What is the real number of Lyme disease cases in Canada?

N H Ogden,S P Elias,J K Koffi,D Webster,D Webster,T F Hatchette,L R Lindsay,C B Lubelczyk,C Bouchard,J Badcock,A S Peregrine,M A Drebot,R P Smith,P A Leighton

doi:10.1186/s12889-019-7219-x

N H Ogden, S P Elias + Show 12 more

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BackgroundLyme disease is emerging in Canada due to expansion of the range of the tick vector Ixodes scapularis from the United States. National surveillance for human Lyme disease cases began in Canada in 2009. Reported numbers of cases increased from 144 cases in 2009 to 2025 in 2017. It has been claimed that few (< 10%) Lyme disease cases are reported associated with i) supposed under-diagnosis resulting from perceived inadequacies of serological testing for Lyme disease, ii) expectation that incidence in Canadian provinces and neighbouring US states should be similar, and iii) analysis of serological responses of dogs to the agent of Lyme disease, Borrelia burgdorferi.We argue that performance of serological testing for Lyme disease is well studied, and variations in test performance at different disease stages are accounted for in clinical diagnosis of Lyme disease, and in surveillance case definitions. Extensive surveillance for tick vectors has taken place in Canada providing a clear picture of the emergence of risk in the Canadian environment. This surveillance shows that the geographic scope of I. scapularis populations and Lyme disease risk is limited but increasing in Canada. The reported incidence of Lyme disease in Canada is consistent with this pattern of environmental risk, and the differences in Lyme disease incidence between US states and neighbouring Canadian provinces are consistent with geographic differences in environmental risk. Data on serological responses in dogs from Canada and the US are consistent with known differences in environmental risk, and in numbers of reported Lyme disease cases, between the US and Canada.ConclusionThe high level of consistency in data from human case and tick surveillance, and data on serological responses in dogs, suggests that a high degree of under-reporting in Canada is unlikely. We speculate that approximately one third of cases are reported in regions of emergence of Lyme disease, although prospective studies are needed to fully quantify under-reporting. In the meantime, surveillance continues to identify and track the ongoing emergence of Lyme disease, and the risk to the public, in Canada.

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Serological diagnosis failure as a potential source of under-detection and under-reporting Some patient advocacy groups suggest that the “twotier” serological algorithm used for Lyme disease (LD) diagnosis; screening enzyme immunoassays (EIA) followed by confirmatory immunoblot testing recommended by US Centers for Disease Control and Prevention (CDC) and supported by the Association of Medical Microbiology and Infectious Diseases (AMMI) Canada and the Public Health Agency of Canada (PHAC), is too insensitive and misses many cases [22]
This increase in incidence is consistent with the range spread of I. scapularis ticks in Canada [13], but other factors may modulate the risk of LD to the Canadian public via effects on rates of their exposure to tick bites
Our comparisons of human case LD incidence, as reported in surveillance and as suggested by canine seroprevalence, in the US and Canada do not support the idea that there is a high degree of under-reporting (< 10% of cases reported) in Canadian human case surveillance

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Our comparisons of human case LD incidence, as reported in surveillance and as suggested by canine seroprevalence, in the US and Canada do not support the idea that there is a high degree of under-reporting (< 10% of cases reported) in Canadian human case surveillance. Even if it could be argued that these comparisons are themselves plagued by underreporting, the extensive surveillance efforts conducted in Canada mean that we have a strong understanding of where LD risk is occurring, how it has evolved and in what ways it is similar to, and different from, that occurring in neighbouring US states. Tick surveillance data, combined with evidence from serological studies in dogs, do not suggest high levels of under-reporting in Canada.

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