DAO 73:131-139 (2006)  -  doi:10.3354/dao073131

ABSTRACT: White seabass Atractoscion nobilis surviving experimental exposure to Piscirickettsia salmonis harbored the bacterium for periods up to at least 123 d post injection (dpi). Intraperitoneal injections of juvenile white seabass with 1.26 × 10² TCID₅₀ P. salmonis fish^–1 resulted in a 29% cumulative mortality over a 27 d period. Both molecular and histologic methods provided evidence for persistence of the bacterium in fish sampled sequentially from the surviving population. Throughout the period of acute mortality, the bacterium was detected in all impression smears of liver tissue stained with Giemsa and was reisolated in cell cultures from all dead fish sampled. Polymerase chain reaction (PCR) assays detected P. salmonis-specific DNA in 13.3 to 50% of the fish sampled at time points between 28 and 123 dpi, while cell culture reisolation was largely ineffective in detecting the bacterium. An enzyme-linked immunosorbent assay (ELISA) detected serum anti-P. salmonis antibodies in 48 of 59 white seabass exposed to P. salmonis but not in fish which were not exposed to the bacterium. At the end of the 4 mo experiment, microscopic lesions consisting of single to multiple and coalescing granulomas were found in liver and kidney tissues of 9 of 10 fish examined from the exposure group, while no lesions were detected in the fish from the control group. Immunohistochemical staining with anti-P. salmonis polyclonal antibodies detected bacterial antigens in some but not all granulomas examined from the exposure group at 4 mo. This study demonstrates that P. salmonis may persist among white seabass following infection and thus provide a potential reservoir of infection contributing to transmission both within and between fish species in the marine environment.

KEY WORDS: Piscirickettsia salmonis · White seabass · Persistent infection · Carrier state