The protozoan parasite Giardia intestinalis (formerly known as G. lamblia or G. duodenalis).
By ingesting fecally contaminated food or water, including water swallowed while swimming; through contact with fecally contaminated environmental surfaces; or through person-to-person contact, such as caring for an infected person or sexual contact.
Transmission occurs worldwide, most commonly diagnosed in travelers returning from south Asia, the Middle East, and South America. Risk of infection increases with duration of travel.
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 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 (DFA) 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.
Tinidazole, metronidazole, nitazoxanide, paromomycin, furazolidone, and quinacrine are known to be effective in treating giardiasis. Because making a definitive diagnosis is difficult, empiric treatment can be used in patients with the appropriate history and typical symptoms.
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.
Okhuysen PC. Traveler’s diarrhea due to intestinal protozoa. Clin Infect Dis. 2001 Jul 1;33(1):110–4.
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.