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FROM 1977 through May 1989, 30 cases of Lyme disease (LD [Lyme borreliosis]) were reported to Canada's Laboratory Centre for Disease Control (LCDC). Ontario is the only province in which LD is a reportable condition; however, active laboratory surveillance for LD is conducted in other provinces. In Ontario, the diagnosis of LD is used on recognition of erythema migrans (EM), with involvement of at least two of the three organ systems usually affected by LD (joints, nervous system, and cardiovascular system), or EM and an indirect immunofluorescence antibody titer greater than or equal to 1:128 or an enzyme-linked immunosorbent assay optical density greater than or equal to 0.40, or EM and isolation of Borrelia burgdorferi. Without EM, diagnosis is based on involvement of at least one organ system and positive serology or isolation (C. LeBer, Ontario Ministry of Health, personal communication, 1989).
For 25 (83%) of the 30 cases, exposure