EXPERIMENTAL-INFECTION OF BROWN TROUT, SALMO-TRUTTA L, USING A FINNISH YERSINIA-RUCKERI ISOLATE

The pathogenicity of a Finnish Yersinia ruckeri isolate to 1-year-old brown trout was tested using both immersion and injection mode of infe ction. The occurrence of bacteria and antibody production were monitor ed for up to 25 days. No disease signs or mortality due to Y. ruckeri occurred in the immersion experiment where the fish were further stres sed with Ichthyophthirius multifiliis, but Y. ruckeri was isolated fro m certain organs in half of the fish. The intestine, kidney and spleen were the most favoured sites of infection, only one fish having bacte ria in the liver. The injection of brown trout caused 100% Y. ruckeri infection and some mortality was recorded during the first week post i nfection. All dead fish exhibited signs of bacterial infection but non e had inflammation around the mouth. Antibodies against Y. ruckeri wer e produced in both the groups subjected to immersion and that subjecte d to injection but the production was higher in the injected fish. The se results confirm earlier findings that the Finnish Y. ruckeri is les s pathogenic than, for example, Y. ruckeri in USA, and indicate that m ore experimental studies are needed to examine the significance of geo graphical variation of Y. ruckeri strains in terms of their pathogenic ity.