Research

The immune response of rainbow trout to Flavobacterium psychrophilum following immersion-challenge model with and without hydrogen peroxide pre-treatment

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Abstract

The bacterial fish pathogen Flavobacterium psychrophilum is a major cause of mortality in farmed rainbow trout (Oncorhynchus mykiss) and other salmonid fish. The disease following infection is called bacterial coldwater disease (BCWD) or rainbow trout fry syndrome (RTFS). To our knowledge, no commercial vaccine is currently available and the disease is treated with antibiotics. Injection-based challenges with F. psychrophilum are standardized but the route of infection does not reflect a natural situation. Therefore, we established an immersion-based model investigating if hydrogen peroxide (H2O2) pre-treatment could elevate infection and mortality. The model consisted of four groups: 1) Un-exposed control, 2) H2O2 exposure, 3) F. psychrophilum immersion and 4) H2O2 + F. psychrophilum. Pre-treatment with H2O2 increased mortality two-fold if fish also were exposed to F. psychrophilum after pretreatment. Tissue samples were taken from the involved groups 4 h, 48 h, 125 h and 192 h post-exposure and investigated for regulation of immune genes. Following genes were examined in the head kidney and gills by qPCR: IgT, IgM, CD8, CD4, MHC I, MHC II, IL-4/13A, TcR-β, IL-10, IL-6, IL-1β, IL-17, SAA and FoxP3. A pro-inflammatory response was indicated, but only a weak indication of an adaptive response was recorded (most evident in the F. psychrophilum group). Further, pre-treatment with H2O2 affected the correlation gene expression and pathogen load in several cases. Morphological changes in the gill tissue were evaluated using hematoxylin and eosin stained tissue sections. Exposure to both H2O2 and F. psychrophilum intensified tissue damage and postponed healing. The results indicate that F. psychrophilum may have an immunosuppressive action and that environmental stress may be one of several factors playing a role in RTFS outbreaks.