Abstract
Flavobacterium psychrophilum, the causative agent of bacterial cold-water disease (BCWD) in freshwater-reared salmonids, is also a common commensal organism of healthy fish. The virulence potential of F.psychrophilum isolates obtained from BCWD cases in Ontario between 1994 and 2009 was evaluated. In preliminary infection trials of rainbow trout juveniles, significant differences (0% to 63% mortality) in the virulence of the 22 isolates tested were noted following intraperitoneal injection with 108 cfu/fish. A highly virulent strain, FPG 101, was selected for further study. When fish were injected intraperitoneally with a 106 , 107 or 108 cfu/fish of F.psychrophilum FPG 101, the 108 cfu/fish dose produced significantly greater mortality (p<0.05). The bacterial load in spleen samples collected from fish every 3days after infection was determined using rpoC quantitative polymerase chain reaction amplification and by plate counting. Bacterial culture and rpoC qPCR were highly correlated (R2 =0.92); however, culture was more sensitive than the qPCR assay for the detection of F.psychrophilum in spleen tissue. Ninety-seven per cent of the asymptomatic and the morbid fish had splenic bacterial loads of <2.8 log10 gene/copies and >3.0 log10 gene copies/reaction, respectively, following infection with 108 cfu/fish.
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