Incidence of Lyme Borreliosis in Finland: Exploring Observed Trends Over Time Using Public Surveillance Data, 2015-2020

Abstract

Background: Lyme borreliosis (LB) is a tick-borne zoonotic disease endemic in many European countries, including Finland. We describe the incidence, time trends, and geographical distribution of LB in Finland for the period 2015-2020. The data generated can help inform public health policy, including prevention strategies. Methods: We retrieved online-available LB cases and incidence from two Finnish national databases. Microbiologically confirmed LB cases were identified from the National Infectious Disease Register and clinically diagnosed LB cases from the National Register of Primary Health Care Visits (Avohilmo), with the total LB cases equal to the sum from these two sources. Results: A total of 33,185 LB cases were reported for the 2015-2020 period, of which 12,590 (38%) were microbiologically confirmed and 20,595 (62%) were clinically diagnosed. The average annual national incidences for total, microbiologically confirmed, and clinically diagnosed LB were, respectively, 99.6, 38.1, and 61.4 per 100,000 population. The total LB incidence was highest in the south to southwestern coastal areas by the Baltic Sea and in eastern areas, with average annual incidences of 109.0 to 207.3/100,000. The Åland Islands were a hyperendemic region with an average annual incidence of 2473.9/100,000. The highest incidence was among persons aged >60 years, peaking at age 70-74 years. Most cases were reported between May and October, with a peak in July and August. Conclusions: The incidence of LB varied substantially by hospital district, and many regions reached incidences comparable with other high incidence countries, suggesting preventive measures such as vaccines may be an efficient use of resources.

Keywords: Finland; Lyme borreliosis; incidence; spatial epidemiology; surveillance; tick-borne disease.

FIG. 1.

Annual incidence (per 100,000 population, ±95% CI) of clinically diagnosed LB cases reported in the Register for Primary Health Care Visits (Avohilmo), microbiologically confirmed LB cases reported in the NIDR, and both combined, Finland 2015–2020. CI, confidence interval; LB, Lyme borreliosis; NIDR, National Infectious Disease Register.

FIG. 2.

Average annual incidence (per 100,000 population) by 21 Finnish HDs for the years 2015–2020. (A) Clinically diagnosed LB. (B) Microbiologically confirmed LB. (C) Combined clinically diagnosed and microbiologically confirmed LB. HDs, hospital districts.

FIG. 3.

Annual incidence (per 100,000 population) by 21 Finnish hospital districts during 2015–2020. (A) Clinically diagnosed LB. (B) Microbiologically confirmed LB.

FIG. 4.

Average annual incidence (per 100,000 population, ±95% CI) of clinically diagnosed LB cases reported in the Register for Primary Health Care Visits (Avohilmo) and of microbiologically confirmed LB cases reported in the NIDR by age groups in Finland in the 2015–2020 period.

FIG. 5.

Average number of weekly clinically diagnosed cases reported in the Register for Primary Health Care Visits (Avohilmo) and of weekly microbiologically confirmed LB cases reported in the NIDR in Finland in the 2015–2020 period.