AN EPIZOOTIC IN FARMED, MARKET-SIZE RAINBOW-TROUT IN SPAIN CAUSED BY A STRAIN OF CARNOBACTERIUM-PISCICOLA OF UNUSUAL VIRULENCE

We report here the first description in Spain of a Carnobacterium stra in causing important mortalities in market-size rainbow trout Oncorhyn chus mykiss. Relevant clinical signs in affected fish were a pronounce d bilateral exophthalmia with periocular hemorrhages, accumulation of ascitic fluid, and hemorrhages in the liver, swimbladder, muscle, and intestine. Taxonomic studies conducted in comparison with reference st rains indicated that the present isolate (PT-31) was Carnobacterium pi scicola. Agglutination assays demonstrated that this isolate was not s erologically related to the reference strains. In addition, an analysi s of the surface proteins revealed that different patterns occurred am ong the C. piscicola isolates. Although the immunoblotting assays supp orted the antigenic heterogeneity within this species, all strains did share 2 major antigenic proteins of 30 and 57 kDa. Interestingly, thi s immunoreactive 57 kDa protein is also produced by Renibacterium salm oninarum and Corynebacterium aquaticum. Regardless of the challenge me thod (injection or water-borne), isolate PT-31 proved to be highly pat hogenic for rainbow trout (LD50 < 5-6 x 10(4) cells), and the moribund fish displayed the external and internal signs observed in the natura l disease. The inoculated strain could be recovered from various organ s of both dead and surviving fish. The extracellular products (ECP) of strain PT-31 contained exotoxins lethal for rainbow trout (LD50 = 4.5 mug protein g-1 fish). However, the toxic ECP lacked enzymatic, cytot oxic, and hemolytic activities when tested in vitro. Histopathological examination of naturally and experimentally infected trout showed ext ensive and/or intensive lesions in most organs. The most marked tissue damage (acute hemorrhages and necrosis) was observed in eyes, kidney, liver, spleen, pancreas, and muscle. These findings should alert fish farmers to the existence of C. piscicola strains with a pathogenic ca pability higher than that previously recorded.