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Epidemiology of Lyme Disease in Canada 2009-2019

While the current numbers likely underrepresent the number of people living with Lyme disease in Canada, it is still important to note that even with passive surveillance, the number of cases per year has increased, showing the need for more accurate surveillance methods as well as public health initiatives for prevention and early detection and treatment.

Geographic distribution of Lyme disease cases and proportion of cases by disease stage, Canada, 2009–2019*.

A 2023 study examined the epidemiology and clinical manifestations of Lyme disease (LD) cases in Canada. The study included 4,701 individuals residing in seven provinces. Participants had been diagnosed with LD between 2009-2019. The median age was 53 years and nearly 57% were male. “There was a significant decrease in the proportion of late disseminated cases reported after 2016.”1

LD cases reported in seven Canadian provinces from 2009 to 2019 are described by age groups, geography, time, and season. The proportion of males was greater than females. Dual peaks in incidence were observed in children and older adults (≥60 years of age) for all clinical signs except cardiac manifestations, which were more evenly distributed across age groups. Proportions of disease stages varied between provinces: Atlantic provinces reported mainly early LD, while Ontario reported equal proportions of early and late-stage LD. Early LD cases were mainly reported between May through November, whereas late LD were reported in December through April. The authors conclude that increased awareness over time may have contributed to a decrease in the proportion of cases reporting late disseminated LD. These analyses help better describe clinical features of reported LD cases in Canada.

Citation

Murison K, Wilson CH, Clow KM, Gasmi S, Hatchette TF, Bourgeois AC, Evans GA, Koffi JK. Epidemiology and clinical manifestations of reported Lyme disease cases: Data from the Canadian Lyme disease enhanced surveillance system. PLoS One. 2023 Dec 15;18(12):e0295909. https://doi.org/10.1371/journal.pone.0295909

This paper does make note of the limitations of the analysis in that the findings are not generalizable to all LD cases in Canada, and only to those submitted to the surveillance system with available data on clinical manifestations, and that the surveillance system is passive in nature. The data is reported on a voluntary basis, meaning that the case numbers will likely be incomplete and potentially missing clinical manifestation information. Additionally, the study also points out that some provinces do not report clinical signs to the surveillance system, thus they were excluded from the analysis. It is evident that a more robust reporting system with wider criteria would be beneficial for accurate reporting of LD in Canada. 

Canadians, particularly those living in Lyme hotspots, have long complained that officials are low-balling the number of cases in Canada. Officials only count a few of the cases that they are absolutely sure meet their highly selective case definition and pass the test (PDF) that may miss a third of those that truly do have the disease. There is consensus that cases are under-detected or under-reported but no consensus on what multiplier to use.23

Under-detected and under-reported are two separate problems with different solutions. Epidemiologists have a different use for the data they collect, to track disease trends but not to provide absolute case numbers.

The CDC’s chief epidemiologist, Dr. Paul S. Mead agrees; he stated “for a variety of reasons, reportable diseases are always under-reported” and went on to explain the CDC just tracks some of the cases to establish changing trends and patterns. The CDC itself gives the number of LD cases in the U.S as 476,000, rather than the 63,000 reported cases. It’s more accurate to say 476,000 is the estimated number of people treated for LD and likely excludes untold thousands of patients who were actually infected. The reported case numbers in the U.S. could be compared to commercial insurance data related to LD claims.

There isn’t an alternate data source in Canada and thus we may never know the true number; however, a more recent paper estimating the incidence and economic costs of LD in Canada uses a multiplier of 13.7 to adjust the numbers.4

While the current numbers likely underrepresent the number of people living with LD in Canada, it is still important to note that even with passive surveillance, the number of cases per year has increased, showing the need for more accurate surveillance methods as well as public health initiatives for prevention and early detection and treatment.5

Footnotes

  1. Murison K, Wilson CH, Clow KM, Gasmi S, Hatchette TF, Bourgeois AC, Evans GA, Koffi JK. Epidemiology and clinical manifestations of reported Lyme disease cases: Data from the Canadian Lyme disease enhanced surveillance system. PLoS One. 2023 Dec 15;18(12):e0295909. https://doi.org/10.1371/journal.pone.0295909
  2. Lloyd VK, Hawkins RG. Under-detection of Lyme disease in Canada. InHealthcare 2018 Oct 15 (Vol. 6, No. 4, p. 125). MDPI. https://doi.org/10.3390/healthcare8030322
  3. Ogden NH, Bouchard C, Badcock J, Drebot MA, Elias SP, Hatchette TF, et al. 2019. What is the real number of Lyme disease cases in Canada? BMC Public Health 19(1):849. https://doi.org/10.1186/s12889-019-7219-x
  4. Kugeler KJ, Schwartz AM, Delorey MJ, Mead PS, Hinckley AF. 2021. Estimating the frequency of Lyme disease diagnoses, United States, 2010–2018. Emerg Infect Dis 27(2):616-619: https://pubmed.ncbi.nlm.nih.gov/33496229/
  5. Ogden NH, Dumas A, Gachon P, Rafferty E. Estimating the Incidence and Economic Cost of Lyme Disease Cases in Canada in the 21st Century with Projected Climate Change. Environmental Health Perspectives. 2024 Feb 13;132(2):027005. https://doi.org/10.1289/EHP13759

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