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Clinical Characteristics

The vast majority of people infected with G. lamblia are asymptomatic. Giardia cysts may be found in varying proportions of the normal population in countries around the world, thus indicating that many healthy people harbor this parasite in a commensal relationship. Clinical experiments, using human volunteers who ingested G. lamblia cysts, have demonstrated that most people acquire only a slight infection of short duration, possibly because the host becomes resistant to the parasite.

In a minority of individuals, usually children or those with deficient immunity, there may be a variety of symptoms ranging from mild abdominal discomfort to recurrent diarrhea, malabsorption and steatorrhea, usually (but not invariably) associated with heavy infection.

Children are more susceptible to giardiasis than adults and usually have more severe symptoms.The stools and clinical manifestations in children may resemble those of sprue or celiac disease. In addition to sprue-like stools, there may be weight loss and defective fat and xylose absorption. However, anemia is absent and the overall effects of malabsorption are not as severe as in sprue. The diarrhea may be mild with a few semiformed stools or there may be a severe watery diarrhea, which can be chronic for many months with exacerbations and remissions.

Adult patients who are symptomatic usually have chronic diarrhea as the most common clinical finding, sometimes associated with steatorrhea and malabsorption. Dyspepsia with flatulence and dull epigastric and right upper quadrant pain, vague abdominal distress, nausea, anorexia, fever and weight loss may also be present. Rarely, G. lamblia may invade the gallbladder and cause symptoms referable to cholecystitis and jaundice.

The individual response of a patient to infection with Giardia is quite variable and depends on the number of organisms present and other factors such as age, nutritional state, associated bacterial infection in the gut, and immunoglobulin deficiency. The mere presence of G. lamblia in the stool does not indicate that these organisms are the cause of the patient's symptoms, and other causes should not be ruled out. Thus, it is well recognized that infection with Entamoeba histolytica can be overlooked when there are large numbers of G. lamblia present.

Giardia infections are particularly common and often symptomatic in patients with agammaglobulinemia or dysgammaglobulinemia; the latter have a decreased level of IgG and virtual absence of IgA and IgM immunoglobulins in their serum. There is an increased prevalence of giardiasis among male homosexuals, contributing to "gay bowel syndrome". Sexually active gay men show cyst passage rates as high as 20%, and of the many enteric pathogens found in this group, one study noted that diarrheal symptoms correlated most closely with the presence of G. lamblia.

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