Serum And Cutaneous Antibody Responses Of Channel Catfish (Ictalurus punctatus) Against Ichthyophthirius multifiliis Following Immunization By Infection Or Injection Of Immobilization Antigen (I-Ag)     Joanne L. Maki and Harry W. Dickerson   Department of Medical Microbiology and Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA     We are using channel catfish infected with Ichthyophthirius multifiliis (Ich) to investigate mechanisms of mucosal immunity. In this study, serum and cutaneous antibody (Ab) responses of catfish fingerlings (n=20/group) were evaluated following immunization by either Ich infection or inoculation with purified i-Ag (5mg/fish). Sera and cutaneous mucus were collected bi-weekly for 14 weeks and Ab levels measured by ELISA. Abs in sera (1:80) and mucus (undiluted) were detected with a biotinylated-goat anti-catfish Ig Ab. Both groups (Ich-exposed and i-Ag inoculated) had mean sera absorbance (A405) values that were significantly higher than pre-treatment (0.241+0.047) at 5 weeks and increased through the sampling period to 0.754+.264. In mucus samples, Ich-exposed fish had A405 values above pre-treatment levels (0.210+0.041) at 3-9 weeks with a peak value (0.301+0.066) at 7 weeks. Mucus Ab in inoculated fish varied by tank and week. In one tank (n=10 fish) A405 values were >0.251 at 2,3 and 11 weeks. Three fish had A405 values >0.300 at 9 weeks (n=1) and 14 weeks (n=2). In a replicate tank (n=10), mucus Abs were not detected above background even though fish in both tanks had similar serum Ab levels. In summary, fish immunized by Ich-exposure or i-Ag inoculation had relatively high serum Ab levels that increased over time.  Mucus Ab levels were relatively low in both groups, were detected transiently in the Ich-exposed fish and occurred sporadically in the inoculated fish. Production of cutaneous and serum Ab against Ich does not appear to occur synchronously.