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Amebae are identified microscopically in the stool in 18% of cases at the time of diagnosis of liver abscess, although they can be identified in the stool by culture in the majority of patients. Microscopic examination of a single stool specimen for amebic cysts and trophozoites in a patient with amebic colitis is only 33-50%
Detection of pathogenic Entamoeba histolytica in stool
■ • • Pathogenic
• + Other/no parasites
+ •• • +
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Fig. 9.7 Antigen detection test for E. histolytica in stool. Stool specimens with culture-confirmed E. dispar (open circles) or E. histolytica (closed circles) infection, or stools without detectable Entamoeba detected by microscopy (crosses), were assayed using an ELISA containing mAb specific for E. histolytica
sensitive, and is unable to distinguish pathogenic E. histolytica from the morphologically identical non-pathogenic E. dispar (Haque et al., 1995, 1997; Petri and Mann, 1993). Erythrophagocytic amebae are more likely to be E. histolytica than E. dispar (Gonzalez-Ruis et al., 1994), but E. dispar trophozoites have also been found to contain ingested red blood cells (Haque et al.,
In patients with amebic liver abscess, microscopic examination of the stool is even less efficacious, as repeated stool examinations in patients with amebic liver abscess were only able to detect the parasite in 8-44% of cases (Katzenstein et al., 1982; Petri et al., 1990). Aspiration of the abscess is occasionally required to diagnose amebiasis, and although amebae are visualized in the pus in only the minority ofcases, antigen detection on the abscess fluid appears to be very sensitive. In pyogenic abscesses, bacteria will be seen and/or cultured from the aspirated fluid. Antibodies to E. histolytica are present in the serum of 92-97% of patients upon acute presentation with amebic liver abscess, and therefore are very useful diagnostically. Because
a significant proportion of the population in developing countries is seropositive, however, antibody tests are less specific in residents or immigrants from the developed world. Identification of the parasite in aspirated pus from liver abscesses, even in the most experienced hands, is only 20% sensitive (Haque et al., 1995; Katzenstein et al., 1982).
Antigen detection is a recently developed tool which exploits the molecular differences between E. histolytica and E. dispar. A stool antigen detection test that is specific for E. histolytica is now commercially available for clinical use from TechLab Inc. (Blacksburg, VA, USA) (Figure 9.7) (Haque et al., 1993, 1995, 1997). The E. histolytica antigen test is rapid, has improved sensitivity compared to microscopy, and is only slightly less sensitive than the ‘gold standard’ of culture/isoenzyme analysis. The TechLab E. histolytica test is based on detection of
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
the Gal/GalNAc lectin in stool; another E. histolytica specimen antigen test is also available (Gonzalez-Ruiz et al., 1994). A test to detect the Gal/GalNAc lectin in serum appears promising for the diagnosis of amebic liver abscess (a situation where it is more difficult than amebic colitis to demonstrate the parasite in stool), with a reported sensitivity in initial tests of 67%. Detection of Gal/GalNAc lectin in liver abscess aspirates has demonstrated to be a sensitive test in a small number of patients (R. Haque and W. Petri, unpublished).
Polymerase Chain Reaction
Polymerase chain reaction (PCR) can now be used to diagnose E. histolytica in stool samples with sensitivity comparable to antigen detection (Haque et al., 1998). Preliminary results for PCR detection of E. histolytica DNA in liver abscess pus appear encouraging. Use of PCR can be used also to distinguish amongst isolates of E. histolytica, which should prove useful for epidemiologic purposes, as well as determining the virulence characteristics of different isolates (Garfinkel et al., 1989; Tachibana et al., 1991; Tannich et al., 1991; Acuna-Solo et al., 1993).
Serology using an indirect hemagglutination (IHA) test for anti-amebic antibody is eventually reportedly 99% sensitive for amebic liver abscess and 88% sensitive for amebic colitis (Kagan, 1970). However, early in the course of amebic liver abscess the indirect hemagglutination test may be negative. The major problem limiting clinical use of the current serologic tests is that they remain positive for years after an episode of amebiasis. As a result, a substantial number (1035%) of residents of developing countries have anti-amebic antibodies detected by current serologic tests (Caballero-Salcedo et al., 1994; Choudhuri et al., 1991). This has resulted in a situation where it is impossible to distinguish current from past infection in individuals from countries with high prevalence of infection (Ximenez et al., 1993). In five recent series of