Reinfection with Lyme borreliosis: a retrospective follow-up study in southern Sweden.

In a 5-y retrospective follow-up study in southern Sweden that initially included > 1000 individuals with previously diagnosed erythema migrans, factors that influenced the risk of reinfection with Lyme borreliosis were elucidated. The total frequency of reinfection was 4% and the number of tick bites influenced the risk of reinfection: those bitten > 10 times during the observation period had a 4-8-fold increased risk compared with those bitten <5 times. Women manifested to a greater extent than men although both genders were bitten equally by ticks, thus indicating that women may be more susceptible to reinfection. All reinfected women were > 44 y old. The county of Kalmar including Oland was found to be highly endemic for reinfection with Lyme borreliosis. Thus the number of tick bites, gender, age and study area influenced the risk of reinfection.