A SECOND COMMUNITY OUTBREAK OF WATERBORNE GIARDIASIS IN CANADA AND SEROLOGICAL INVESTIGATION OF PATIENTS

A waterborne outbreak of giardiasis which occurred 5 years after anoth er in the same town in Canada was investigated. Sera from residents de fined as cases or non-cases were tested by enzyme-linked immunosorbent assay (ELISA) and compared with sera from symptomatic and asymptomati c control groups. The outbreak-associated Giardia isolate was retrieve d from contaminated drinking water and antigen from this strain was us ed in the serological investigation. Up to 84% of cases were identifie d by ELISA. More cases were identified by elevated immunoglobulin (Ig) G than by either elevated anti-Giardia IgA or IgM levels. Residents o f the community infected during the first outbreak were significantly less likely to have been reinfected during the second outbreak. This i s the first report of a second waterborne outbreak occurring in a comm unity and results of the investigations are consistent with an acquire d, protective immunity lasting at least 5 years.