This disorder is the result of infection with the pathogen Giardia lamblia, and it is one of the most common parasitic infections worldwide. It is associated with epidemics of water-borne diarrhea, but person-to-person and food-borne transmission may also occur. Outbreaks of infection have originated in contaminated municipal water supplies, infected animals, swimming pools, restaurants, nursing homes, and day-care centers.

The life cycle of G. lamblia entails two stages, trophozoite and cyst. Infection occurs when cysts are ingested (as few as 10 cysts may initiate infection) via contaminated food or water. In the stomach, the cysts rupture releasing trophozoites which then migrate into the duodenum and jejunum. In these portions of the small intestine, the trophozoites attach to the brush border of the intestinal epithelial cells. Cysts are then produced and excreted in the feces. Cellular invasion does not occur.

Giardiasis manifests as diarrhea, nausea, anorexia, flatulence, and weight loss. Patients often complain of greasy, foul-smelling stools (this is secondary to hydrogen sulfide production). Symptoms develop 1 to 2 weeks after ingestion, and if effective therapy is not administered, they may persist for months.

Associated laboratory abnormalities are uncommon. The CBC usually does not show leukocytosis or other abnormalities; however, when children are infected, they may manifest a mild normocytic anemia. Fecal specimens are without leukocytes or blood. The first step in establishing the diagnosis is to examine a stool sample for  Giardia antigen. The diagnosis may also be established by small-bowel biopsy in patients who are undergoing endoscopic examination or via stool examination for parasites (multiple stools may be required).

Either quinacrine hydrochloride (Atabrine 100 mg PO TID x 7-10 days) or metronidazole (Flagyl, Prostat 250 mg PO TID x 7-10 days) are effective first-line therapy. Cure is established in 90-95% of patients who receive therapy. If initial therapy is ineffective, then therapy with the other first-line agent should be offered. If this is also ineffective, options for cure include administering both first-line agents together at full dose for 2 weeks, or another course of therapy with metronidazole may be administered concomitantly with propranolol hydrochloride (Inderal 10-40 mg PO BID). Propranolol serves to inhibit the growth and motility of G. lamblia. If symptoms still persist, a second etiology for the patients symptoms should be suspected.

Prophylaxis entails routine boiling of water or ingesting only bottled water when traveling to areas where the water supply may be contaminated. Also, the ingestion of uncooked fruits or vegetables should be discouraged while in endemic areas. Household and sexual contacts of infected individuals should undergo evaluation for possible infection.