We determined whether
epitope-specific
monoclonal antibodies to the
galactose-inhibitable adherence
protein (GIAP) of Entamoeba histolytica could be used in an
enzyme-linked
immunosorbent assay (ELISA) to detect
antigen in serum and feces and differentiate between nonpathogenic zymodemes and the potentially invasive pathogenic organisms that require treatment. Overall, 57% of subjects from Cairo, Egypt, with symptomatic
intestinal amebiasis and 42% with
asymptomatic infection possessed GIAP
antigen in their sera, whereas 4% of uninfected controls or subjects with other
parasitic infections possessed GIAP
antigen in their sera (P < 0.001). In subjects from Durban, South Africa, only 6% of uninfected controls or those with nonpathogenic E. histolytica
infection were positive for GIAP in serum, whereas 3 of 4 with asymptomatic pathogenic intestinal
infection and 75% with
amebic liver abscess were positive for GIAP in serum. Fifteen stool samples from patients with
intestinal amebiasis were available for study; all had a positive ELISA result for fecal GIAP
antigen.
Epitope-specific
monoclonal antibodies identified 8 of 15 subjects with fecal
antigen from pathogenic strains. Seven of those eight subjects had adherence
protein antigen in their sera, whereas none of seven with apparent nonpathogenic E. histolytica
infection had adherence
protein antigen in their sera. In summary, we were able to detect E. histolytica adherence
protein antigen directly in serum and fecal samples by ELISA. The presence of amebic
antigen in serum demonstrated 94% specificity for pathogenic E. histolytica
infection, and amebic
antigen is present during asymptomatic intestinal
infection. In conjunction with antibody detection, this method should be very useful in the diagnosis and management of
intestinal amebiasis.