Abstract

In the past decade, Piscirickettsia salmonis has caused significant mortality among salmonid fish in Chile, Europe, and Canada.^2,3,4,5,6 Recently, a rickettsial pathogen very similar to P. salmonis has been isolated from Atractoscion nobilis (white seabass) in southern California.¹ The infected white seabass were being reared in a private hatchery as part of a restoration effort of a popular sport and commercial fishery. A potential threat is spread of this white seabass Piscirickettsia salmonis-like organism (WSPSLO) from the natural host to other important commercial species such as salmonids. The aim of this research was to experimentally infect white seabass and then re-isolate the bacteria following mortality to fulfill Koch's Postulates, and to evaluate transmission of the agent to two salmonids: coho (Oncorhynchus kisutch) and chinook (O. tshawytscha).

Juvenile white seabass used in this study originated from a hatchery in southern California. Fall run chinook and coho salmon were originally obtained as eyed eggs from California Department of Fish and Game hatcheries in northern California. All three species were reared in pathogen containment facilities at Bodega Marine Laboratory. For the transmission experiments, white sea bass were maintained in seawater at 20-21°C while the coho and chinook salmon were held in fresh water at 11°C. All fish were fed a commercial pelleted diet twice daily. The WSPSLO, originally isolated from liver tissue of moribund white sea bass^l, was maintained at 20°C in culture flasks containing monolayers of CHSE-214 (chinook salmon embryo) cells and minimum essential medium supplemented with 2% fetal bovine serum. The flasks were subcultured when 80% destruction of the monolayer was evident. Following sedation with 50 mg/L tricaine methanesulfonate (MS-222), eight white sea bass (6 g) and ten each of coho (5 g) and chinook (6 g) salmon were inoculated by intraperitoneal injection with 104 50% tissue culture infectious doses (TCIDs0) WSPSLO per fish with the culture medium from a 6-day infected flask of CHSE-214 cells. Control groups were injected with culture medium from uninfected CHSE-214 cells. Subsequent to mortality or the termination of the experiment, the liver and kidney of each fish were aseptically removed, tissue imprints were made, and the organs were homogenized for bacterial re-isolation. Culture plates containing monolayers of CHSE-214 cells were inoculated with the homogenate, incubated at 20°C and evaluated daily for 14 days for cytopathic effect.

All of the white sea bass died within 6-8 days post inoculation. None of the coho or Chinook salmon died during the course of the 4-week experiment, but the rickettsial agent was recovered from 80% and 100% of the fish, respectively (Table 1). All of the control fish survived and no WSPSLO was recovered from these fish at the end of the trial. This experiment fulfilled Koch's Postulates for WSPSLO infections in white sea bass and the bacteria produced clinical and pathological signs consistent with those described by Chen et al.¹ Moreover, recovery of WSPSLO from the coho and chinook demonstrates that this organism is transmissible to these species and is a potential marine pathogen for anadromous salmonids.

Table 1. The TCIDs0 of recovered Piscirickettsia salmonis-like organism from infected white seabass, chinook salmon, and coho salmon. ND= no cytopathic effect.

Acknowledgements

Funding for this work was provided by the California Sea Grant College Program. Support for S. Kozlowicz was provided by a research fellowship from the NCSU College of Veterinary Medicine.

References

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