endemic countries (22).
Emerging and reemerging disease risk interacts with socioeconomic vulnerability to determine the populations at highest risk for infection and those most likely to import pathogens into the country. In the context of changing patterns in international travel, immigration, and global disease spread, understanding the socioeconomic determinants of existing disease risks is prudent.
In this study, we first conducted a descriptive spatial analysis to identify the geographic and individual determinants of malaria incidence in Ontario, Canada. We then tested the hypothesis that malaria case-patients do not differ significantly from controls in terms of their individual and geographic characteristics.