The anaerobic protozoan parasite Giardia duodenalis (formerly known as G. lamblia or G. intestinalis).
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 contact with feces (for example, when providing direct patient care or during sexual activity), eating contaminated food, or contact with fecally contaminated surfaces.
Giardia is endemic worldwide, including in the United States. 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 people are asymptomatic, though if symptoms develop, they typically develop 1–2 weeks after exposure 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 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 11, Travelers’ Diarrhea in Returned Travelers).
Giardia cysts or trophozoites are not consistently seen in the stools of infected patients. Diagnostic sensitivity can be increased by examining up to 3 stool specimens over several days. New molecular enteric panel assays generally include Giardia as a target pathogen. Diagnostic techniques include microscopy with direct fluorescent antibody testing (considered the gold standard), rapid immunochromatographic cartridge assays, enzyme immunoassay kits, microscopy with trichrome staining, and molecular assays. Only molecular testing (such as PCR) can be used to identify the genotypes and subtypes of Giardia. Retesting is recommended only if symptoms persist after treatment. In the United States, 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.
The best defense against giardiasis is thorough frequent handwashing, strict adherence to standard food and water precautions (see Chapter 2, Food & Water Precautions), and minimizing fecal–oral exposures during sexual activity.
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