In vivo study of phagocytosis, intracellular survival and multiplication of Flavobacterium psychrophilum in rainbow trout, Oncorhynchus mykiss (Walbaum), spleen phagocytes

The intracellular behaviour of a Flavobacterium psychrophilum strain, inges ted by spleen phagocytes of rainbow trout, Oncorhynchus mykiss, of differen t ages, was assessed in vivo. Three groups of rainbow trout weighing 1 g (a ged 10 weeks), 25 g (aged 20 weeks) and 300 g (aged 15 months), respectivel y, were injected intraperitoneally with 1 x 10(6) cfu of a F. psychrophilum strain. It was found that only fry, aged 10 weeks, displayed clinical sign s and suffered mortality. Bacteriological colony plating of different organ s demonstrated that the spleen and to a lesser extent the kidney of only th e fry were affected. The number of colony forming units per gram of spleen tissue increased with time. No bacteria were found in the trout aged 5 mont hs and older. Light microscopical examination and epifluorescence microscop y revealed that the fry spleen phagocytes; contained an increasing number o f viable intracellular F. psychrophilum bacteria over time. Again, no bacte ria were encountered in the phagocytes collected from older fish.