Cytophaga psychrophila, the causative agent of bacterial cold-water disease in salmonid fish Public Deposited

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  • Morphological, cultural, biochemical and serological characteristics of 33 bacterial strains, thought to be similar to Cytophaga psychrophila, the causative agent of bacterial cold-water disease were compared. Bacterial strains identified as C. psychrophila were obtained from diseased salmonids collected at widely separated geographic locations, and from fish with pathological signs different from bacterial cold-water disease. These included coho salmon with spinal deformities and nervous tissue involvement, chinook salmon with necrotic white liver or mixed infections (viral anemia), and rainbow trout with gill lesions. Cytophaga psychrophila was also found in tissues of sexually mature adult salmon including ovarian fluid. Presence of the bacterium in ovarian fluid suggests an opportunity for vertical transmission. Cytophaga psychrophila strains exhibited gliding motility, were actively proteolytic and grew only at temperatures of 25°C or lower. All strains possessed common antigens but serological differences were detected. Shieh, tryptone-yeast extract and modified Cytophaga broth provided improved growth when compared to Cytophaga broth. The percent guanine plus cytosine in DNA was 34.3. Cellular protein patterns determined by polyacrylamide gel electrophoresis and fatty acid composition were found to be similar for all C. psychrophilia strains but distinctly different from other cytophagals. Flexibacter aurantiacus strains were sufficiently different from C. psychrophila, to be considered separate species. The existence of strains with high or low virulence were demonstrated by infectivity experiments. The effect of water temperature on progress of infection in three species of salmonids was determined. Mortality was highest for all three species at temperatures of 3-15°C but decreased progressively up to 23°C. The mean time from infection to death was shortest at 15°C which correlates with the most rapid doubling time for this bacterium in culture medium. Deaths from experimental infections of C.psychrophila could be prevented by exposure of infected steelhead trout to a water temperature of 22°C. Vaccination by intraperitoneal injection of formalin-killed cells and adjuvant or by direct immersion in formalin-killed cells effectively protected coho salmon against experimental challenge with this bacterium. With the immersion method, best protection was obtained in fish at 1.5 g mean weight or larger. Serological type of C. psychrophila, incorporated in bacterin preparations influenced efficacy of the bacterin.
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