Risk of acquiring tick bites in south-eastern Sweden

The incidence and the temporal pattern of tick bites were studied in a population frequently out-of-doors in a tick-endemic area in south-eastern Sweden between May 2000 and March 2001. The participants, who were well aware of tick-borne diseases, inspected their skin daily from May until September and completed a diary sheet, registering visited geographical places, time out-of-doors, observed tick bites, etc. The participants were also given questionnaires in both the initial and final stages of the study, asking questions about their earlier history of tick bites, previous tick-borne diseases and out-of-doors activities. The incidence was 0.04 [95% confidence interval (95% CI) 0.02-0.061 tick-bitten participants/10 h spent out-of-doors. In total, the participants registered 1767 tick bites, i.e. an incidence of 0.14 (95% CI 0.10-0.18) tick bites/10 h out-of-doors. Within 6 months after the registration period, 8/235 (3%) had been treated for physician-diagnosed Lyme borreliosis. In conclusion, this study found a 4% risk of being tick-bitten per 10 h spent out-of-doors. The risk of contracting Lyme borreliosis was 1/221 tick bites (0.5%, 95% CI 0.44-0.56). Thus, the results indicate a low risk of acquiring Lyme borreliosis when using daily tick checks and we underline the opinion of not recommending routine prophylactic treatment for observed tick bites.