Giardiasis has become an increasingly common cause of diarrhea and malabsorption in humans. Thirty-four outbreaks of giardiasis were reported in the United States from 1993 to 1994, compared with just six outbreaks in 1984. Ortega and Adam review the epidemiology and life cycle of Giardia and discuss current treatment and prevention.

Giardia lamblia is most often transmitted through contaminated water or food, or by the fecal-oral route. Outbreaks peak in late summer, except in day care centers, where no seasonal patterns have been observed. G. lamblia occurs throughout the world and is most prevalent in areas with poor water treatment or unsanitary conditions. The seroprevalence in developing countries ranges from 20 to 30 percent. Most of these persons are asymptomatic. A seroprevalence rate as high as 35 percent has been reported in children attending day care centers. Although many of these children are asymptomatic, they may transmit the infection to family members.

The usual incubation period for symptomatic giardiasis is one to two weeks but may vary from one to 45 days. However, as many as 60 percent of persons exposed to the infection remain asymptomatic. Patients who develop symptoms usually have loose diarrhea with foul-smelling, non-bloody stools. Other common symptoms include flatulence, abdominal cramps, bloating, anorexia, nausea and weight loss. Fever sometimes occurs at the onset of infection. Malabsorption is common and is the likely cause of the substantial weight loss that may occur. Unlike those with other forms of infectious diarrhea, patients with giardiasis will usually be symptomatic for one to two weeks before seeking medical attention. The illness may resolve spontaneously, but symptoms may persist for weeks and sometimes for months. Chronic infection occurs despite the presence of an antibody-mediated immune response. The reasons for this are unclear; however, the antibodies do seem to provide protection against newly acquired infection or reinfection.

The diagnosis of giardiasis is usually confirmed by the presence of cysts or, less frequently, trophozoites in stool specimens stained with trichome or iron hematoxylin. The sensitivity of this test can be improved by repeating the stool examination on one or two additional specimens. Giardia antigens can be detected in stool specimens using monoclonal antibodies or direct fluorescent assays. These tests should be considered if the results of routine stool examinations are nondiagnostic. Serodiagnosis is not useful because current antibody tests are unable to differentiate between present and prior infection.

In patients with persistent symptoms, a string test may be useful. In this test, the patient swallows a capsule on the end of a string which migrates to the jejunum, where the trophozoites attach. The string is withdrawn after four hours or more and can then be examined for trophozoites. Some clinicians prefer to proceed to esophagogastroduodenoscopy with duodenal aspiration and biopsy. This method aids in the detection of other diseases that may cause similar symptoms, such as lymphoma, Whipple's disease, cryptosporidiosis, isosporiasis or Crohn's disease.

Several effective treatments are currently available for patients with symptomatic giardiasis (see the accompanying table). The majority of patients will respond to a five-day course of metronidazole. Quinacrine hydrochloride had been considered the drug of choice for giardiasis but, due to the side effects of hemolysis and toxic psychosis in some patients with glucose-6 phosphate dehydrogenase deficiency, it is no longer commercially available in the United States. Tinidazole, given as a single dose, is widely used throughout the world but is not labeled for this use by the U.S. Food and Drug Administration. Furazolidone is currently the only drug labeled for the treatment of giardiasis in the United States and is often used to treat children. Albendazole seems to be comparable to metronidazole in efficacy and has fewer side effects. Paromomycin is a non-absorbable aminoglycoside that is not as effective as the other available agents but is commonly used for the treatment of giardiasis in pregnant women.

The prevention of infection with G. lamblia should focus primarily on the avoidance of contaminated water. Vigorous hand-washing and proper disposal of soiled diapers should be practiced in day care settings. Outbreaks of giardiasis have usually been associated with contaminated surface water or shallow wells. The most effective method of rendering Giardia cysts nonviable is boiling the water. Chlorination is not effective. Filtration with a pore size of 2 μm or smaller is also effective for removing Giardia cysts from water.