Genetic heterogeneity at the β-giardin locus among human and animal isolates of Giardia duodenalis and identification of potentially zoonotic subgenotypes

Human giardiasis, caused by the intestinal flagellate Giardia duodenalis, is considered a zoonotic infection, although the role of animals in the transmission to humans is still unclear. Molecular characterisation of cysts of human and animal origin represents an objective means to validate or reject this hypothesis. In the present work, cysts were collected in Italy from humans (n=37) and animals (dogs, one cat and calves, n=46), and were characterised by PCR amplification and sequencing of the β-giardin gene. As expected, only Assemblages A and B were identified among human isolates. The host-specific Assemblages C and D were found in the majority of dog isolates; however, 6 dog isolates were typed as Assemblage A. The cat-specific Assemblage F has been identified in the single feline isolate available. Among calf isolates, most were typed as Assemblages A (n=12) and B (n=5), whereas the host-specific Assemblage E was rarely found (n=3). Sequence heterogeneity in the β-giardin gene allowed a number of subgenotypes to be identified within Assemblage A (8 subgenotypes), B (6 subgenotypes), D (2 subgenotypes), and E (3 subgenotypes). Five of these subgenotypes, namely A1, A2, A3, A4 and B3, were found to be associated with infections of humans, of dogs and of calves; these data, therefore, supported the role of these animals as a source of infection for humans.