This protozoan lives in the duodenum and proximal jejunum and is said to be the most frequent intestinal parasite in the United States. An estimated 3%-7% of U.S. adults may have the disease. The organism is usually transmitted through fecal (sewage) contamination of water. Chloridation will not kill Giardia lamblia, but iodine will. Some reports suggest a high incidence in male homosexuals. G. lamblia can also be transmitted by fecal-oral contact, especially in day-care centers, where it has been estimated that 5%-15% of young children of diaper age become infected. Acute infection typically shows foul-smelling watery diarrhea, usually accompanied by greasy floating stools, considerable intestinal gas, and epigastric pain. The symptoms may last 3-4 days or 1-3 weeks or may become chronic. In severe cases, steatorrhea and intestinal malabsorption have been reported.

Diagnosis. Diagnosis is made through the same type of stool examinations discussed under amebiasis. Overall detection rate of stool microscopy (direct examination plus stained slide) is usually considered to be 50%-70%. Cathartics do not increase detection rates. Permanent stain techniques detect about one-third more cases than wet mounts. Cyst excretion in the stool may be irregular; Giardia cysts may be passed at 1, 2-3, or even 7-8 day intervals. Although three specimens (one specimen collected every 2 days) are usually sufficient, more may be necessary. The first specimen detects about 75% of cases diagnosed through multiple specimens. Duodenal aspiration has been found to detect more infections (about 80% of cases) than repeated stool specimens. A commercial company has a string apparatus (Entero-Test) that can be swallowed instead of resorting to duodenal aspiration.

Serologic tests. ELISA tests for antibody have not been widely used, as current tests do not differentiate between past or currently active infection. A commercial test (Pro Spec T Giardia) for antigen detection in stool is commercially available and in several evaluations was found to have a sensitivity of 92%-100% compared to the total positive patients by ELISA plus standard O& P examinations. A commercial indirect immunofluorescent test is also available, and the manufacturer claims 97% sensitivity. To date, I have not seen any independent evaluations.