Immune response of channel catfish to ciliary antigens of Ichthyophthirius multifiliis

Channel catfish were rendered immune to the protozoan pathogen, Ichthyophthirius multifiliis, by exposure to sublethal infections. Sera from test animals were then screened for antibodies against the parasite using enzyme-linked immunoassays. Ichthyophthirius cilia were blotted onto nitrocellulose filters and reacted with catfish sera, followed by rabbit anti-catfish Ig antibodies coupled to horseradish peroxidase. Subsequent color development revealed the presence of anti-ciliary antibodies in a number of fish tested. Reactions appeared to be highly specific; little cross-reactivity was seen in equivalent assays with heterologous cilia from Tetrahymena. Ciliary antigens were associated predominantly with a membrane polypeptide fraction isolated from intact cilia by phase separation in solutions of the nonionic detergent, Triton X-114. The relative levels of anti-ciliary antibodies in sera from individual fish were quantitated by photometric scanning of immunoblot assays. A strong correlation (P less than .03) was found between antibody levels and the ability of sera to agglutinate live parasites in vitro.