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

Abstract

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 × 106 CFU/g of fish (optical density at 525 nm [OD525] = 1.779) and a low dose containing 6.86 × 105 CFU/g of fish (OD525 = 1.077); alternatively, fish were immersed in a solution containing 4.5 × 107 CFU/mL of fish (OD525 = 0.886). The ATCC 33739 strain (originating from Brook Trout Salvelinus fontinalis) was also included and intracoelomically injected at 3.58 × 105 CFU/g of fish (OD525 = 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: OD405 = 0.71; low dose: OD405 = 0.66) was significantly higher than that for fish injected with ATCC 33739 (OD405 = 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.

spaB-positive Erysipelothrix rhusiopathiae, a novel teleost pathogen isolated from cultured barramundi

“… 18 Furthermore, intracoelomic challenges also have advantages over immersion challenges, including standardization of inoculum and predictable induction of acute infection, because the bacteria can bypass the fish’s natural defenses of mucus and epithelium. 13 , 27 Although immersion challenges may better mimic natural routes of infection, they tend to have more variable morbidity and mortality rates and require higher concentrations of bacteria, which can confound pathology assessments. 3 , 18 However, future studies evaluating the pathogenesis of this E. rhusiopathiae isolate via immersion challenge are warranted.…”

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) 2016/429): Bacterial kidney disease (BKD)

“…Parameter 2 -Presence and duration of latent infection period R. salmoninarum is often transmitted vertically through eggs in aquaculture settings, but clinical signs of disease usually do not develop until salmonids are 6-12 months old (Evelyn, 1993;Wiens, 2011). Clinical signs begin at 10 days post inoculation (dpi) in artificially infected rainbow trout (Watson et al, 2023). The interval from infection until onset of clinical signs depends on multiple environmental and management factors, such as the season of stocking and husbandry (Boerlage et al, 2018), as does bacterial shedding (Purcell et al, 2016).…”

“…For that reason, qPCR is more commonly used in detecting low levels of infection to prevent vertical transmission via contaminated eggs or horizontal transmission via subclinically infected individuals (Delghandi et al., 2020a ). Primers targeting the p57 gene are most commonly used, as formerly recommended by WOAH (Brown et al., 1994 ) and they show high sensitivity and specificity (Watson et al., 2023 ). Loop‐mediated isothermal amplification (LAMP) assays to detect R. salmoninarum are also available (Brown et al., 1994 ).…”