Changing case definition: An analysis of its impact on Lyme disease surveillance in Nova Scotia

Nova Scotia has experienced a growing number of Lyme disease (LD) cases since 2002. From 2009 to 2022, Nova Scotia adopted a LD case definition that aligned with the Public Health Agency of Canada's definition. On January 1, 2023, Nova Scotia transitioned to a LD definition that relies on laboratory evidence alone.

To describe and compare historic trends in confirmed LD case counts and incidence under the former and current LD case definitions between 2018 and 2023 and assess the impact of the case definition change on LD surveillance.

Confirmed LD cases were extracted from Nova Scotia's Electronic Public Health Information System, software Panorama, according to the former case definition for the years 2018-2022 and the current case definition for the years 2019-2023. As the 2018 laboratory data in Panorama was incomplete, raw data for 2018 were obtained from Nova Scotia's Provincial Public Health Laboratory Network. Confirmed case counts and incidence rates per 100,000 population were calculated by year, sex, age group and geographic zone, under both case definitions. Seasonality was determined by the reporting date of the case.

From 2018-2022, the current case definition identified 4,238 cases, a substantial increase of 2,493 cases over the 1,745 reported by the former case definition, with an additional 2,058 cases in 2023 under the current case definition. This led to a clear upward trend in confirmed incidence rates with the current case definition, unlike the variable pattern seen with the former case definition. Males and individuals aged 5-14, 40-59, and ≥60 years experienced consistently higher sex and age-specific rates under both case definitions. The Western Zone consistently reported the highest incidence rates. Seasonally, both case definitions showed reporting peaks from June to September, with the peak occurring two-week later peak with the current case definition.

When the current LD case definition was applied to historic surveillance data, past rates of confirmed LD increased suggesting under-reporting of clinical presentation of LD to public health in high incidence jurisdictions.