Difficulties in experimental infection studies with Flavobacterium psychrophilum in rainbow trout (Oncorhynchus mykiss) using immersion, oral and anal challenges

This study was carried out in order to try to establish an efficacious and reliable experimental infection model for Flavobacterium psychrophilum, the causative agent of rainbow trout fry syndrome, using contact, oral and ana l challenges. Ten F psychrophilum strains of different origin were included . The influence of water temperature, scarification, water quality, stress and growth conditions of the pathogen on the experimental infection was ass essed. For each challenge protocol, all strains failed to reproduce disease signs or mortality in rainbow trout (Oncorhynchus mykiss L.) fry. Histolog ical and bacteriological examination of the skin, gills and internal organs of the fish 3 weeks following inoculation were found to be negative. Diffe rent hypotheses to explain the inability of the challenge models to reprodu ce the disease experimentally are discussed. (C) 2000 Harcourt Publishers L td.