DAO 60:97-103 (2004)  -  doi:10.3354/dao060097

Infectivity of a Scottish isolate of Piscirickettsia salmonis for Atlantic salmon Salmo salar and immune response of salmon to this agent

T. Harry Birkbeck1,*, Shona Rennie1, Douglas Hunter2, L. Anthony Laidler2, Simon Wadsworth2,3

1Institute of Biomedical and Life Sciences, Division of Infection and Immunity, Joseph Black Building,University of Glasgow, Glasgow G12 8QQ, Scotland, UK
2Marine Harvest Scotland, Lochailort, Inverness-shire PH38 4LZ, Scotland, UK
3Present address: EWOS Innovation, 4335 Dirdal, Norway

ABSTRACT: A Scottish isolate of Piscirickettsia salmonis (SCO-95A), previously shown by intraperitoneal injection to have a lethal dose (LD50) of <2 × 103 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 × 105 tissue culture infective doses (TCID) of P. salmonis ml–1 for 1 h in a bath challenge resulted in only 1 mortality, 18 d later, in 10 exposed fish. Application of 2.5 × 106 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 109 heat-inactivated (100°C, 30 min) or 109 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.

KEY WORDS: Piscirickettsia salmonis · Piscirickettiosis · Salmonid rickettsial septicaemia · SRS · Immune response · Vaccine

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