Studies on rainbow trout fry syndrome (RTFS)

Abstract

A comprehensive survey of representative rainbow trout (Oncorhynchus mykiss) hatcheries revealed that the Gram negative, yellow-pigmented, filamentous bacterium Cytophaga psychrophila was implicated in a single disease in the U.K. and other European states. The involvement of C. psychrophila as the aetiological agent of the syndrome was substantiated by the fulfilment of Koch's postulates. Infectivity studies with isolates of C. psychrophila, carried out under natural and laboratory conditions successfully reproduced clinical signs and gross pathological changes analogous to those observed during field outbreaks of the condition. Histopathological examination of artificially and naturally infected fish tissues demonstrated several features that were considered to be pathognomonic for RTFS. Preliminary electron microscopic studies described the ultra-structure of C. psychrophila and partially elucidated the cellular response to the pathogen. Environmental sampling across selected hatchery sites demonstrated that members of the family Cytophagaceae formed a substantial element of the bacterial flora from natural waters, although recovery of C. psychrophila was restricted to areas where the substantial mortalities in fry attributable to RTFS had occurred. C. psychrophila was isolated from the sexual fluids of broodstock and additionally the bacterium was demonstrated associated with surfaces of eyed ova following various disinfection regimes. The minimum inhibitory concentrations of a range of antimicrobial agents both existing in, and novel to, aquaculture were examined, revealing compounds which would potentially mitigate losses attributable to RTFS during field outbreaks. The emergence of bacterial resistance to chemotherapeutants was discussed. The minimum inhibitory and bactericidal concentrations and required exposure times to a number of disinfecting agents were demonstrated. The efficacy of these agents as disinfectants of egg surfaces and equipment associated with fish production was assessed. The potential of a number of serodiagnostic techniques were evaluated as a means of rapid detection of C. psychrophila in diseased fish.