Immunostimulants added to injected Aeromonas salmonicida bacterin enhance the defense mechanisms and protection in rainbow trout (Oncorhynchus mykiss).

Immunostimulants were given to rainbow trout for assaying effects on modulating non-specific defense mechanisms, specific immune response, and protection levels against pathogen challenge with Aeromonas salmonicida. Three drugs, levamisole (an approved veterinary drug in the USA), a quaternary ammonium compound (QAC), and a short-chain polypeptide (ISK) were found to affect the non-specific defense mechanism activities, which were measured by changes in circulatory neutrophil and phagocytic activity levels, and the specific immune response factors, which were measured by numbers of plaque-forming cells, and circulatory antibody levels. When given alone, the immunostimulants elevated the non-specific factors. When injected in combination with an A. salmonicida O-antigen bacterin, the non-specific factors were further elevated, and the specific response was raised over samples taken from fish given the bacterin without the immunostimulants. Challenge tests with the virulent pathogen, A. salmonicida, showed a 5-6 day delay in the onset of mortalities in the fish given the immunostimulants alone, and a 12-14 day delay when immunostimulants given were combined with the bacterin. In the groups given the QAC or ISK with the bacterin, there was a 20% and 40% survival rate, respectively.