Research

Immersion challenge with Flavobacterium psychrophilum in rainbow trout fry (Oncorhynchus mykiss)

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Abstract

The fish pathogen Flavobacterium psychrophilum is one of the main causes of mortality in farmed rainbow trout (Oncorhynchus mykiss) and other salmonid fish. Presently no commercial vaccine exists, although several are under development. Various models for experimental infection have been carried out with varying success, including challenge through injection, bath and cohabitation. Intraperitoneal challenge and bath challenges combined with various forms of stress have shown to be reproducible. Bath challenge is more appropriate for vaccine testing, since natural transmission of infection is imitated and is also more suitable due to the small size of the fry. Various chemicals are used against pathogens in aquacultures, including copper sulphate, chloramine-T, sodium carbonates, sodium chloride, formalin and hydrogen peroxide (H2O2). One of the more successful immersion models made use of formalin as a stressor, but a less harmful chemical was desirable, since the use of formalin is to be phased out in the Danish farms by 2014 due to health considerations. The aim of this study was to establish a reproducible method for immersion challenge of rainbow trout fry to be used in investigations concerning the immune response and vaccine testing. Various concentrations of H2O2 were tested, before being combined with immersion exposure to the wellknown, virulent strain 950106-1/1, which was used for all infections. The mortality of F. psychrophilum infection in fry was increased by pre-treating with H2O2, although variation was very high.