Application of genotyping during an extensive outbreak of waterborne giardiasis in Bergen, Norway, during autumn and winter 2004.

During the autumn and winter of 2004 and 2005, an extensive outbreak of waterborne giardiasis occurred in Bergen, Norway. Over 1,500 patients were diagnosed with giardiasis. Analysis of water from the implicated source revealed low numbers of Giardia cysts, but the initial contamination event probably occurred up to 10 weeks previously. While sewage leakage from a residential area is now considered to be the probable source of contamination, during the episode waste from one particular septic tank was thought to be a possible source. Genotyping of cysts from the septic tank demonstrated that they were assemblage A cysts, although the sequences were not identical to any previously published sequences. For the beta-giardin gene, the closest published subgenotype was subgenotype A3; for the gdh gene, the closest published subgenotype was subgenotype A2. Genotyping of cysts from 21 patient samples revealed that they were assemblage B cysts; thus, the septic tank was unlikely to be the contamination source. Sequencing of the beta-giardin and gdh genes from patient samples and a comparison of the sequences gave complex results. For the beta-giardin gene, three isolates had sequences identical to subgenotype B3 sequences. However, other isolates had between one and four single-nucleotide polymorphisms (SNPs). For the gdh gene, none of the sequences were identical to the sequence published for subgenotype B3, and the sequences had between one and three SNPs. One isolate, which was identical to subgenotype B3 at the beta-giardin gene, was more similar to subgenotype B2 at the gdh gene. Grouping the isolates on the basis of SNPs resulted in different groups for the two genes. The results are discussed in relation to giardiasis in Norway and to other Giardia genotyping studies.