Comparative studies on the immunogenicity of theronts, tomonts and trophonts of Cryptocaryon irritans in grouper.

Cryptocaryon irritans is a ciliated obligate parasite of marine fish, causing white spot disease in tropical and sub-tropical regions. Recent studies have shown that fish can mount immune responses against C. irritans and acquire protection. The present study compared the protective immunity in grouper (Epinephelus coioides) against C. irritans induced by theronts, trophonts and tomonts via intraperitoneal (i.p.) injection and tentatively identified an immobilization antigen. Specific antibody titres of immunized fish serum were determined by enzyme-linked immunosorbent assay and immobilization assays at weeks 0, 1, 2, 4, 6 and 8 post-immunization, and immunized grouper were challenged with theronts to detect the survival percentage of fish. Fish immunized with theronts produced higher levels of serum antibody against C. irritans than fish immunized with trophonts or tomonts. A significantly higher level of immune protection was achieved in fish immunized with theronts but not in fish immunized with tomonts or trophonts. Western blot analysis using polyclonal immobilization antibodies of rabbit immunized with theront revealed an immunoreactive band of the protective (immobilization) antigens of C. irritans, the size of which is approximately 34 kDa. The sera of mice immunized with the protein could immobilize theronts of C. irritans. These results demonstrated that the theront stage of C. irritans elicited stronger protective immunity than the trophont and tomont stages in grouper. The tentative identification of protective antigen provides the solid foundation for the development of a defined vaccine against C. irritans.