Estimated incidence of symptomatic Lyme borreliosis cases in five southern coastal counties in Norway, 2022

Lyme borreliosis (LB), the most common tick-borne disease in Europe, is endemic to southern coastal Norway. LB commonly presents as erythema migrans, which can disseminate, resulting in more severe disease such as Lyme neuroborreliosis or arthritis. In Norway, public health LB surveillance is conducted via mandatory reporting of laboratory-confirmed disseminated cases. From 2012 to 2022, Norway's surveillance-reported incidence of laboratory-confirmed disseminated LB increased by 78%. Although surveillance provides estimates of the incidence of disseminated LB, this study sought to estimate the incidence of symptomatic LB to better understand Norway's LB disease burden. Two studies were identified that, when combined, estimated an LB seroprevalence of 6.8% in the general adult population in southern Norway. Utilizing data from these seroprevalence studies, public health surveillance, and results from literature searches indicating that 37% of seroconverted LB cases are symptomatic and that the duration of LB antibody detection ranges from 10 to 20 years, we estimated that there were 315–630 symptomatic LB cases per 100,000 adult population in five southern coastal counties in Norway in 2022 and 24–48 cases of symptomatic LB for every public health surveillance-reported LB case in adults in these five counties in Norway.