Nguyen

Virulence of Flavobacterium psychrophilum Sequence Type 78, 85 and 319 to Different Cultured Rainbow Trout Oncorhynchus mykiss Strains in California, USA

Diem Thu Nguyen1,2, Mark Adkison3, Thomas Loch4, Christopher Knupp4, Esteban Soto1

1Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California Davis, 95616, Davis, CA, USA; 2Department of Pathobiology, St. George's University School of Veterinary Medicine, Grenada; 3California Department of Fish and Wildlife Fish Health Laboratory, Rancho Cordova, California, USA; 4Michigan State University - Aquatic Animal Health Laboratory, Aquatic Animal Disease Ecology Program (Department of Fisheries and Wildlife and Department of Pathobiology and Diagnostic Investigation, Michigan State University)

ABSTRACT

Flavobacterium psychrophilum is a psychrophilic, Gram-negative bacterium responsible for significant economic losses globally for the aquaculture industry, particularly in cultured salmonids. Flavobacterium psychrophilum isolates recovered in California have been assigned into 11 distinct multilocus sequence typing (MLST) sequence types (STs) with the majority of genotyped isolates belonging to a single clonal complex (CC), CC-ST10. Although CC-ST10 is the largest CC worldwide and has been associated with disease outbreaks on several continents, few studies have investigated the susceptibility of rainbow trout strains to F. psychrophilum genotypes within CC-ST10. This study evaluated the susceptibility of four different rainbow trout (Oncorhynchus mykiss) strains cultured in California to two isolates typed as F. psychrophilum STs 78, 85 or 319, the three most common STs collected in salmonid hatcheries in this state. The rainbow trout strains investigated are either native trout strains to California raised for conservation purposes, hatchery hybrid trout raised for sport fisheries or are imported strains selected for disease resistance or fast growth phenotypes. Central valley strain Steelhead trout, Eel river strain Steelhead trout, Hofer rainbow trout and Shasta strain rainbow trout (3.7 ± 1.3g/fish) challenged with F. psychrophilum ST78, ST85 and ST319 by intramuscular injection (~107 CFU/fish) appeared equally susceptible under the experimental conditions, leading to 95-100% cumulative percent mortality (CPM) within 21 days post challenge. Flavobacterium psychrophilum was recovered from posterior kidney of all sampled moribund and dead fish. Post-mortem evaluation of up to 5 survivors in each treatment was performed to determine carrier status in posterior kidney. The prevalence of F. psychrophilum in survivors of ST85 and ST319 were 33% (1/3 fish) and 100% (2/2 fish), respectively. There were no surviving fish challenged with ST78 group. The results suggest similar susceptibility to F. psychrophilum STs 78, 85 and 319 in all trout strains evaluated in this study; however careful interpretation is needed as susceptibility to F. psychrophilum infections have been shown to be age dependent; thus, further studies are warranted comparing different age classes and other environmental conditions.

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