A Scottish isolate of Piscirickettsia salmonis (SCO-95A), previously shown by intraperitoneal injection to have a lethal dose (LD 50 ) of < 2 × 10 3 infectious rickettsial units, was tested for virulence by bath challenge, surface application to the skin, or dorsal median sinus injection. Atlantic salmon Salmo salar post-smolts were used in all experiments, and exposure to 1 × 10 5 tissue culture infective doses (TCID) of P. salmonis ml -1 for 1 h in a bath challenge resulted in only 1

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... , 18 d later, in 10 exposed fish. Application of 2.5 × 10 6 TCID of P. salmonis SCO-95A to paper discs on the skin failed to induce any mortalities within 42 d. Intraperitoneally, fish were administered vaccines containing 10 9 heat-inactivated (100°C, 30 min) or 10 9 formalin-inactivated P. salmonis SCO-95A in adjuvant, with a control group receiving phosphate-buffered saline (PBS) in adjuvant. After an induction period of over 6 mo fish were challenged by injection of P. salmonis into the dorsal median sinus. Mortalities in the control group reached 81.8% and the heat-inactivated and formalin-inactivated vaccines gave significant protection from P. salmonis, with relative percentage survivals of 70.7 and 49.6%, respectively. The nature of the protective antigen is unknown, but could be lipopolysaccharide or a heat-stable outer membrane protein. Fish that survived a dorsal median sinus challenge of P. salmonis or were cohabitants showed a strong immune response to P. salmonis.