Comparison of injection and immersion challenges of Renibacterium salmoninarum strains in rainbow trout (Oncorhynchus mykiss).

Renibacterium salmoninarum is a pathogenic gram-positive bacterium and 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). The CK-90 strain was previously isolated in steelhead trout (O. mykiss) from Dworshak National Fish Hatchery (NFH) in 1990. It was intracoelomically injected (100 μL) at a high dose containing 4.80 x 10 6 CFU g fish -1 (OD 525 =1.779) and a low dose containing 6.86 x10 5 CFU g fish -1 (OD 525 = 1.077), or fish were immersed in a solution containing 2.21 x 10 5 CFU g fish -1 (OD 525 =0.886) of R. salmoninarum. The ATCC 33739 strain, originating from brook trout (Salvelinus fontinalis), was also included and intracoelomically-injected at 3.58 x 10 5 CFU/g (OD 525 =1.431) to discern differences in virulence between the strains. Clinical signs of BKD manifested approximately ten days post-challenge, and mortalities began 19 days post-challenge. To confirm infection and quantify R. salmonirarum antigen load, an enzyme-linked immunosorbent assay (ELISA) was conducted using kidney tissue collected post-challenge. Injection-challenged rainbow trout with CK-90 (both high and low groups) exhibited significantly higher mortality (P<0.001) than fish injected with ATCC 33739 or those exposed to CK-90 via immersion challenge. The R. salmoninarum antigen, p57-kDa, was confirmed via ELISA. Antigen load for fish injected with the CK-90 strain (high dose: OD 405 =0.71, low dose: OD 405 =0.66) was significantly higher than for fish injected with the ATCC 33739 strain (OD 405 =0.34). It was found that the CK-90 strain (both high and low doses) was more virulent than ATCC 33739 strain, which exhibited no mortalities over 28 days. Although there were no mortalities during the 28-day trial, the ELISA confirmed that the R. salmoninarum antigen infiltrated kidney tissue in ATCC 33739 fish. Further, the immersion challenge methodology for these R. salmoninarum strains was ineffective in inducing mortalities at the abovementioned doses.