Giardia lamblia causes the most common gastrointestinal parasite infection in the United States, one with a highly variable presentation. Symptoms are usually chronic and intermittent, characterized by diarrhea without blood or mucus, flatulence, abdominal pain, belching, malabsorption, and failure to thrive or lack of weight gain. Less commonly, patients have upper gastrointestinal symptoms including nausea, vomiting and dehydration. Fever is uncommon and usually suggests another, perhaps accompanying, pathogen. Nash reviews the diagnosis and treatment of children with G. lamblia infection.

When G. lamblia is present in reasonable numbers, results of stool examination easily provide the diagnosis. Cysts are excreted, but the motile trophozoite—the form of the parasite that multiplies and causes disease—can be found in looser stools. If the number of organisms is small, stool examination can be difficult and produce inaccurate results. Stool Giardia antigen detection assays that are better than 90 percent sensitive and specific have recently become available. Rarely, patients who have repeated negative assay results but are highly suspect on a clinical basis may require more invasive procedures to make the diagnosis. Asymptomatic G. lamblia infections are relatively frequent; therefore, other pathogens should be sought in symptomatic patients.

The drugs commonly used for treatment of G. lamblia in children include metronidazole, quinacrine, furazolidone and paromomycin. Furazolidone, a nitrofuran compound available in liquid form, is more acceptable and tolerable in young children than the other agents. Cure rates range from 80 to 100 percent but, in older children, are usually lower with furazolidone than with metronidazole and quinacrine. Adverse effects of furazolidone include nausea and/or vomiting (approximately 7 percent of patients), hemolysis secondary to glucose 6-phosphatase deficiency and nitrofuran sensitivity reactions. Furazolidone is administered at 6 to 8 mg per kg per day in three to four divided doses for seven to 10 days.

Metronidazole is the drug most commonly used in adults to treat G. lamblia infection although it is not approved by the U.S. Food and Drug Administration (FDA) for this indication. It has a highly unpleasant, bitter taste and is not well tolerated by children. Side effects include dyspepsia. In children, it is administered at 15 mg per kg per day divided into three doses for five to seven days. Cure rates range from 60 to 100 percent. Quinacrine is no longer routinely available in the United States, but it can still be obtained to manage difficult cases; it is administered in doses of 2 mg per kg per day (maximal dosage, 300 mg per day) in three divided doses, for a cure rate that exceeds 90 percent. Adverse effects include upper gastrointestinal symptoms, yellow skin and urine, and occasional reversible psychoses.

Paromomycin is a nonabsorbed aminoglycoside that is preferred in situations where absorption of a drug is undesirable (e.g., during pregnancy). Although there is clear efficacy in vitro and in vivo, its usefulness has not been well studied. It is usually administered at 25 to 35 mg per kg per day divided into three doses for seven days. Albendazole is not FDA-approved for this indication. It has broad activity against a variety of helminths and has some in vitro and in vivo activity against G. lamblia, but there is little experience with use of this drug in children six years or younger in developed countries. If standard therapeutic regimens fail because of immunodeficiency or other issues, combinations of metronidazole and quinacrine can be administered for three weeks.

The author concludes that all symptomatic or likely symptomatic patients should be treated. Treatment should also be considered in asymptomatic patients who may be a source of infection to others. If reinfection is likely, withholding therapy should be considered. In heavily infected endemic populations, asymptomatic G. lamblia infections are common, and reinfection is practically universal by three months after treatment, meaning that medical treatment is not useful.