An epizootic in farmed, market-size rainbow trout in Spain caused by a strain of Carnobacterium piscicola of unusual virulence

We report here the f~rs t description In Spaln of a Carnobactenum straln causing important mor ta l~ t~es in market-size rainbow trout Oncorhynchus mykiss Relevant cllnical signs In affected fish were a pronounced bilateral exophthalmia w ~ t h per~ocular hemorrhages, accurnulat~on of ascitic f lu~d . and hemorrhages in the liver, swimbladder muscle, and lntestlne Taxononuc studies conducted In companson with reference strains indicated that the present Isolate (PT-31) was Carnobactenuin p ~ s c i cola Agglutination assays demonstrated that this isolate was not serologically related to the reference stralns In addition, an analysis of the surface protelns revealed that different patterns occurred among the C piscicola isolates Although the ~mmunoblotting assays supported the antigenic heterogeneity within this specles all strains did share 2 major anhgenic proteins of 30 and 57 kDa Interestingly, this ~mmunoreactlve 57 kDa protein 1s also produced by Renibactenum salmon~narum and Corynebacterium aquat~cum Regardless of the challenge method (~njection or water-borne) isolate PT-31 proved to be hlghly pathogen~c for ralnbow trout (LD,, < 5-6 X 104 cells), and the monbund fish dlsplayed the external and internal slgns observed in the natural dlsease The inoculated strain could be recovered from varlous organs of both dead and surviving fish The extracellular products (ECP) of straln PT-31 contained exotoxins lethal for r a~nbow trout = 4 5 kg protein g' fish) However the tonc ECP lacked enzymatic, cytotoxic and hemolytic activities when tested In vitro H~stopathological examination of naturally and experimentally infected trout showed extensive and/or intenslve lesions in most organs The most marked tissue damage (acute hemorrhages and necrosis) was observed in eyes, kidney, l~ver , spleen pancreas, and muscle These findings should alert flsh farmers to the exlstence of C piscicola strains with a pathogenic capabll~ty higher than that previously recorded