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Diseases of Aquatic Organisms

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DAO 134:175-187 (2019)  -  DOI:

Pathogenicity of the bacterium New Zealand rickettsia-like organism (NZ-RLO2) in Chinook salmon Oncorhynchus tshawytscha smolt

C. L. Brosnahan1,2,*, J. S. Munday2, P. S. Davie2, L. Kennedy1, M. Preece3, S. Barnes3, J. B. Jones4, W. L. McDonald1

1Animal Health Laboratory, Ministry for Primary Industries, Ward Street, Wallaceville, Upper Hutt, 5018, New Zealand
2School of Veterinary Sciences, Massey University, Tennent Drive, Palmerston North 4442, New Zealand
3New Zealand King Salmon, Dublin Street, Picton 7220, New Zealand
4Murdoch University School of Veterinary and Life Sciences, Perth, WA 6150, Australia
*Corresponding author:

ABSTRACT: Farmed New Zealand Chinook salmon Oncorhynchus tshawytscha Walbaum have been found to be infected by rickettsia-like organisms (NZ-RLO). While these Gram-negative intra-cellular bacteria are closely related to Piscirickettsia salmonis, a significant pathogen for farmed salmon globally, the pathogenicity of NZ-RLO is unknown. The aim of the present study was to determine if one strain, NZ-RLO2, causes disease in Chinook salmon. Post-smolt salmon were inoculated with NZ-RLO2 by intraperitoneal injection at high, medium and low doses and observed for 30 d. All fish in the high and medium dosed groups died by the end of the study and 63% of the low dose group died within 30 d of inoculation. Necropsy revealed the fish inoculated with NZ-RLO2 had internal multifocal haemorrhages. The most consistent histological finding in fish inoculated with NZ-RLO2 was neutrophilic and necrotizing pancreatitis and steatitis with intra-cytoplasmic organisms often visible within areas of inflammation. Other histological lesions included multifocal hepatic necrosis, haematopoietic cell necrosis and splenic and renal lymphoid depletion. The presence of NZ-RLO2 within the inoculated fish was confirmed by replication in cell culture and qPCR. The results suggest NZ-RLO2 can cause disease in Chinook salmon and therefore could be a significant pathogen in farmed Chinook salmon.

KEY WORDS: Chinook · Rickettsia-like · Piscirickettsia salmonis · Histopathology · qPCR · Culture

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Cite this article as: Brosnahan CL, Munday JS, Davie PS, Kennedy L and others (2019) Pathogenicity of the bacterium New Zealand rickettsia-like organism (NZ-RLO2) in Chinook salmon Oncorhynchus tshawytscha smolt. Dis Aquat Org 134:175-187.

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