Ixodes ricinus spirochete infection as the cause of postinfectious arthritis in Sweden

Antibodies to Swedish Ixodes ricinus spirochete were determined by an enzyme-linked immunosorbent assay (ELISA) in sera from 298 patients with postinfectious arthritis. Sera from healthy individuals, patients with acute infectious meningitis of proven etiology and patients with multiple sclerosis served as controls. With the upper limit of normal values set at the 95 percentile of controls, 18 of 298 (6%) arthritis patients had positive serum antibody titres. Titres above the 100 percentile of controls were found in 5 of 298 (2%) arthritis patients. Two of the arthritis patients had extremely high titres--higher than any earlier found in Swedish patients with spirochetal meningitis. The clinical manifestations and laboratory findings in the 5 patients with high spirochetal antibody titres are described. It is concluded that a spirochetal etiology should be considered in patients with reactive or postinfectious arthritis of unknown origin.