The presence of Giardia cysts in the feces of dogs and cats is fairly common. Depending on the population sampled, the prevalence in dogs and cats can vary from 0% to more than 50%. Not all infected animals have clinical signs; some will have only mild or transitory diarrhea, and some may have severe and chronic diarrhea. Regardless, both dogs and cats are routinely infected by this flagellated protozoan (Figure 1, above).
Indications In the recent past, the taxonomy of Giardia was ruled by those who believed that nearly all Giardia species in domestic animals were the same; this species was called Giardia intestinalis. However, current molecular epidemiologic assays have indicated that dogs, cats, and humans each essentially have their own set, or assemblage, of organisms. Some crossover between the organisms in cats and dogs and other hosts does occur, but this is relatively uncommon. In the old nomenclature, we would have called the species in dogs Giardia canis, in cats Giardia felis, and in humans Giardia lamblia. Veterinarians should note two important points. First, the zoonotic threat of Giardia is not considered to be as great as it was several years ago. Second, the antigen from which the new SNAP Giardia test for dogs and cats has been developed should detect any of the Giardia assemblages. Thus, the test should be robust and accurate in a cat or a dog regardless of the original source of infection.
This protozoan parasite has two forms: the motile flagellate stage (i.e., the trophozoite) and the nonmotile transmission stage (i.e., the cyst). The cyst is the stage most commonly found in samples from animals with formed feces. Trophozoites are sometimes found in semiformed or liquid feces and are best visualized in direct fecal smears diluted in saline. Trophozoites are harder to find than cysts, especially when present in small numbers.