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

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Abstract
Fish acquire protective immunity against the ciliated protozoan parasiteIchthyophthirius multifiliisfollowing sublethal infection or inoculation withI. multifiliisimmobilization 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 (α = 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. multifiliisinfection 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 againstI. multifiliisdid not occur synchronously.

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