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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Flagellates

Giardiasis : Clinical Aspects

In endemic situations, over two thirds of infected patients are asymptomatic.



In endemic situations, over two thirds of infected patients are asymptomatic. In acute out-breaks, this ratio of asymptomatic to symptomatic patients is usually reversed. When they do occur, symptoms begin 1 to 3 weeks after exposure; they typically include diarrhea, which is sudden in onset and explosive in character. The stool is foul smelling, greasy in appearance, and floats on water. It is devoid of blood or mucus. Upper abdominal cramp-ing is common. Large quantities of intestinal gas produce abdominal distention, sulfuric eructations, and abundant flatus. Nausea, vomiting, and low-grade fever may be present. The acute illness generally resolves in 1 to 4 weeks; in children, however, it may persist for months, leading to significant malabsorption, weight loss, and malnutrition.

In many adults, the acute phase is often followed by a subacute or chronic phase char-acterized by intermittent bouts of mushy stools, flatulence, and “heartburn” and weight loss that persist for weeks or months. At times, patients presenting in this fashion deny having experienced the acute syndrome described previously. In the majority, symptoms and organisms eventually disappear spontaneously. It is not uncommon for lactose intol-erance to persist after eradication of the organisms. This condition may be confused with an ongoing infection, and the patient may be subjected to unnecessary treatment.


The diagnosis is made by finding the cyst in formed stool or the trophozoite in diarrheal stools, duodenal secretions, or jejunal biopsy specimens. In acutely symptomatic patients, the parasite can usually be demonstrated by examining one to three stool specimens, pro-viding appropriate concentration and staining procedures are used. In chronic cases, ex-cretion of the organism is often intermittent, making parasitologic confirmation more difficult. Many of these patients can be diagnosed by examining specimens taken at weekly intervals over 4 to 5 weeks. Alternatively, duodenal secretions can be collected and examined for trophozoites in trichrome or Giemsa-stained preparations. There are now a number of reliable, commercially available, enzyme immunoassays (EIAs) for the direct detection of parasite antigen in stool. They appear to be as sensitive and specific as microscopic examinations. The organism can be grown in culture, but the methods are not currently adaptable to routine diagnostic work.


Four drugs are currently available for the treatment of giardiasis in the United States: quinacrine hydrochloride, metronidazole, furazolidone, and paromomycin. Quinacrine and metronidazole are somewhat more effective (70 to 95%) and are preferred for pa-tients capable of ingesting tablets. Furazolidone is used by pediatricians because of its availability as a liquid suspension, but it has the lowest cure rate. These three agents re-quire 5 to 7 days of therapy. Tinidazole, an oral agent not yet available in the United States, is safe and effective in single-dose treatment. Because of the potential of giardiasis for person-to-person spread, it is important to examine and, if necessary, treat close phys-ical contacts of the infected patient, including playmates at nursery school, household members, and sexual contacts. None of the aforementioned agents should be used in pregnant women because of their potential teratogenicity. Paromomycin, a nonabsorbed but somewhat less effective agent, may be used in this circumstance.


Hikers should avoid ingestion of untreated surface water, even in remote areas, because of the possibility of contamination by feces of infected animals. Adequate disinfection can be accomplished with halogen tablets yielding concentrations higher than that generally achieved in municipal water systems. The safety of the latter results from additional floc-culation and filtration procedures.

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