Although LD incidence in Ottawa had reached ≈7 cases/100,000 population by 2015–2016, the observed incidence rates in KFL and LGL during this period were 4-fold higher (≈30 cases/100,000 population). By comparison, these rates are still far below the ≈110 cases/100,000 population observed in the bordering St. Lawrence County of New York state. Given the ongoing emergence process, LD incidence will likely continue to increase in eastern Ontario as I. scapularis tick populations and B. burgdorferi continue to establish and fill in suitable habitats. This pattern highlights the importance of fine-scale studies to identify patterns and determinants of LD and other tickborne pathogens in different regions and populations.
Our study was limited by the availability of information on location of tick acquisition and patient exposure location. As such, we aggregated data at the FSA level on the basis of location of patient and tick submitter residence and excluded case-patients and tick submitters with reported travel outside their municipality of residence. Spatiotemporal analysis based on the location of exposure would help to more precisely determine the timing and rate of spread.
Altogether, our findings indicate that LD has emerged in eastern Ontario over a relatively short timescale after the invasion of I. scapularis ticks and B. burgdorferi. Tick surveillance data can serve to identify areas of risk for LD emergence.
The authors would like to thank Dara Spatz Friedman, John Cunningham, and Adam van Dijk for their epidemiologic assistance and data provision. We also thank Ann Stanton-Loucks, Michael Bennitz, and the Ottawa Public Health Innovation Fund for their collaboration and data sharing on tick surveillance activities in Ottawa.
M.A.K. is supported by a grant from the Canadian Institutes for Health Research and by an Early Researcher Award from the Ontario Ministry of Research and Innovation.
Ethics approval for this study was obtained from the Ottawa Public Health Research Ethics Board (no. 226–16) and the University of Ottawa Science and Health Sciences Research Ethics Board (H06-16-22). Access to Integrated Public Health Information System data was granted through data-sharing agreements with participating public health units. All data were aggregated to preserve anonymity.
Emerging Infectious Diseases. 2019;25(2):328-332. © 2019 Centers for Disease Control and Prevention (CDC)