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Diseases of Aquatic Organisms

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DAO 63:139-149 (2005)  -  doi:10.3354/dao063139

Genetic characterization and experimental pathogenesis of Piscirickettsia salmonis isolated from white seabass Atractoscion nobilis

Kristen D. Arkush1,2,*, Anne M. McBride1, Holly L. Mendonca1, Mark S. Okihiro3, Karl B. Andree2, Sergio Marshall4, Vitalia Henriquez4, Ronald P. Hedrick2

1Bodega Marine Laboratory, PO Box 247, Bodega Bay, California 94923, USA
2Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, One Shields Avenue, Davis, California 95616, USA
3California Department of Fish and Game, 4065 Oceanside Boulevard, Suite G, Oceanside, California 92056, USA
4Laboratorio de Genetica e Inmunologia Molecular, Instituto de Biología, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
*Present address: Bodega Bay. Email:

ABSTRACT: An intracellular bacterium originally isolated from hatchery-reared juvenile white seabass Atractoscion nobilis in southern California, USA, was identified by sequences of the small and large subunit ribosomal (16S and 23S) DNA and the internal transcribed spacer (ITS) as Piscirickettsia salmonis. Considering all rDNA sequences compared, the white seabass isolate (WSB-98) had a 96.3 to 98.7% homology with 4 previously described strains of P. salmonis isolated from salmon in Chile, Norway, and British Columbia, Canada. Experimental infections induced by intraperitoneal injections of juvenile white seabass with WSB-98 resulted in disease and mortality similar to that observed in P. salmonis infections in salmon. After 60 d, the cumulative mortality among P. salmonisinjected white seabass was 82 and 40%, respectively, following a high (1.99 × 104 TCID50) or low (3.98 × 102 TCID50) dose-challenge with WSB-98. The bacterium was recovered by isolation in cell culture or was observed in stains from tissues of injected white seabass but not from control fish. There were no external signs of infection. Internally, the most common gross lesion was a mottled appearance of the liver, sometimes with distinct nodules. Microscopic lesions were evident in both the capsule and parenchyma of the liver and were characterized by multifocal necrosis, often with infiltration of mononuclear leukocytes. Macrophages filled with bacteria were present at tissue sites exhibiting focal necrosis. Foreign body-type granulomas were prevalent in livers of experimentally infected white seabass, but not in control fish. Similar granulomatous lesions were observed in the spleen, kidney, intestine and gills, but these organs were considered secondary sites of infection, with significantly fewer and less severe histologic lesions compared to the liver. The results from this study clearly indicate that infections with P. salmonis are not restricted to salmonid fishes and that the bacterium can cause a disease similar to piscirickettsiosis in nonsalmonid hosts.

KEY WORDS: Rickettsia-like · Seabass · Piscirickettsia salmonis · 16S rRNA

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