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


Giardiasis, an intestinal infection acquired from untreated water sources, is most often symptomatic.


Giardiasis, an intestinal infection acquired from untreated water sources, is most often symptomatic. When disease occurs, it is in the form of a diarrhea lasting up to 4 weeks with foul-smelling, greasy stools. Abdominal pain, nausea, and vomit- ing are also present.


Giardiasis has a cosmopolitan distribution; its prevalence is highest in areas with poor sanitation and among populations unable to maintain adequate personal hygiene. In de-veloping countries, infection rates may reach 25 to 30%; in the United States, G. lamblia is found in 4% of stools submitted for parasitologic examination, making it this country’s most frequently identified intestinal parasite. All ages and economic groups are repre-sented, but young children and young adults are preferentially involved. Children with immunoglobulin deficiencies are more likely to acquire the flagellate, possibly because of a deficiency in intestinal immunoglobulin A. Giardiasis is also common among attendees of day-care centers. Attack rates of over 90% have been seen in the ambulatory non – toilet-trained population (age, 1 to 2 years) of these institutions, suggesting direct person-to-person transmission of the parasite. The frequency with which secondary cases are seen among family contacts reinforces this probability. Undoubtedly, direct fecal spread is also responsible for the high infection rate among male homosexuals. In several recent studies, the prevalence of giardiasis and/or amebiasis in that population has ranged from 11 to 40% and is correlated closely with the number of oral – anal sexual contacts.

Water-borne and, less frequently, food-borne transmission of G. lamblia has also been documented, and probably accounts for the frequency with which American travelers to third world nations acquire infection. Unlike the typical bacterial diarrhea syndrome seen in travelers, the diarrhea begins late in the course of travel and may persist for several weeks. More than 20 water-borne outbreaks of giardiasis have also been reported in the United States. The sources have included untreated pond or stream water, sewage-contaminated municipal water supplies, and chlorinated but inadequately filtered water. In a few of these outbreaks, epidemiologic data have suggested that wild mammals, particularly beavers, served as the reservoir hosts. Domestic cats and dogs, which have recently been shown to have a high prevalence of G. lamblia, may also act as reservoirs for human infections.


Disease manifestations appear related to intestinal malabsorption, particularly of fat and carbohydrates. Disaccharidase deficiency with lactose intolerance, altered levels of intesti-nal peptidases, and decreased vitamin B12 absorption have been demonstrated. The precise pathogenetic mechanisms responsible for these changes remain poorly understood. Mechanical blockade of the intestinal mucosa by large numbers of Giardia, damage to the brush border of the microvilli by the parasite’s sucking disc, organism-induced deconjuga-tion of bile salts, altered intestinal motility, accelerated turnover of mucosal epithelium, and mucosal invasion have all been suggested. None of these correlates well with clinical manifestations. Patients with severe malabsorption have jejunal colonization with enteric bacteria or yeasts, suggesting that these organisms may act synergistically with Giardia. Eradication of the associated microorganism, however, has not uniformly resulted in clini-cal improvement. Jejunal biopsies sometimes reveal a flattening of the microvilli and an inflammatory infiltrate, the severity of which correlates roughly with that of the clinical disease. Generally, both malabsorption and the jejunal lesions have been reversed with specific treatment. The demonstration of occasional trophozoites in the submucosa raises the possibility that these changes reflect T lymphocyte – mediated damage.


Susceptibility to giardiasis has been related to several factors, including strain virulence,inoculum size, achlorhydria or hypochlorhydria, and immunologic abnormalities. In oneexperimental study, humans were challenged with varying doses from as few as 10 cysts.They were uniformly parasitized when 100 or more were ingested. Several workers havenoted the frequency with which giardiasis occurs in achlorhydric and hypochlorhydric in- dividuals. Although reinfection is common, the frequent occurrence of giardiasis in pa- tients with immunologic diseases, plus the rarity with which it is seen in older adults, suggests that protective immunity, albeit incomplete, does develop in humans. Animalstudies have demonstrated that Giardia-specific, secretory IgA (sIgA) antibodies inhibitattachment of trophozoites to intestinal epithelium, perhaps by blocking parasite surfacelectins. Moreover, antitrophozoite IgM or IgG antibodies, plus complement, are known tobe capable of killing Giardia trophozoites.

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