Piscirickettsia salmonis is the etiologic agent of Salmonid Rickettsial Septicemia

We have used the expression library immunization technology to study the protection of Coho salmon Oncorhynchus kisutch to the infection with Piscirickettsia salmonis. Purified DNA from this bacterium was sonicated and the fragments were cloned in the expression vector pCMV-Bios. Two libraries were obtained containing 22,000 and 28,000 colonies and corresponding to approximately 8 and 10 times the genome of the pathogen, respectively. On average, the size of the inserts ranged between 300 and 1,000 bp. The plasmid DNA isolated from one of these libraries was purified and 20 μg were injected intramuscularly into 60 fish followed by a second dose of 10 μg applied 40 days later. As control, fish were injected with the same amount of DNA of the vector pCMV-Bios without insert. The titer of IgM anti-P. salmonis of vaccinated fish, evaluated 60 days post-injection, was significantly higher than that of the control group injected with the vector alone. Moreover, this response was specific against P. salmonis antigens, since no cross reaction was detected with Renibacterium salmoninarum and Yersinia ruckeri. The vaccinated and control fish were challenged 60 days after the second dose of DNA with 2.5 x 10 P. salmonis corresponding to 7.5 times the LD 50 . At 30 days post-challenge, 100% mortality was obtained with the control fish while 20% of the vaccinated animals survived. All surviving fish exhibited a lower bacterial load in the kidney than control fish. The expression library was also tested in Balb/c mice and it was found that the humoral immune response was specific to P. salmonis and it was dependent on the amount of DNA injected. Key terms: Piscirickettsia salmonis, genetic vaccine, expression library immunization. coccoid in shape and ranging 0.5 to 1.0 μm in diameter (Fryer et al., 1992). The pathogen was initially isolated from the kidney of a diseased adult coho salmon and successfully cultured in a cell lined derived from Chinook salmon (Oncorhynchus tshawytscha) embryos, known as CHSE214 cells (Fryer et al., 1990). Presently, the SRS has been partially cont ro l led my means of the use of antibiotics. These are not fully effective and have problems derived from their toxicity and generation of resistance. A more convenient procedure to prevent the infection is the use of vaccines. However, no effective vaccines are currently available against P. salmonis. This pathogen grows very slowly, taking 15 to 20 days to develop a full cytopathic effect making it difficult to produce large amounts of cells and to