Life Cycle:
The life cycle of G. lamblia (also known as Giardia intestinalis or Giardia duodenalis) is composed of 2 stages: trophozoites and cysts.
Giardia infects humans after ingestion of as few as 10–100 cysts. Ingested cysts, which measure 8–10 mm in diameter, each produce 2 trophozoites in the duodenum. After excystation, trophozoites colonize the lumen of the duodenum and proximal jejunum, where they attach to the brush border of the intestinal epithelial cells and multiply by binary fission. The body of the trophozoite is teardrop shaped, measuring 10–20 mm in length and 5–15 mm in width. Giardia trophozoites contain 2 oval nuclei anteriorly, a large ventral disk, a curved median body posteriorly, and 4 pairs of flagella. As detached trophozoites pass down the intestinal tract, they encyst to form oval cysts that contain 4 nuclei.
Cysts are passed in stools of infected individuals and may remain viable in water for as long as 2 months. Their viability often is not affected by the usual concentrations of chlorine used to purify water for drinking.
Clinical Manifestations:
The incubation period of Giardia infection usually is 1–2 wk but may be longer. A broad spectrum of clinical manifestations occurs, depending on the interaction between G. lamblia and the host. Children who are exposed to G. lamblia may experience asymptomatic excretion of the organism, acute infectious diarrhea, or chronic diarrhea with persistent gastrointestinal tract signs and symptoms, including failure to thrive and abdominal pain or cramping. Most infections in both children and adults are asymptomatic.
Symptomatic infections occur more frequently in children than in adults. Most symptomatic patients usually have a limited period of acute diarrheal disease with or without low-grade fever, nausea, and anorexia; in a small proportion of patients, an intermittent or more protracted course characterized by diarrhea, abdominal distention and cramps, bloating, malaise, flatulence, nausea, anorexia, and weight loss develops .
Initially, stools may be profuse and watery and later become greasy and foul smelling and may float. Stools do not contain blood, mucus, or fecal leukocytes.
Varying degrees of malabsorption may occur.
Diagnosis:
Giardiasis should be considered in young children in child care or in any person who has had contact with an index case or a history of recent travel to an endemic area who has persistent diarrhea, intermittent diarrhea and constipation, malabsorption, crampy abdominal pain and bloating, failure to thrive, or weight loss.
Traditionally, a diagnosis of giardiasis has been established by microscopy documentation of trophozoites or cysts in stool specimens, but 3 stool specimens are required to achieve a sensitivity of >90%.
Stool enzyme immunoassay (EIA) or direct fluorescent antibody tests for Giardia antigens are less reader dependent and more sensitive for detection of Giardia than microscopy, and are now the tests of choice for giardiasis in most situations.
Treatment:
Children with acute diarrhea in whom Giardia organisms are identified should receive therapy. In addition, children who manifest failure to thrive or exhibit malabsorption or gastrointestinal tract symptoms such as chronic diarrhea should be treated.
The Food and Drug Administration (FDA) has approved tinidazole and nitazoxanide for the treatment of Giardia in the United States. Both medications have been used to treat Giardia in thousands of patients in other countries and have excellent safety and efficacy against Giardia.
Tinidazole has the advantage of single-dose treatment and very high efficacy (>90%), while nitazoxanide has the advantage of a suspension form, high efficacy (80–90%), and very few adverse effects.
Metronidazole was the treatment of choice for Giardia infection in the United States for many years, although it was never approved by the FDA for treatment of Giardia. When a full course of therapy is taken, metronidazole is highly effective (80–90% cure rate), and the generic form is considerably less expensive than tinidazole or nitazoxanide. However, frequent adverse effects are seen with metronidazole therapy, and it requires 3 times a day dosing for 5–7 days. Suspension forms of tinidazole and metronidazole must be compounded by a pharmacy; neither drug is sold in suspension form
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