Low risk of developing Borrelia burgdorferi infection in the south-east of Sweden after being bitten by a Borrelia burgdorferi-infected tick.

Objectives: The risk of developing Lyme borreliosis (LB) from Borrelia burgdorferi sensu lato (Bb)-infected ticks in Sweden is largely unknown. In the current study, we investigated the prevalence of Bb in ticks that had bitten humans and the risk of developing LB from Bb-infected ticks.

Methods: Health questionnaires, blood samples, and ticks were collected from 394 tick-bitten study subjects in the County of Östergötland, Sweden, at the time of the tick bite. Questionnaires and blood samples were also collected 3 months later. Ticks were screened for Bb DNA with PCR, while sera were analyzed for antibodies against Bb using two ELISA assays. Seroconversion, i.e., an at least two-fold increase in anti-Bb antibodies after 3 months, was confirmed using a Strip-Immunoassay.

Results: Seventy-five of 397 ticks collected from the study subjects were determined to be Bb-positive. Sixty-four of the tick-bitten subjects had been bitten by Bb-infected ticks. Four of them showed seroconversion and were therefore considered to have an active Bb infection. None of these four subjects had sought health care due to symptoms, but one reported symptoms.

Conclusions: Our data suggest that the risk of developing LB after being bitten by a Bb-infected tick is low, and asymptomatic Bb infections appear to be more frequent than symptomatic infections.