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Giardia lamblia
Entamoeba histolytica
Other Organisms

Information obtained by history and physical examination may provide clues to parasitic infections of the gastrointestinal tract. Diarrhea, bloating, weight loss, or eosinophilia in the context of an appropriate epidemiologic history requires that a stool sample be examined for ova and parasites.
The number and frequency of fecal specimens to be collected varies with the diagnosis suspected. Because many pathogens are shed sporadically, it is generally advisable to inspect multiple specimens collected during several days. Many drugs and other substances can interfere with proper stool examination. Iron, bismuth, and mineral oil may interfere with stool examination for 1 week or more; barium and dyes, iodine preparations, and antimicrobials may cause difficulty for as long as several weeks. Fresh stool specimens are important to maximize the yield of trophozoites, such as Entamoeba histolytica. Prompt examination or fixation is particularly important for soft or watery specimens. Specimens that cannot be examined or fixed quickly should be refrigerated. Fecal specimens should not be incubated or frozen and thawed.
Fecal specimens should be collected in a wide-mouthed container or carton. Urine is harmful to some parasites and must not contaminate the specimen. Specimens should not be retrieved from toilet water because water is destructive to some eggs and protozoa. Purgation may increase the yield of some parasites, such as amebae, Giardia, and Strongyloides, and it should be considered when their presence is strongly suspected clinically but specimens have been negative.
The physician may want to screen the sample before submitting it to the laboratory. Macroscopic examination may reveal Ascaris or tapeworm proglottids. Wet mounts can be prepared with saline or iodine solution and examined for trophozoites, cysts, ova, or Strongyloides larvae. A fecal fleck is obtained with a wooden applicator stick. Flecks of mucus, pus, or blood will have a higher yield. The sample is mixed with a drop of saline solution and put on a microscope slide with cover slip. An iodine solution (1 g potassium iodide, 1.5 g iodine crystals to 100 mL of water) will stain protozoa and helminths.
The laboratory will usually perform three microscopic examinations, depending on the specimen submitted: a direct wet mount, a concentrated specimen wet mount, and permanent stain. Concentration techniques will vary depending on the parasite that is suspected. For example, a Baermann concentration method is used to detect Strongyloides. A Sheather sugar flotation method is recommended for concentration of Cryptosporidium cysts.
Giardia lamblia
Cyst excretion in Giardia infection varies from one specimen to another and is observer-dependent. Only about 80% of infections are detected even when three daily specimens are studied. Cyst concentration techniques will improve overall yield. A recent study suggests that duodenal fluid obtained by aspiration is not superior to fecal specimens. Trophozoites can also be demonstrated in jejunal mucosal biopsy specimens. Immunologic methods to detect Giardia antigen in feces are under investigation and will likely provide additional sensitivity. A 65-kd Giardia antigen can be identified in feces by means of a counterimmunoelectrophoresis technique. DNA probes are also being developed.
Entamoeba histolytica
Multiple stool specimens are necessary in the diagnosis of intestinal amebiasis. Specimens obtained by endoscopy with scraping of ulcers have a high yield. Biopsy material stained with paraaminosalicylic acid will demonstrate trophozoites. Wet preparations can identify amebae with their linear motility. Permanent stains with trichrome or iron hematoxylin are used to identify E. histolytica by nuclear morphology and ingestion of RBCs. Concentration methods are superior to direct stool examination for detection of cysts. Results of an indirect hemagglutination serum test are positive in active intestinal infection.
Stool examination is useful in the diagnosis of cestode infections. Eggs or proglottids of Taenia species are demonstrated with concentration methods and thick smears. A perianal swab or adhesive tape method detects 90% of cases. These thick smears are diagnostic in more than 95% of patients with infection. Specific detection of Taenia eggs by DNA hybridization studies has recently been reported.
The thick-smear technique is also the procedure of choice to detect Schistosoma eggs. Rectal biopsy may be necessary in some patients. When biopsy is performed, tissue should be crushed and examined with low-power microscopy.
Concentration techniques such as formalin-ethylacetate and zinc sulfate flotation lack sensitivity in diagnosis of infection with this nematode. Multiple stool samples and long periods of inspection are often inadequate to ensure diagnosis. The Baermann technique uses the hydrothermotropism of the larva to improve sensitivity. In one study of 736 patients, the Baermann technique found 243 positive cases, compared with 55 cases when a concentration method was used. An agar-plate method has recently been devised. In this method, nematodes are suspected by the burrows in agar that they leave behind. The larvae themselves are viewed with a dissecting microscope. This method appears to be more sensitive than the Baermann technique.
Serodiagnostic studies in which antigens derived from filariform larvae are used have a sensitivity greater than 80% and should be performed when the disease is suspected but cannot be confirmed by stool analysis. After treatment, titers fall or become negative altogether.
Hookworm infection is usually easily diagnosed by finding the typical oval eggs on direct stool smear. Concentration methods are helpful for mild infections. Ascaris infection also is relatively easy to diagnose by direct smear. Adult worms at times will be seen by barium studies or cholangiography.
Antibody and antigen capture assays in dipstick dot enzyme-linked immunosorbent assay format are now being developed for the detection of Strongyloides in feces. These systems utilize parasite antigens that do not cross-react and can therefore identify organisms even in multiple infections.
Other Organisms
Cryptosporidium infection is diagnosed by finding cysts in stool or biopsy material. Modified acid-fast stain will demonstrate the organism. Yeasts, similar in shape and size, will not stain with acid-fast stain. A direct immunofluorescence antibody stain based on a murine monoclonal antibody to the cyst cell wall should improve sensitivity. An enzyme-linked immunosorbent assay method has also been used to detect anticryptosporidial immunoglobulin in serum.
Isospora belli, also a cause of diarrhea in AIDS patients, will stain with acid-fast stain but is larger and ellipsoid in shape compared with Cryptosporidium. (S.L.B.)
Arakaki T, et al. A new method to detect Strongyloides stercoralis from human stool. Jpn J Trop Med Hyg 1988;16:11.
The agar-plate method is probably the most sensitive approach to the diagnosis of Strongyloides infection.
Arakaki T, et al. Efficacy of agar-plate culture in detection of Strongyloides stercoralis infection. J Parasitol 1990;76:425.
Strongyloides furrows could be distinguished from those of Necator, which were thicker.
Berk SL, et al. Clinical and epidemiologic features of strongyloidiasis: a prospective study in rural Tennessee. Arch Intern Med 1987;147:1257.
Repeated stool samples and use of the ether extraction method did not improve Strongyloides detection (only 2 of 512 repeated specimens were positive).
Blumencranz H, et al. The role of endoscopy in suspected amebiasis. Am J Gastroenterol 1983;78:15.
Endoscopy may be diagnostic in amebiasis. When results of stool studies are negative, it may also detect concomitant disease, such as inflammatory bowel disease.
Butcher PD, Clark C, Farthing MJG. Giardia lamblia cloned genomic DNA probes: uses in faecal diagnosis and genetic analysis of clinical isolates. Gut 1988;29:A722.
Giardia species can be detected by DNA probe.
Craft JC, Nelson JD. Diagnosis of giardiasis by counterimmunoelectrophoresis of feces. J Infect Dis 1982;145:499.
A countercurrent immunoelectrophoresis test for Giardia fecal antigen was as sensitive as combined examination of feces and duodenal fluid.
Flisser A, et al. Specific detection of Taenia saginata eggs by DNA hybridization. Lancet 1988;2:1429.
DNA probe has been used to distinguish Taenia solium from T. saginata.
Genta RM. Global prevalence of strongyloidiasis: critical review with epidemiologic insights into the prevention of disseminated disease. Rev Infect Dis 1989;11:755.
Excellent review of laboratory diagnosis of strongyloidiasis.
Goka AKJ, et al. Diagnosis of Strongyloides and hookworm infections: comparison of faecal and duodenal fluid microscopy. Trans R Soc Trop Med Hyg 1990;84:829.
Fecal microscopy was insensitive in the diagnosis of Strongyloides and hookworm infection. Examination of duodenal fluid improved diagnosis.
Hruska JF. Gastrointestinal and intra-abdominal infection. In: Reese RE, Betts RF, eds. A practical approach to infectious diseases, 3rd ed. Boston: Little, Brown, 1991.
Excellent review of the diagnosis and treatment of gastrointestinal parasites.
Kamath KR, Murugasu R. A comparative study of four methods for detecting Giardia lamblia in children with diarrheal disease and malabsorption. Gastroenterology 1974;66:16.
Mucosal impression smears and biopsy gave the highest yield of positive test results. Duodenal aspiration was more commonly positive than stool examination.
Koga K, et al. An evaluation of the agar-plate method for the detection of Strongyloides stercoralis in northern Thailand. J Trop Med Hyg 1990;93:183.
The agar-plate method was most sensitive. A few cases were missed by this method. A smear was required to distinguish Strongyloides from hookworm.
Lima JP, Delgado PG. Diagnosis of strongyloidiasis: importance of Baermann’s method. Am J Dig Dis 1961;6:899.
Baermann’s method, which is based on the thermohydrotropism of Strongyloides, was more sensitive than concentration methods.
Madanagopalan N, et al. A correlative study of duodenal aspirate and faeces examination in giardiasis, before and after treatment with metronidazole. Curr Med Res Opin 1975;3:99.
Duodenal aspiration and stool examination supplement the diagnostic yield in Giardia infection.
Pelletier LL Jr. Chronic strongyloidiasis in World War II Far East ex-prisoners of war. Am J Trop Med Hyg 1984;33:55.
Five hours of microscopy per case resulted in a 90% rate of detection of positive cases.
Rosenthal P, Liebman WM. Comparative study of stool examinations, duodenal aspiration, and pediatric Entero-Test for giardiasis in children. J Pediatr 1980;96:278.
Entero-Test was an efficient, safe method for diagnosis of giardiasis in children.
Rosoff JD, Stibbs HH. Isolation and identification of a Giardia lamblia-specific stool antigen (GSA 65) useful in coprodiagnosis of giardiasis. J Clin Microbiol 1986; 23:905.
A Giardia lamblia-specific antigen was demonstrated in 36 of 40 infected patients. Antigen is detected by countercurrent immunoelectrophoresis with rabbit monospecific antiserum.
Siddiqui AA, et al. Strongyloides stercoralis: identification of antigens in natural human infections from endemic areas of the United States. Parasitol Res 1997; 83:655.
Describes eight immunodominant antigens found in immunoprecipitates of infected patients. These antigens may be targets for immunodiagnosis by testing of stool samples.
Smith JW, Bartlett MS. Diagnostic parasitology: introduction and methods. In: Balows A, ed. Manual of clinical microbiology, 5th ed. Washington, DC: American Society for Microbiology, 1991.
An excellent overview of stool examination. Describes wet-mount concentration procedures and permanent stains.
Thornton SA, et al. Comparison of methods for identification of Giardia lamblia. Am J Clin Pathol 1983;80:858.
Trichrome was the superior stain for cysts and trophozoite in giardiasis.
Ungar BLP, et al. Enzyme-linked immunosorbent assay for the detection of Giardia lamblia in fecal specimens. J Infect Dis 1984;149:90.
An enzyme-linked immunosorbent assay is sensitive and specific for detection of G. lamblia in human feces. Stools became negative after successful treatment.


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