The anaerobic protozoan parasite Giardia intestinalis (formerly known as G. lamblia or G. duodenalis).
Giardia is transmitted via the fecal-oral route. Its low infectious dose, protracted communicability, and moderate chlorine tolerance make Giardia ideally suited for transmission through drinking and recreational water. Transmission also occurs through person-to-person contact, such as caring for an infected person, through sexual contact, through eating food contaminated by infected food handlers or by contaminated water used for irrigation or washing food, and by contact with fecally contaminated surfaces.
Giardia is endemic worldwide. Giardia-related acute diarrhea was a top 10 diagnosis in ill US travelers returning from the Caribbean, Middle East, Eastern Europe, Central America, South America, North Africa, sub-Saharan Africa, and South-Central Asia. The risk of infection increases with duration of travel. Backpackers or campers who drink untreated water from lakes or rivers are also more likely to be infected. Giardia is commonly identified in routine screening of refugees and internationally adopted children, although many are asymptomatic.
Many infected are asymptomatic, though if symptoms develop, they typically develop 1–2 weeks after infection and generally resolve within 2–4 weeks. Symptoms include diarrhea (often with foul-smelling, greasy stools), abdominal cramps, bloating, flatulence, fatigue, anorexia, and nausea. Usually, a patient presents with the gradual onset of 2–5 loose stools per day and gradually increasing fatigue. Sometimes upper gastrointestinal symptoms are more prominent. Weight loss may occur over time. Fever and vomiting are uncommon. Reactive arthritis, irritable bowel syndrome, and other chronic symptoms sometimes occur after infection with Giardia (see Chapter 5, Persistent Travelers’ Diarrhea).
Giardia cysts or trophozoites are not consistently seen in the stools of infected patients. Diagnostic yield can be increased by examining up to 3 stool samples over several days. Direct fluorescent antibody testing is extremely sensitive and specific. Rapid immunochromatographic cartridge assays also are available but should not take the place of routine ova and parasite examination. Only molecular testing (such as PCR) can be used to identify the subtypes of Giardia. Retesting is only recommended if symptoms persist after treatment. Giardiasis is a nationally notifiable disease.
Effective treatments include metronidazole, tinidazole, and nitazoxanide. An alternative is paromomycin. Because making a definitive diagnosis is difficult, empiric treatment can be used in patients with the appropriate history and typical symptoms.
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