Standardization of experimental infection with Flavobacterium psychrophilum, the agent of rainbow trout Oncorhynchus mykiss fry syndrome.

Rainbow trout fry syndrome (RTFS) is a septicaemic infection of young rainbow trout Oncorhynchus mykiss occurring at low temperatures and responsible for severe economic losses in European fish farming. The causative agent, Flavobacterium psychrophilum, is a gliding bacterium, and difficulties in culturing it have long been an impediment to investigations on pathogenesis and immunity. Successful attempts at experimentally inducing the disease have been reported, but no experimental model resulting in well-controlled and quantitatively reproducible effects has been described. Recent improvements in F. psychrophilum cultivation made it possible to produce bacterial suspensions with nearly constant viability and to complete challenge injections in rainbow trout fingerlings, using accurately adjusted infective doses. Parenteral injection resulted in significant mortality, which was higher when administered intramuscularly (IM) than intraperitoneally (IP). Lethal doses 50 % lower than 10(3) colony forming units were consistently obtained in trout weighing 3 to 5 g, and the regular shape of the cumulative mortality curves appeared to lend itself to quantitative analyses. Bath experiments produced milder effects, although mortality ranging between 45 and 60 % was obtained in 6 g trout when skin lesions or stressors were induced along with bacterial exposure. Temperature, salinity and the process of preserving isolates (at least over short periods of time) did not seem to be associated with the severity of infection. Nevertheless, infection trials performed at 2 different locations differing both in water quality and in the system of fish maintenance resulted in different mortalities. These findings notwithstanding, the proposed IM model appears easy to apply under standardized experimental conditions and should contribute to effective advances in the study of the disease.