Comparison of injection and immersion challenges of Renibacterium salmoninarum strains in Rainbow Trout.

Objective: Renibacterium salmoninarum is a pathogenic gram-positive bacterium and is the causative agent of bacterial kidney disease (BKD), a malady that mainly impacts salmonid species. Experimental challenges were conducted to assess the virulence and challenge route for select R. salmoninarum strains (CK-90 and ATCC 33739) in Rainbow Trout Oncorhynchus mykiss.

Methods: The CK-90 strain was intracoelomically injected (100 μL) at a high dose containing 4.80 × 10  CFU/g of fish (optical density at 525 nm [OD ] = 1.779) and a low dose containing 6.86 × 10  CFU/g of fish (OD  = 1.077); alternatively, fish were immersed in a solution containing 4.5 × 10  CFU/mL of fish (OD  = 0.886). The ATCC 33739 strain (originating from Brook Trout Salvelinus fontinalis) was also included and intracoelomically injected at 3.58 × 10  CFU/g of fish (OD  = 1.431) to discern differences in virulence between the strains.

Result: Clinical signs of BKD manifested at approximately 10 d postchallenge, and mortalities began at 19 days postchallenge. To confirm infection and quantify R. salmoninarum antigen load, an enzyme-linked immunosorbent assay (ELISA) was conducted using kidney tissue collected after the challenge. Rainbow Trout that were challenged with CK-90 by injection (both high- and low-dose groups) exhibited significantly higher mortality than fish that were injected with ATCC 33739 or those that were exposed to CK-90 via immersion challenge. The R. salmoninarum p57 (57-kDa protein) antigen was confirmed via ELISA. Antigen load for fish injected with CK-90 (high dose: OD  = 0.71; low dose: OD  = 0.66) was significantly higher than that for fish injected with ATCC 33739 (OD  = 0.34). The CK-90 strain (both high and low doses) was more virulent than ATCC 33739, which caused no mortalities over the 28-days trial. Although there were no mortalities among ATCC 33739 fish, the ELISA confirmed that the R. salmoninarum antigen infiltrated kidney tissue in those fish.

Conclusion: The immersion challenge methodology for R. salmoninarum CK-90 was ineffective for inducing mortalities at the examined dose.