Examinations on the serodiagnosis of Sarcocystis infections. II. Class-specific immunoglobulin responses in mice, pigs, and sheep.

Mice, pigs and sheep were experimentally infected with Sarcocystis muris, S. miescheriana (syn. S. suicanis) and S. tenella (syn. S. ovicanis) respectively. Enzyme-linked immunosorbent assays employing conjugates specific for the relevant host immunoglobulin classes M and G were then evaluated for use in serodiagnosing these infections. Soluble antigenic extracts were prepared from cystozoites harvested from macroscopic S. muris and S. gigantea (syn. S. ovifelis) cysts and from microscopic S. miescheriana cysts. Selected sera were firstly screened in the immunoassays for false positive and negative reactions which may have resulted from competition or interference occurring between the different antibody classes. No false reactions were detected as evidenced by the reactions of the sera before and after mercaptoethanol reduction of IgM and gel fractionation of IgM and IgG. The attempted absorption of IgG from the sera with Protein A proved ineffective in all 3 host animal species. Sera from the experimentally infected animals were then tested in the immunoassays against homologous and heterologous antigens and the kinetics of specific antibody development were determined. None of the antigens proved to be species-specific although heterologous reactions were usually weaker. Specific IgM-antibodies were detected early in the courses of all infections but they did not persist. These transitory responses occurred simultaneously with parasite merogonous proliferation in mice and sheep but later in pigs. Such responses may be of some value in the serodiagnosis of acute sarcocystosis. Specific IgG-antibodies were detected later in all infected animals and elevated levels were prominent after cyst formation by the parasite had occurred. These responses were therefore suitable for the serodiagnosis of latent and chronic Sarcocystis infections.