Isolation and identification of a Giardia lamblia-specific stool antigen (GSA 65) useful in coprodiagnosis of giardiasis.

A Giardia lamblia-specific antigen (GSA 65) was isolated from stools of G. lamblia-positive patients by crossed- and line-immunoelectrophoresis and counterimmunoelectrophoresis (CIE) in agarose by using rabbit antiserum prepared against G. lamblia cysts. CIE with rabbit anti-GSA 65 monospecific antiserum revealed that GSA 65 was present in aqueous stool eluates of giardiasis patients and in cysts and trophozoites of the parasite. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of immunoaffinity-purified antigen followed by Western blotting showed that the molecular weight of this molecule was about 65,000. GSA 65 was detectable by CIE in stool eluates of 36 of 40 giardiasis patients but not in eluates of 10 G. lamblia-negative asymptomatic controls. GSA 65 was detected in stool eluates of 2 of 18 individuals with chronic diarrhea who were negative for parasites by microscopic examination. Cross-specificity studies with other genera of parasitic protozoa performed by using CIE and immunofluorescence indicated that GSA 65 was present only in strains of G. lamblia. Based on these findings, GSA 65 may prove to have an important application in the design of sensitive diagnostic tests for giardiasis.