RIBOSOMAL-RNA SEQUENCING REVEALS DIFFERENCES BETWEEN THE GENOTYPES OFGIARDIA ISOLATES RECOVERED FROM HUMANS AND DOGS LIVING IN THE SAME LOCALITY

A polymerase chain reaction-based method for genotyping Giardia duoden alis isolates using a polymorphic region near the 5' end of the small subunit ribosomal (SSU) RNA gene is described. Analysis was performed using Giardia cysts purified directly from feces. isolates were collec ted from humans and dogs living in isolated Aboriginal communities whe re Ginrrlin infections are highly endemic. This is the first report of the genetic characterization of Giardia from dogs and humans living i n the same locality. Comparison of the SSU-rRNA sequences from 13 huma n and 9 dog isolates revealed 4 different genetic groups. Groups 1 and 2 contained all of the human isolates. whereas groups 3 and 4 consist ed entirely of Giardia samples recovered from dogs. One dog sample con tained templates from both groups 2 and 3. These results suggest that zoonotic transmission of Giardia infections between humans and dogs do es not occur frequently in these communities. The dog-associated SSU-r RNA sequences have not been reported before, suggesting a new G. duode nalis subgroup. A genetic basis for the differences observed between t he groups was supported by sequence analysis of 9 in vitro cultured is olates that were placed into the same genetic groups established by en zyme electrophoresis.