Isolates of Flavobacterium psychrophiluln (formerly Cytophaga psychrophila and Flexlbacter psychrophilus) mainly originating from clinical outbreaks of either coldwater disease (CWD) or rainbow trout fry syndrome (RTFS) were studied for selected biochemical, physiological, rnorphological and genomic characteristics, and compared with previously characterized French and American strains. DNA hybridization studies showed that the Danish isolates were highly related to the type strain, F. psychrophilum NCIMB 1 9 4 7~ Plasmid profiling of Danish isolates and those from other European countries revealed differences, which might be related to differences in pathogenicity. European ~solates originating from clinical outbreaks of either RTFS or CWD usually harboured one plasmid of 3.2 kb, whereas isolates originating from fish with different or no disease slgns had other profiles. Phenotypically, the Danish isolates appeared very homogeneous and shared most characteristics with the type strain, and with French and American strains studied by other authors. Further studies on the importance of the plasmids and the proteolytic activities of the bacterium might help in elucidating possible virulence factors.

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The possibility of serological differentiation between isolates of Flavobacterium psychrophilum was analyzed by ELISA and slide agglutination. Twenty-five Dan~sh Isolates and 20 isolates from other European countries were studied using polyclonal rabbit antisera and whole-cell preparations. Unabsorbed as well as reciprocally absorbed antisera and purified Ig preparations derived from the antisera were included. Most of the isolates originated from clinical outbreaks of rainbow trout fry syndrome (RTFS) or coldwater disease (CWD), but some were isolated from asymptomatic fish or from other fish species with different dlsease signs. The ELISA showed the existence of different serotypes most distinctly, but slide agglutination supported the ELISA results. Three serotypes were found among the isolates studied: 1 major serotype (serotype Th) represented most of the Danish isolates and isolates from other European countries; 2 minor serotypes (serotypes Fd and FpT) also occurred. Serotype Th could be further divided into a major subtype, Th-1, and a minor subtype, Th-2. Serotype FpT was deflned by the type strain F. psychrophilurn NCIMB 1 9 4 7~, and seemed to include mostly ~solates from asymptomatic f~s h or from fish species other than rainbow trout

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ABSTRACT. Eleven European National Reference Laboratories participated in an inter-laboratory comparison of the susceptibility of 5 selected cell lines to 3 fish pathogenic viruses. The test included viral hemorrhagic septicaemia virus (VHSV), infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV), and the cell llnes derived from bluegill fry (BF-2), chinook salmon embryo (CHSE-214), epithelioma papulosum cyprin~ (EPC), fathead minnow (FHh4) and rainbow trout gonad (RTG-2) The results showed that for isolation of VHSV, BF-2 and RTG-2 cells performed equally well and had higher sensitivity compared to the other cell lines. For 1HNV. EPC and FHM cells gave the best results, and for IPNV it was BF-2 and CHSE-214 cells. FHM cells showed the largest variability among laboratories, whereas EPC was the cell line showing the smallest variability.

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