Genotyping Giardia duodenalis isolates from dogs: lessons from a multilocus sequence typing study.

Giardiasis is a common infection of dogs, and the occurrence of both zoonotic and host-adapted assemblages of Giardia duodenalis is well documented in this host. In the current study, G. duodenalis isolates from dogs collected in Croatia from both private owners (n=44) and kennels (n=52) were analyzed at four genetic loci: the ITS1-5.8S-ITS2 (ITS), the glutamate dehydrogenase (gdh), the triosephosphate isomerase (tpi), and the beta-giardin (bg). Both generic and assemblage D specific primers were used for the amplification of the tpi gene. All data were stored in a dedicated database, and analyzed to evaluate (1) the rate of amplification of G. duodenalis DNA from dogs at the four loci; (2) the distribution of assemblages and the occurrence of mixed infections; (3) the genetic variability at the intra-assemblage level; and (4) the zoonotic potential. We found that only half of the isolates could be amplified at either the gdh or the bg gene, whereas the combined use of generic and D-specific tpi primers yielded the highest amplification rate (85%). Sequence analysis showed that assemblages C and D are largely predominant in both kennel and household dogs, thus suggesting a minor role of dogs in zoonotic transmission of giardiasis. However, in many kennel dogs, incongruent results were obtained by using different markers, a result that is more likely explained by mixed infections rather than by genetic recombination. Phylogenetic analysis based on single or multiple loci failed to reveal the presence of distinct subpopulations within assemblages C and D. Our study illustrates the problems associated with the characterization of G. duodenalis isolates from dogs, and it casts doubts on the interpretation of genotyping results based on the analysis of single markers. We concluded that the current typing scheme is not suited to distinguish between recombinants and mixed infections in field isolates.