Response of Rainbow Trout (Salmo gairdneri) to Antigens of Aeromonas hydrophila

Abstract

Rainbow trout (Salmo gairdneri) produced antibodies specific for Aeromonas hydrophila. Two parenteral routes yielded no measurable difference in rate of antibody synthesis. Method of bacterin suspension indicated adjuvant suspension was superior in maintenance of humoral antibody and challenge protection when compared to saline suspension of the organism.Antibody synthesis from adjuvant-suspended bacterin lagged approximately 30 days behind the rate of synthesis by fish receiving saline suspended bacteria. The highest measurable antibody titer reached by an individual was 1:64. Individuals receiving saline suspended bacterin showed evidence of loss of titer approximately 2 months after the last antigen inoculation. Definite decrease in titer was noted 5 months after the last antigen inoculation and the return to a probable nonimmune class by approximately one-half of the individuals 7 months after the last inoculation. All individuals receiving adjuvant-suspended bacterin remained in the probable immune class for as long as 6 months following the last bacterin inoculation, but only approximately one-half of the individuals remained in the probable immune class 7 months postinoculation.All fish receiving a weighed quantity of killed A. hydrophila orally each day for 9 consecutive months showed presence of humoral A. hydrophila antibody. This immunization procedure produced 50% of the individuals with humoral antibody titers considered to be in the probable immune class.Fish receiving no immunization remained with negative antibody titers throughout the experimental period.Definite protection was given fish which had been immunized by parenteral inoculation.Similar protection was given after immunization by either intramuscular or intraperitoneal routes. Fish receiving either saline or adjuvant-suspended bacterin parenterally showed analogous protection when challenged by the specific bacteria. Fish receiving oral bacterin received questionable protection against parenteral challenge of the specific organism.Phagocytosis was found to occur to a greater extent in immunized rainbow trout than in nonimmunized rainbow trout. An average phagocytic index of parenterally immunized rainbow trout was 1.08. Nonimmunized rainbow trout of the same age and egg source had an average phagocytic index of 0.30. The opsonic index was 3.6.