Molecular characterisation of Giardia isolates from clinical infections following a waterborne outbreak

During autumn/winter 2004/2005 an outbreak of waterborne giardiasis occurred in Bergen, Norway. Genetic characterisation at 2 genes of Giardia duodenalis isolates from samples from the outbreak peak showed significant variations between isolates. Characterisation of further isolates from patients diagnosed in the subsequent months was conducted to determine whether isolates with particular sequences might predominate, or whether the sequence variation would continue.

Genetic characterisation was conducted on 63 isolates from patients diagnosed in the 12 months subsequent to the outbreak peak.

At the beta-giardin gene and glutamate dehydrogenase gene, particular isolate sequences within Assemblage B, gradually predominated over time. These sequences had not been the most frequently identified amongst 21 isolates from the outbreak peak. Nor were they apparently associated with a particular sequence at the triose phosphate isomerase gene.

The predominance of particular sequences at the beta-giardin gene and glutamate dehydrogenase gene over time suggests that these sequences may be associated with enhanced transmission characteristics such as higher virulence, greater cyst environmental resistance, increased proliferation, or a combination of these factors. Alternatively greater association with clinical disease may have led to increased submission of samples with these sequences. Whether these sequences may be associated with particular symptom characteristics such as overt clinical disease, infection persistence or unresponsiveness to treatment warrants further study.