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Detection of Parasite Antigens

The diagnosis of human intestinal protozoa depends on microscopic detection of the various parasite stages in feces, duodenal fluid, or small intestine biopsy specimens.  Since fecal examination is very labor-intensive and requires a skilled microscopist, antigen detection tests have been developed as alternatives using direct fluorescent antibody (DFA), enzyme immunoassay (EIA), and rapid, dipstick-like tests.  Antigen detection methods can be performed quickly and do not require an experienced and skilled morphologist.  Much work has been accomplished on the development of antigen detection tests, resulting in commercially available reagents for the intestinal parasites Cryptosporidium spp., Entamoeba histolytica, Giardia intestinalis (lamblia), and Trichomonas vaginalis.  In addition, antigen detection tests using blood or serum are available for Plasmodium and Wuchereria bancrofti.

Specimens for antigen detection
Fresh or preserved stool samples are the appropriate specimen for antigen detection testing with most kits, but refer to the recommended collection procedures included with each specific kit.

Amebiasis
EIA kits are commercially available for detection of fecal antigens for the diagnosis of intestinal amebiasis.  Organisms of both the pathogenic E. histolytica and the nonpathogenic Entamoeba dispar strains are morphologically identical.  These assays use monoclonal antibodies that detect the galactose-inhibitable adherence protein in the pathogenic E. histolytica.  The primary drawback of these assays is the requirement for fresh, unpreserved stool specimens.  Several EIA kits for antigen detection of the E. histolytica/E. dispar group are available in the U.S., but only the TechLab kit is specific for E. histolytica.

Cryptosporidiosis
Immunodetection of antigens on the surface of organisms in stool specimens, using monoclonal antibody-based DFA assays, is the current test of choice for diagnosis of cryptosporidiosis and provides increased sensitivity over modified acid-fast staining techniques.  There are commercial products (DFA, IFA, EIA, and rapid tests) available in the United States for the diagnosis of cryptosporidiosis.  Several kits are combined tests for Cryptosporidium, Giardia, and E. histolytica.  Factors such as ease of use, technical skill and time, single versus batch testing, and test cost must be considered when determining the test of choice for individual laboratories.  The most sensitive (99%) and specific (100%) method is reported to be the DFA test, which identifies oocysts in concentrated or unconcentrated fecal samples by using a fluorescein isothiocyanate (FITC)-labeled monoclonal antibody.  A combined DFA test for the simultaneous detection of Cryptosporidium oocysts and Giardia cysts is available.

Some commercial EIA tests are available in the microplate format for the detection of Cryptosporidium antigens in fresh or frozen stool samples and also in stool specimens preserved in formalin, or sodium acetate-acetic acid-formalin (SAF) fixed stool specimens.  Concentrated or polyvinyl alcohol-treated (PVA) samples are unsuitable for testing with available antigen detection EIA kits.  The kits are reportedly superior to microscopy, especially acid-fast staining, and show good correlation with the DFA test.  Kit sensitivities and specificities reportedly range from 93 to 100% when used in a clinical setting.  Laboratories which use these EIA kits need to be aware of potential problems with false-positive results and take steps to monitor kit performance.

Rapid immunochromatographic assays are available for the combined antigen detection of either Cryptosporidium and Giardia or Cryptosporidium, Giardia, and E. histolytica.  These offer the advantage of short test time and multiple results in one reaction device.  Initial evaluations indicate comparable sensitivity and specificity to previously available tests.

The Meridian Merifluor DFA Kit for Cryptosporidium/Giardia, modified acid-fast stain for Cryptosporidium spp., or Wheatley's trichrome stain for Giardia spp. are used at CDC for routine identification of these parasites.  These techniques can be used to confirm suspicious or discrepant diagnostic results.

Giardiasis
Detection of antigens on the surface of organisms in stool specimens is the current test of choice for diagnosis of giardiasis and provides increased sensitivity over more common microscopy techniques.  Commercial products (DFA, EIAs, and rapid tests) are available in the United States for the immunodiagnosis of giardiasis.  DFA assays may be purchased that employ FITC-labeled monoclonal antibody for detection of Giardia cysts alone or in a combined kit for the simultaneous detection of Giardia cysts and Cryptosporidium oocysts.  The sensitivity and specificity of these kits were both 100% compared to those of microscopy.  They may be used for quantitation of cysts and oocysts, and thus may be useful for epidemiologic and control studies.

Some commercial EIA tests are available in the microplate format for the detection of Giardia antigen in fresh or frozen stool samples and also in stool specimens preserved in formalin, MIF, or SAF fixatives.  Concentrated or PVA samples are not suitable for testing with EIA kits.  EIA kit sensitivity rates were recently reported as ranging from 94-100% while specificity rates were all 100%.

Rapid immunochromatographic assays are available for the combined antigen detection of either Cryptosporidium and Giardia or Cryptosporidium, Giardia, and E. histolytica.  These offer the advantage of short test time and multiple results in one reaction device.  Initial evaluations indicate comparable sensitivity and specificity to previously available tests.

The Meridian Merifluor DFA Kit for Cryptosporidium/Giardia, modified acid-fast stain for Cryptosporidium spp., or Wheatley's trichrome stain for Giardia spp. are used at CDC for routine identification of these parasites.  These techniques can be used to confirm suspicious or discrepant diagnostic results.

Trichomoniasis
Trichomoniasis, an infection caused by Trichomonas vaginalis, is a common sexually transmitted disease.  Diagnosis is made by detection of trophozoites in vaginal secretions or urethral specimens by wet mount microscopic examination, DFA staining of specimens, or culture.  Sensitivity of the assays were reported as 60% for wet mounts and 86% for DFA when compared to cultures.  A kit which employs FITC- or enzyme-labeled monoclonal antibodies for use in a DFA or EIA procedure is available for detection of whole parasites in fluids.  A latex agglutination test for antigen detection in vaginal swab specimens is available; the manufacturer's evaluation indicated good sensitivity and specificity. 

^a Antibodies, Inc., P O Box 1560, Davis, CA 95617-1560
Becton Dickinson, 1 Becton Dr., Franklin Lakes, NJ 07417
BioSite, 11030 Roselle St., San Diego, CA 92121
Cellabs, P O Box 421, Brookvale, NSW 2100, Australia
Genzyme Virotech, Gmbh, Lowenplatz 5, 66248, Russelheim, Germany
Medical Chemical Corporation, 19430 Van Ness Avenue, Torrance, CA 90501
Meridian Bioscience, Inc., 3471 River Hills Dr., Cincinnati, OH 45244
Novocastra, 30 Ingold Rd., Burlingame, CA 94010
Remel, 12076 Santa Fe Drive, Lenexa, KS 66215
TechLab, VPI Research Park, 1861 Pratt Dr., Blacksburg, VA 24060
Wampole Laboratories, P O Box 1001, Cranbury, NJ 08512

^b EIA = enzyme immunoassay; Rapid = immunochromatographic cartridge; DFA = direct immunofluorescence assay; IFA = indirect immunofluorescence assay.