Effect Of Cutaneous Antibodies From Ichthyophthirius Immune Fish On Ichthyophthirius Development In Catfish     De-Hai Xu, Phillip H. Klesius, and Richard A. Shelby   USDA, Agricultural Research Service, Aquatic Animal Health Research Lab, P.O. Box 952, Auburn, AL     This study determined the effect of cutaneous antibodies to Ichthyophthirius multifiliis (Ich) development in channel catfish.  Culture conditions were established to observe the attachment of theronts and development of trophonts in the excised catfish tissues.  Many important events involved for the same population of theronts and trophonts can be observed continuously with this in vitro model, such as theront invasion, shape transformation, trophonts rotation, relocation, development and survival.  Using tissue culture technique, cutaneous antibodies were collected from the culture fluid of excised skin from fish immune to Ich.  Culture fluids from excised skin of immune fish immobilized theronts and immobility of theronts was not observed after immunoadsorption.  Immunofluorescent staining of theronts treated with immune culture fluid revealed strong and uniform fluorescence on the cilia and cell surface of theronts.  Western blot analysis of the immune culture fluid revealed a 70 kDa band which corresponded to the molecular weight of catfish immunoglobulin heavy chain.  The treatment of theronts with the immune culture fluid greatly reduced the attachment of theronts, trophont size and survival in excised tissues compared to those treated with the culture fluid from naive fish.  In vivo, fewer fish were infected and the infection density was less for fish exposed to theronts treated with immune culture fluid.  Results of this study show that cutaneous antibodies against Ich were present in and released from the excised skin from Ich immune fish.  The antibodies significantly reduced theront infectivity by immobilizing and weakening theronts.