Experimental studies on circulating antigen of Toxoplasma gondii in intermediate hosts: criteria for detection and structural properties.

The present study was performed to demonstrate circulating antigen (cag) of Toxoplasma gondii in the sera of orally and intraperitoneally infected rabbits and swine, to determine the time of their appearance after infection, and to characterize the antigenic components of the cag by means of affinity chromatography, SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and electrophoretic blot onto nitrocellulose sheets. Cag, as detected by an indirect ELISA, was found in the sera of both rabbits and swine from week 5 to week 8 after infection. Electrophoretic separation of cag extracted from swine and human sera showed 5 and 8 distinct protein bands, respectively, the molecular weight of which ranged between 25 and 100 kD. After Western blot, 2 of the 5 protein bands of swine-cag (27 and 57 kD) and 3 of the 8 protein bands of human cag (27, 32, and 57 kD) reacted with the anti-Toxoplasma antibody used in the ELISA. On the basis of the data presented, the influence of the dose and mode of infection as well as that of the preparation method of antisera on cag detection is discussed.