The majority of infections are asymptomatic even in children, although disease most often occurs in children with an impaired humoral immunity. The incubation period is 3-25 days and symptoms of disease are mainly diarrhea and bloating, with abdominal pain. Occasionally, nausea and vomiting occur, and the acute phase of disease typically lasts several days. In some patients, giardiasis may be a prolonged gastrointestinal illness associated with food intolerance, episodes of recurring diarrhea and flatulence, and urticaria. The presence of concurrent bacterial infections apparently does not influence the pathogenicity of infection.

As with the human disease, is usually asymptomatic in animals and more commonly seen in the young. Symptoms in dogs and cats are similar to those in people, whereas only mild diarrhea occurs in calves and lambs.

Diagnosis is confirmed by finding Giardia cysts in feces. Cysts are seen in formed stool, whereas trophozoites are usually seen in diarrheal stools. As cysts are eliminated intermittently, at least three samples taken every other day should be examined to rule out infection. Stool concentration methods are helpful in detecting infection. For trophozoites, fresh and stained stool preparations are examined for the presence of the flagellate form of the parasite. Tests using specific fluorescent antibodies or ELISA are also available and afford increased sensitivity in detecting past and present exposure. It is important to remember that the finding Giardia cysts in an individual with gastrointestinal illness may not be causally related, so that other etiologies need to be investigated as well.

Public water suppies should be protected from contamination with human and animal feces that could harbor Giardia spp. Removal and sanitation of human and animal waste in the environment is also important. Water should be boiled in situations of a potential exposure, and travelers should drink only bottled water. While there is no evidence that domestic animals are a significant source of infection for humans, dogs and cats infected with Giardia cysts should be treated because of the likelihood of contact with children.

A killed vaccine for canine giardiasis is available but is currently not recommended for routine use by the 2006 Canine Vaccine Guidelines of the American Animal Hospital Association. The vaccine may prevent oocyst shedding but does not prevent infection. There is no duration of immunity data for this vaccine, because it doesn't prevent infection. Most puppies and kittens exposed to Giardia spp. develop sub-clinical infection, and are not considered a zoonotic risk to immunocompetent humans.

Metronidazole or fenbendazole are commonly recommended treatments for giardiasis. Bathing during treatment is also advised to prevent re-infection from ingestion of Giardia oocysts during grooming. Control measures should be employed to prevent re-infection.

References: Shedlock and Weiner, J Leuk Biol 68: 973-806, 2000; Giardiasis ICD-10 AO7.1. Pan American Health Organization, Zoonoses and Communicable Diseases, vol. 3, pp 52-58; Paul et al, AAHA Canine Vaccine Task Force, JAAHA March 2006; pp 8, 27.

Test Profiles: Giardia ELISA Antigen Test (Profile # T820 [East & West]); Fecal Combo Test (Profile # 405 [East]; # 85862 [West]), includes ZnSO4 Fecal Centrifugal Flotation Test and Giardia ELISATest.