Humoral and peritoneal cell responses of rainbow trout (Oncorhynchus mykiss) to ovalbumin, Vibrio anguillarum and Freund's complete adjuvant following intraperitoneal and bath immunisation

ELISA methods were used to evaluate the humoral immune responses of rainbow trout (Oncorhynchus mykiss) to ovalbumin and Vibrio anguillarum. Antibody responses to ovalbumin administered intraperitoneally (i.p.) were inconsistent even when Freund's complete adjuvant (FCA) was used and the induction phase (4-6 weeks) of the response was longer compared with the response to V. anguillarum (<4 weeks). Significant elevation in antibody level was noted 3 weeks after bath vaccination with V. anguillarum but levels decreased thereafter. Humoral responses of greatest magnitude occurred where V. anguillarum was given in an emulsion with Freund's complete adjuvant (FCA). However, i.p. administration of FCA alone 3 weeks prior to i.p. immunisation with non-adjuvanted V. anguillarum resulted at 5 weeks in similar elevations in antibody to those in fish given V. anguillarum and FCA concurrently, suggesting that the effects of FCA were not limited to creation of an antigen depot. Proliferation of peritoneal inflammatory cell populations which included macrophages and plasma cells was detected histologically within 3 weeks of administration of FCA, but changes were not detected in the spleen or haematopoietic kidney.