Abstract
The immune response in rainbow trout fry against Flavobacterium psychrophilum was elucidated using an immersion‐based challenge with or without prior exposure to hydrogen peroxide (H 2 O 2 ). Samples were taken from the head kidney 4, 48, 125 and 192 h after immersion, and the regulation of several genes was examined. Bacterial load was assessed based on the presence of 16S rRNA and correlated with gene expression, and the levels of specific antibodies in the blood were measured 50 days post‐infection. Separately, both H 2 O 2 and F. psychrophilum influenced gene expression, and pre‐treatment with H 2 O 2 influenced the response to infection with F. psychrophilum . Pre‐treatment with H 2 O 2 also affected correlation between gene regulation and pathogen load for several genes. A delay in antibody production in H 2 O 2 ‐treated fish in the early phase of infection was indicated, but H 2 O 2 exposure did not affect antibody levels 50 days post‐infection. An increasing amount of F. psychrophilum 16S rRNA was found in the head kidneys of infected fish pre‐treated with H 2 O 2 relative to the F. psychrophilum group. The results show that a single pre‐treatment with H 2 O 2 impairs the response against F. psychrophilum and may intensify infection.