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From May 1991 to May 1994,
Lyme borreliosis was studied prospectively in 301 residents living on Aspö, a highly endemic area for the
disease. The study included annual questionnaires and blood samples for serology. Immunoglobulin G (IgG) antibodies to Borrelia burgdorferi sensu lato were detected by enzyme-linked immunosorbent assay in 63/301 (21%) of the residents at the start of the study. Seropositivity rates increased with time, and 3 years later 101/301 (34%) were positive. A total of 34 individuals developed physician-verified manifestations of
Lyme borreliosis during the study period. Nine individuals developed an erythema migrans, despite a previously treated
Lyme borreliosis or pre-existing high levels of IgG antibodies to B. burgdorferi s.l.