Entamoeba histolytica causes invasive amebiasis, a major parasitic disease of the developing world, whose primary symptoms are liver abscess and colitis. All strains of E. histolytica express a 260-kDa surface Gal/GalNAc lectin that is antigenically conserved and immunogenic. The lectin is required for adherence to human intestinal epithelial cells and contact-dependent killing of immune effector cells. By expression cloning, the carbohydrate recognition domain (CRD) was identified within the lectin heavy-subunit cysteine-rich region. Of interest for a hepatic parasite, the CRD had sequence identity to the receptor-binding domain of hepatocyte growth factor (HGF) and competed with HGF for binding to the c-Met HGF receptor. In an animal model of invasive disease, immunization with the CRD inhibited liver-abscess formation, yet in humans, a naturally acquired immune response against the CRD did not persist.

A noninvasive diagnostic test for amebic liver abscess is needed, because amebic and bacterial abscesses appear identical on ultrasound or computer tomography and because it is rarely possible to identifyEntamoeba histolytica in stool specimens from patients with amebic liver abscess. Here we report a method of detection in serum of circulating E. histolytica Gal/GalNAc lectin to diagnose amebic liver abscess, which was used in patients from Dhaka, Bangladesh. The TechLab E. histolytica II test (which differentiates the true pathogen E. histolytica fromEntamoeba dispar) detected Gal/GalNAc lectin in the sera of 22 of 23 (96%) amebic liver abscess patients tested prior to treatment with the antiamebic drug metronidazole and 0 of 70 (0%) controls. After 1 week of treatment with metronidazole, 9 of 11 (82%) patients became serum lectin antigen negative. The sensitivity of the E. histolytica II antigen detection test for intestinal infection was also evaluated. Antigen detection identified E. histolytica infection in 50 samples from 1,164 asymptomatic preschool children aged 2 to 5 years, including 16 of 16 (100%) culture-positive specimens. PCR analysis of stool specimens was used to confirm that most antigen-positive but culture-negative specimens were true-positive: PCR identified parasite DNA in 27 of 34 (79%) of the antigen-positive, culture-negative stool specimens. Antigen detection was a more sensitive test for infection than antilectin antibodies, which were detected in only 76 of 98 (78%) amebic liver abscess patients and in 26 of 50 (52%) patients with intestinal infection. We conclude that the TechLab E. histolytica II kit is a sensitive means to diagnose hepatic and intestinal amebiasis prior to the institution of metronidazole treatment.

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