Giardiasis is a well-recognized problem in special groups including travelers, campers, male homosexuals, and persons with impaired immune states. After the cysts are ingested, trophozoites emerge in the duodenum and jejunum. They can cause epithelial damage, atrophy of villi, hypertrophic crypts, and extensive cellular infiltration of the lamina propria by lymphocytes, plasma cells, and neutrophils. Giardia infections in dogs and cats may be inapparent or produce weight loss and chronic diarrhea or steatorrhea, which can be continuous or intermittent, particularly in puppies and kittens. Calves with clinical giardiasis have been reported.
Feces are usually soft, poorly formed, pale, and contain mucus.
Gross intestinal lesions are seldom evident, although microscopic lesions, consisting of villous atrophy and cuboidal enterocytes, may be present. Most infections are asymptomatic. In some cases, acute or chronic diarrhea, mild to severe, with bulky, greasy, frothy, malodorous stools, free of pus and blood. Upper abdominal discomfort, cramps, distention, excessive flatus, and lassitude. Diagnosis is by identifying cysts or trophozoites in feces or duodenal fluid.