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The development and duration of immune protection against Aeromonas hydrophila infections with garlic as an immunostimulant in rainbow trout Oncorhynchus mykiss was studied. Rainbow trout fingerlings of 14 g average weight were fed with 0 g (=Control), 0.5 g and 1.0 g of garlic 100 g−1 of feed for 14 days. Physiological factors, biochemical, immunological, haematological parameters and electrolyte indices were evaluated after a further 14, 21 and 28 days before challenge with Aeromonas hydrophila. Fourteen days after the cessation of feeding with garlic, mortality rates of 12% (relative percent survival [RPS] = 86%) and 16% (RPS = 80%) were recorded in groups which received 0.5 g and 1.0 g of garlic 100 g−1 of feed, respectively, compared to 84% mortalities in the controls. The corresponding RPS 21 days after ending the feeding regime was 75% and 68, respectively. One week later, the RPS had dropped to 55% and 46% in the groups fed with 0.5 g and 1.0 g garlic 100 g−1 of feed, respectively.

Sera from rainbow trout (Oncorhynchus mykiss, Walbaum), which had been immunised with live cells ofAeromonas hydrophila, caused strong agglutination of formalised and live cells ofA. hydrophila. In contrast, lower titres were recorded with sheared and pasteurised cells. Extremely weak agglutination occurred with boiled or lipopolysaccharide (LPS) deficient cells. By means of enzyme linked immunosorbent assays (ELISA), it was revealed that trout antiserum reacted with LPS, extracellular products (ECP), exopolysaccharides (EPS), and subcellular components derived from cells which had been pasteurised, sheared, boiled or extracted with EDTA and/or guanidine. It was concluded that LPS is the major antigenic component of whole cells, whereas EPS and a haemolytic component of the ECP could have induced an antibody response in the fish.

Rainbow trout immunised with live cells of Aeromonas hydrophila produced very low antibody titres, i.e. ≤1:6, against one strain (i.e. strain 168) but very high titres (1:5120–1:40 960) against the rough variant of strain 152 (=152r). However, the fish were unable to respond to two other colony variants of strain 152, coined as 152s and 152c. The inter-strain variation in antigenicity corresponded with morphological, serological, surface-associated and biochemical differences. The variation in antigenicity of the three colony forms of strain 152 was related with differences in hydrophobicity, precipitation after boiling, the ability to pellet, and the binding of Coomassie brilliant blue.

Rainbow trout ( Oncorhynchus mykiss Walbaum), immunised by injection, immersion and orally with killed and live cells of Aeromonas hydrophila , produced antibodies in the serum, bile, skin and gut mucus, and skin and muscle extracts. The antibodies were of a low affinity macroglobulin class, sensitive to 2-mercaptoethanol, and of a structure comprising heavy and light chains. These antibodies reacted maximally with formalised cells and lipopolysaccharide (LPS), and to a lesser extent with exopolysaccharide (EPS), flagella, S-layer protein and a haemolytic component of the extracellular products (ECP) of A. hydrophila .