Nguyen

Co-infection of Mycobacterium salmoniphilum and M. chelonae in Chinook salmon (Oncorhynchus tshawytscha)

Diem Thu Nguyen1,2, Cynthia Ware3, David Marancik2, Matt Griffin3, Esteban Soto1

1Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, 2108 Tupper Hall, Davis, CA 95616-5270, USA; 2Department of Pathobiology, School of Veterinary Medicine, St. George's University, True Blue, Grenada, West Indies; 3 Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, 127 Experiment Station Road, PO Box 197, Stoneville, MS 38776, USA

ABSTRACT

Chinook salmon (Oncorhynchus tshawytscha) are keystone fish species in the Pacific Northwest. In 2019, unusual mortality occurred in cultured Chinook fingerlings in Davis, CA. The culture system consisted of outdoor, closed, flow-through freshwater tanks maintained at 18±1oC. Clinical signs and gross findings included: abnormal swimming, inappetence, lethargy, skin discoloration and the presence of multifocal nodules and ulcerative skin lesions. Histopathological findings demonstrated variable amounts of granulomatous inflammation but lacked granuloma formation more typically associated with mycobacteria infections. Posterior kidney swabs were collected and inoculated in nutrient rich and selective agar media and incubated at 25oC for 2 weeks. Pure bacterial colonies were observed 7-10 d post-inoculation. Partial sequences of 16S rRNA initially identified the recovered bacteria as members of the genus Mycobacterium. However, marked genetic variability was observed among the recovered Mycobacterium spp. isolates using repetitive extragenic palindromic PCR fingerprinting. Amplification and sequencing of the ribosomal RNA internal transcribed spacer region, 65-kDa heat shock protein and RNA polymerase b-subunit gene identified isolates from affected fish as Mycobacterium salmoniphilum and M. chelonae, two different members of the “Abscessus-Chelonae” clade, suggesting a co-infection of these two congeners associated with mortality in captive salmon.

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