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Systemic and Cutaneous Mucus Antibody Responses of Channel Catfish Immunized against the Protozoan Parasite Ichthyophthirius multifiliis

Department of Medical Microbiology and Parasitology, The University of Georgia, Athens, Georgia 30602-7386

Received 30 September 2002/ Returned for modification 30 December 2002/ Accepted 5 June 2003

Fish acquire protective immunity against the ciliated protozoan parasite Ichthyophthirius multifiliis following sublethal infection or inoculation with I. multifiliis immobilization antigens (i-antigens). In both cases, parasite-immobilizing antibodies have been identified in sera and mucosal secretions. To investigate the kinetics of this immune response, antibody levels were determined by enzyme-linked immunosorbent assay (ELISA) in the sera and cutaneous mucus of channel catfish (Ictalurus punctatus) that were either infected with parasites or given a single injection of purified i-antigen (5.0 µg/fish) in Freund's incomplete adjuvant. At 5 weeks, infected and inoculated fish had a mean serum (1:80 dilution) antibody absorbance (A₄₀₅) value of 0.54 ± 0.17 and 0.35 ± 0.03, respectively, which were significantly higher ( = 0.05) than the pretreatment serum (1:80 dilution) antibody absorbance value of 0.24 ± 0.05. At 14 weeks, mean serum (1:80 dilution) ELISA absorbance values in the teo groups of fish increased to 0.79 ± 0.30 and 0.71 ± 0.24, respectively. In both groups of fish, antibody levels in cutaneous mucus (undiluted) were much lower than those in sera. Infected fish had detectable mucus (undiluted) antibody levels from 3 to 9 weeks, with the highest mean value (0.30 ± 0.07) occurring at 7 weeks. Although individual inoculated fish produced serum antibody absorbance values comparable to those seen in infected fish, the mean mucus antibody values in this group did not rise above pretreatment levels. I. multifiliis infection induced a transient mucosal antibody response that coincided with the resolution of infection. Whether elicited by infection or intraperitoneal injection of i-antigen, the serum and mucus antibody responses of channel catfish immunized against I. multifiliis did not occur synchronously.