Rainbow trout Oncorhynchus mykiss (Walbaum) type IV ice-structuring protein LS-12 in the acute-phase response to Flavobacterium psychrophilum infection

A previous proteomic study examining the plasma acute-phase response of rainbow trout to sterile inflammation highlighted an unidentified 9.5-kDa spot using 2D-PAGE, which was dramatically increased. The 15 amino acid sequence obtained from this protein spot allowed rapid amplification of cDNA ends PCR to generate a 443-bp nucleotide sequence that was 98.6% similar to type-4 ice-structuring protein LS-12 from Atlantic salmon Salmo salar Linnaeus. Quantitative reverse translation PCR and an ELISA were used to measure gene expression and plasma concentrations of LS-12 following experimental intraperitoneal injection of rainbow trout with either 10⁶ or 10⁸ colony-forming units (CFU) of Flavobacterium psychrophilum. There was no significant change in the plasma concentration of LS-12 up to 15 days post-infection in any group. Hepatic LS-12 gene expression was significantly reduced at 3 and 6 days (p < 0.001) post-infection in fish injected with 10⁸ CFU of F. psychrophilum relative to control fish, while branchial or head kidney expression was unchanged. Infected fish had significantly increased hepatic gene expression of serum amyloid A, confirming an acute-phase response. Under the conditions used, LS-12 is not a positive acute-phase protein in rainbow trout.

Virulence of Flavobacterium psychrophilum isolates in rainbow trout Oncorhynchus mykiss (Walbaum)

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 3 days 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.