The anaerobic protozoan parasite Giardia intestinalis (formerly known as G. lamblia or G. duodenalis).
By ingesting Giardia cysts in contaminated food or water, including water swallowed while swimming. 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 ingestion of food contaminated by ill food handlers or irrigated or washed with contaminated water, and by contact with fecally contaminated surfaces.
Giardia is endemic worldwide. Giardia was the most commonly diagnosed pathogen in travelers seeking medical attention for a gastrointestinal infection at GeoSentinel surveillance clinics worldwide after returning from the Caribbean, Central and South America, Western Europe, North Africa, sub-Saharan Africa, the Middle East, South Asia, and East Asia and was the second most commonly diagnosed pathogen in travelers returning from Southeast Asia and Australasia. 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 infections are commonly identified in internationally adopted children, although many are asymptomatic.
Symptoms typically develop 1–2 weeks after infection and generally resolve within 2–4 weeks. Signs and symptoms include diarrhea (often with foul-smelling, greasy stools), abdominal cramps, bloating, flatulence, fatigue, anorexia, and nausea. Typically, 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, 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. Other immunodiagnostic kits that do not require microscopy also detect Giardia antigens but do not take the place of ova and parasite examinations. Retesting is only recommended if symptoms persist after treatment.
Tinidazole, metronidazole, and nitazoxanide are the drugs of choice; paromomycin is an alternative drug available in the United States. Because making a definitive diagnosis is difficult, empiric treatment can be used in patients with the appropriate history and typical symptoms.
Abramowicz M, editor. Drugs for Parasitic Infections. New Rochelle, NY: The Medical Letter, Inc.; 2013.
Cantey PT, Roy S, Lee B, Cronquist A, Smith K, Liang J, et al. Study of nonoutbreak giardiasis: novel findings and implications for research. Am J Med. 2011 Dec;124(12):1175 e1–8.
Greenwood Z, Black J, Weld L, O’Brien D, Leder K, Von Sonnenburg F, et al. Gastrointestinal infection among international travelers globally. J Travel Med. 2008 Jul–Aug;15(4):221–8.
Harvey K, Esposito DH, Han P, Kozarsky P, Freedman DO, Plier DA, et al. Surveillance for travel-related disease—GeoSentinel Surveillance System, United States, 1997–2011. MMWR Surveill Summ. 2013 Jul 19;62:1–23.
Okhuysen PC. Traveler’s diarrhea due to intestinal protozoa. Clin Infect Dis. 2001 Jul 1;33(1):110–4.
Ross AG, Cripps AW. Enteropathogens and chronic illness in returning travelers. N Engl J Med. 2013 Aug 22;369(8):784.
Saiman L, Aronson J, Zhou J, Gomez-Duarte C, Gabriel PS, Alonso M, et al. Prevalence of infectious diseases among internationally adopted children. Pediatrics. 2001 Sep 3;108(3):608–12.
Staat MA, Rice M, Donauer S, Mukkada S, Holloway M, Cassedy A, et al. Intestinal parasite screening in internationally adopted children: importance of multiple stool specimens. Pediatrics. 2011 Sep;128(3):e613–22.
Swaminathan A, Torresi J, Schlagenhauf P, Thursky K, Wilder-Smith A, Connor BA, et al. A global study of pathogens and host risk factors associated with infectious gastrointestinal disease in returned international travellers. J Infect. 2009 Jul;59(1):19–27.
Wensaas KA, Langeland N, Hanevik K, Morch K, Eide GE, Rortveit G. Irritable bowel syndrome and chronic fatigue 3 years after acute giardiasis: historic cohort study. Gut. 2012 Feb;61(2):214–9.