Giardiasis is infection with the flagellated protozoan Giardia intestinalis (lamblia). Infection can be asymptomatic or cause symptoms ranging from intermittent flatulence to chronic malabsorption. Diagnosis is by identifying the organism in fresh stool or duodenal contents or by assays of Giardia antigen in stool. Treatment is with metronidazole, tinidazole, or nitazoxanide or, during pregnancy, paromomycin.

Giardia trophozoites firmly attach to the duodenal and proximal jejunal mucosa and multiply by binary fission. Some organisms transform into environmentally resistant cysts that are spread by the fecal-oral route. Waterborne transmission is the major source of giardiasis. Transmission can also occur by ingestion of contaminated food and by direct person-to-person contact, especially in mental institutions and day care centers or between sex partners. Giardia cysts remain viable in surface water and are resistant to routine levels of chlorination. Wild animals may also serve as reservoirs. Thus, mountain streams as well as chlorinated but poorly filtered municipal water supply systems have been implicated in waterborne epidemics.

Symptoms and Signs

Many cases are asymptomatic. However, asymptomatic people can pass infective cysts.

Symptoms of acute giardiasis usually appear 1 to 2 wk after infection. They are usually mild and include watery malodorous diarrhea, abdominal cramps and distention, flatulence, eructation, intermittent nausea, epigastric discomfort, and sometimes low-grade malaise and anorexia. Acute giardiasis usually lasts 1 to 3 wk. Malabsorption of fat and sugars can lead to significant weight loss in severe cases. Neither blood nor WBCs are present in stool.

A subset of infected patients develop chronic diarrhea with foul stools, abdominal distention, and malodorous flatus. Substantial weight loss may occur. Chronic giardiasis occasionally causes failure to thrive in children.

Diagnosis

• Enzyme immunoassay for antigen in stool
• Microscopic examination of stool

Enzyme immunoassay to detect parasite antigen in stool is more sensitive than microscopic examination. Characteristic trophozoites or cysts in stool are diagnostic, but parasite excretion is intermittent and at low levels during chronic infections. Thus, microscopic diagnosis may require repeated stool examinations. Sampling of the upper intestinal contents can also yield trophozoites but is seldom necessary. Specific DNA probes are under evaluation.

Treatment

• TinidazoleSome Trade Names
  TINDAMAX
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  , metronidazoleSome Trade Names
  FLAGYL
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  , or nitazoxanideSome Trade Names
  ALINIA
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For symptomatic infections, metronidazoleSome Trade Names
FLAGYL
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250 mg po tid in adults (5 mg/kg po tid in children) for 5 to 7 days can be used. Adverse effects include nausea, headaches, and a disulfiramSome Trade Names
ANTABUSE
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-like effect if alcohol is consumed concurrently. TinidazoleSome Trade Names
TINDAMAX
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2 g once in adults (50 mg/kg [maximum 2 g] in children) is as effective as and less toxic than metronidazoleSome Trade Names
FLAGYL
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. Neither of these drugs should be taken with alcohol.

NitazoxanideSome Trade Names
ALINIA
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is given orally for 3 days as follows: age 1 to 3 yr, 100 mg bid; age 4 to 11 yr, 200 mg bid; and age > 12 yr (including adults), 500 mg bid. It is available in liquid form for children.

FurazolidoneSome Trade Names
FUROXONE

and quinacrine are effective but are now rarely used because of potential toxicity.

MetronidazoleSome Trade Names
FLAGYL
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and tinidazoleSome Trade Names
TINDAMAX
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should not be given to pregnant women. NitazoxanideSome Trade Names
ALINIA
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is in pregnancy category B. If therapy cannot be delayed because of symptoms, the nonabsorbable aminoglycoside paromomycinSome Trade Names
HUMATIN
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(8 to 11 mg/kg po tid for 5 to 10 days) is an option.

Prevention

Prevention requires appropriate public water treatment, hygienic food preparation, and appropriate fecal-oral hygiene. Water can be decontaminated by boiling. Giardia cysts resist routine levels of chlorination. Disinfection with iodine-containing compounds is variably effective and depends on the turbidity and temperature of the water and duration of treatment. Some handheld filtration devices can remove Giardia cysts from contaminated water, but the efficacy of various filter systems has not been fully assessed.

Treatment of asymptomatic cyst passers can theoretically reduce the spread of infection, but whether it is cost-effective remains unclear.

Last full review/revision December 2009 by Richard D. Pearson, MD

Content last modified February 2012