Author(s): Katharine Benedict, Dawn Roellig

Infectious Agent

Giardiasis is an illness caused by the anaerobic protozoan parasite Giardia duodenalis (formerly known as G. lamblia or G. intestinalis).

Transmission

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 (e.g., when providing direct patient care or during sexual activity), eating contaminated food, or contact with fecally contaminated surfaces.

Epidemiology

Giardia is endemic worldwide, including in the United States. Based on GeoSentinel Global Surveillance Network data from 2000–2012, Giardia-related acute diarrhea was a top 10 diagnosis in ill US travelers returning from destinations in Africa (North Africa and sub-Saharan Africa), the Americas (the Caribbean, Central America, and South America), Asia (South-Central Asia), Eastern Europe, and the Middle East. The risk for infection increases with duration of travel and travel within areas that have poor sanitation. 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.

Clinical Presentation

Many infected people are asymptomatic; if symptoms develop, they typically develop 1–2 weeks after exposure and generally resolve within 2–4 weeks. Symptoms include abdominal cramps, anorexia, bloating, diarrhea (often with foul-smelling, greasy stools), flatulence, and nausea. Patients usually present with a history of gradual onset of 2–5 loose stools per day and increasing fatigue. Sometimes upper gastrointestinal symptoms are prominent. Weight loss can occur over time. Fever and vomiting are uncommon. Reactive arthritis, irritable bowel syndrome, and other chronic symptoms sometimes occur after infection with Giardia (see Sec. 11, Ch. 7, Persistent Diarrhea in Returned Travelers). In children, severe giardiasis can cause development delay, failure to thrive, malnutrition, and stunted growth.

Diagnosis

Giardia cysts or trophozoites are not seen consistently in the stools of infected patients. Diagnostic sensitivity can be increased by examining ≤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), microscopy with trichrome staining, enzyme immunoassay kits, rapid immunochromatographic cartridge assays, and molecular assays. Only molecular testing (e.g., DNA sequencing) can identify the genotypes and subtypes of Giardia. Retesting is recommended only if symptoms persist after treatment.

Health care professionals seeking laboratory support should consult their usual diagnostic laboratory with questions about appropriate testing. If testing beyond the capacity of the diagnostic laboratory is warranted, the diagnostic laboratory should reach out to public health officials (state or county as appropriate) for information and guidance on specimen submission, including submission to the Centers for Disease Control and Prevention (CDC), if appropriate.

Treatment

Effective treatments include metronidazole, tinidazole, and nitazoxanide. Alternative treatments include furozolidone, paromomycin, and quinacrine. Because a definitive diagnosis is difficult, empiric treatment can be used in patients with the appropriate history and typical symptoms.

Prevention

Travelers should follow safe water precautions, use appropriate sanitation, and practice good handwashing to avoid giardiasis. Travelers also should avoid drinking water and recreational water that could be contaminated. If the safety of drinking water is in doubt (e.g., during travel to a location with poor sanitation or lack of water treatment systems), travelers should follow recommended safe water precautions, including drinking commercially bottled water from an unopened factory-sealed container, or treating the water to make it safe for drinking. For more details, see Sec. 2, Ch. 8, Food & Water Precautions, and Sec. 2, Ch. 9, Water Disinfection.

Instruct travelers to avoid swallowing or drinking untreated water (even small amounts) from lakes, the ocean, ponds, rivers, springs, streams, or shallow wells. Travelers also should avoid swallowing water when swimming or recreating in hot tubs, interactive fountains, and swimming pools.

Travelers should wash hands frequently with soap and clean, running water for ≥20 seconds, rubbing hands together to make a lather, and making certain to lather backs of hands and between fingers and to scrub under nails. Travelers should especially wash hands before, during, and after preparing food; before eating; before and after caring for someone who is sick; after using the toilet, changing diapers, or cleaning a child who has used the toilet; and after touching an animal, animal waste, or animal environments.

In addition, travelers should prevent contact and contamination with feces during sex by using a barrier during oral–anal sex, and washing hands immediately after handling a condom used during anal sex and after touching the anus or rectal area of sexual partner(s).

In the United States, giardiasis is a nationally notifiable disease. State health departments should report outbreaks of giardiasis affecting multiple people to the CDC. Clinicians should inform local, state, and federal health authorities about cases of giardiasis so that appropriate public health responses can be taken to help control the spread of this disease.

CDC websites: Giardia; Waterborne Disease & Outbreak Surveillance Reporting: National Notifiable Diseases