Background Lyme borreliosis (LB) is the most common tickborne infection in Sweden and the seroprevalence of Borrelia immunoglobulin G (IgG) antibodies varies between 2% and 26%. The seroprevalence in young Swedish children is unknown and the relation to clinical data has not been previously studied.

Objective To determine the seroprevalence of Borrelia IgG antibodies in serum of young Swedish children and to relate it to gender, geographical location, reported tick bites, symptoms and previous treatment for LB.

Methods 2000 healthy 5-year-old children (n=2000) were randomly selected from among participants of a larger prospective population-based study, the ABIS (All Babies in Southeast Sweden) study. Serum samples were collected and a Borrelia specific ELISA test (Dako) were performed for IgG antibody detection. Clinical data were collected from questionnaires completed by the parents.

Results The seroprevalence of Borrelia IgG antibodies was 3.2% (64/2000). Previous tick bite had been noted in 66% of these seropositive children but the majority (94%) had not previously been treated for LB. In addition, another 55 children reported a history of LB but were negative to Borrelia IgG antibodies in serum. Many of these seronegative children had received treatment for erythema migrans (n=24), which is a clinical diagnosis. Whether children were correctly treated or overtreated for LB is however unknown. No differences in gender, geographical location or reported tick bites were found when comparing Borrelia-seropositive children (n=64) and seronegative children with previous LB (n=55).

Conclusion This population-based study demonstrates a Borrelia IgG antibody seroprevalence of 3.2% in young Swedish children. Very few of these seropositive children report previous symptoms or treatment for LB. Thus the findings suggest that exposure to the Borrelia spirochaete (with subsequent antibody response in serum) does occur in young children, mostly without giving rise to clinical LB. Future studies on cell-mediated immune responses are needed to investigate explanatory immunological mechanisms.