Giardia cysts isolated from fecal samples obtained from humans (Homo sapiens), beaver (Castor canadensis), dogs (Canis familiaris), cats (Felis domesticus), bighorn X mouflon sheep (Ovis canadensis X O. musimon), guinea pig (Cavis porcellus), muskrat (Ondatra ziethica) and mule deer (Odocoileus hemionus) were given to a variety of experimental animals. Human source Giardia established infections in dogs, cats, beaver, rats (Rattus norvegicus), gerbils (Gerbillus gerbillus), guinea pig, raccoon (Procyon lotor), bighorn X mouflon sheep and pronghorn (Antilocapra americana). Giardia cysts from beaver successfully infected dogs. A dog was infected with Giardia from a bighorn X mouflon sheep which was infected with human source Giardia. Human source Giardia was used to infect cats and cysts from the cats were used to successfully infect dogs. Evidence exists that once dogs are treated with metronidazole and then re-exposed to Giardia they become infected yet do not shed cysts. This most likely occurs in natural cases of giardiasis in dogs where the animal stops shedding cysts yet has a latent infection. Female dogs and cats may start shedding Giardia cysts 3-4 weeks after parturition.