Plaque-Forming Cells and Humoral Antibody in Rainbow Trout (Salmo gairdneri) Induced by Immersion in a Yersinia ruckeri O-Antigen Preparation

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Abstract
Rainbrow trout (S. gairdneri) were exposed to the O-antigen[Ag] of Y. ruckeri by various immunization regimens. The passive hemolytic plaque assay showed specific splenic plaque-forming cells (PFC) and passive hemagglutination demonstrated humoral antibody [Ab] titers in fish injected with or immersed in the Ag preparations. Preceding Ag immersion with a 2-min immersion in a 5.2 or a 2.6% NaCl solution did not affect the numbers of PFC or levels of Ab. Fish held for 2 min in ethyl m-aminobenzoate methanesulfonate (MS-222) before Ag immersion showed no significant difference from the control fish. Dosage studies showed that the minimal concentration of Ag for induction of PFC by immersion was 5.0 .mu.g/ml.