Renibacterium salmoninarum—The Causative Agent of Bacterial Kidney Disease in Salmonid Fish

Abstract

Renibacterium salmoninarum is one of the oldest known bacterial pathogens of fish. This Gram-positive bacterium is the causative agent of bacterial kidney disease, a chronic infection that is mostly known to infect salmonid fish at low temperatures. Externally, infected fish can display exophthalmia as well as blebs on the skin and ulcerations alongside haemorrhages at the base of the fins and alongside the lateral line. Internally, the kidney, heart, spleen and liver can show signs of swelling. Granulomas can be seen on various internal organs, as can haemorrhages, and the organs can be covered with a false membrane. Ascites can also accumulate in the abdominal cavity. The bacterium is generally cultivated on specialized media such as kidney disease medium-1 (KDM-1), KDM-2 and selective kidney disease medium (SKDM), and a diagnostic is performed using molecular tools such as PCRs or real-time quantitative PCRs (RT-qPCRs). Several virulence mechanisms have been identified in R. salmoninarum, in particular the protein p57 that is known to play a role in both agglutination and immunosuppression of the host’s defense mechanisms. Control of the disease is difficult; the presence of asymptomatic carriers complicates the eradication of the disease, as does the ability of the bacterium to gain entrance inside the eggs. Bacterin-killed vaccines have proven to be of doubtful efficacy in controlling the disease, and even more recent application of a virulent environmental relative of R. salmoninarum is of limited efficacy. Treatment by antibiotics such as erythromycin, azithromycin and enrofloxacin can be effective but it is slow and requires prolonged treatment. Moreover, antibiotic-resistant strains have been reported. Despite being known for a long time, there is still much to be discovered about R. salmoninarum, notably regarding its virulence mechanisms and its vaccine potential. Consequently, these gaps in knowledge continue to hinder control of this bacterial disease in aquaculture settings.

A review on the recent advances and application of vaccines against fish pathogens in aquaculture

“…A commercial live vaccine has been reported to be licensed under the trade name “Renogen” by Novartis in South Africa (S.A) for BKD prevention (Toranzo et al 2009 ). However, recently, a study reported that bacterin-killed vaccines have proven to possess doubtful efficacy to control this disease since not much is known regarding the potential of neither the vaccine nor its virulence mechanisms (Delghandi et al 2020 ).…”

Factors associated with baseline mortality in Norwegian Atlantic salmon farming

“…However, it is difficult to disentangle the mechanisms that play a role in mortality in general. Major infectious agents and parasites contributing to mortality in farmed Atlantic salmon include salmonid alphavirus (SAV) 10 – 12 , infectious salmon anemia virus (ISAV) 13 , 14 , infectious pancreatic necrosis virus (IPNV) 15 , 16 , piscine myocarditis virus (PMCV) 17 , 18 , Moritella viscosa 19 , Renibacterium salmoninarum 20 , sea lice ( Lepeophtheirus salmonis and Caligus spp.) 21 – 23 , and Paramoeba perurans 24 .…”

“…Detection of IPNV usually occurs during the first 3 months after the salmon transfer to sea 15 , 16 , while PMCV affects larger salmon (> 2 kg), which have spent longer periods at sea 17 , 18 . Both SAV and R. salmoninarum have been detected in salmon across a wide range of ages 20 , 34 , 35 . A factor that can also contribute to greater mortality risk is the lack of vaccinations, for example against SAV 36 and M. viscosa 37 .…”

Isolation and Characterization of a ssDNA Aptamer against Major Soluble Antigen of Renibacterium salmoninarum

“…p57 or major soluble antigen (MSA) is a glycine-rich, 558 amino acid cell surface protein containing two copies of an 81-residue direct repeat, which are 60% homologous, and five copies of a second 25 residue imperfect repeat [ 16 ]. MSA is the best-characterized virulence factor in Rs, although its role in the disease pathogenesis is unclear [ 2 ]. One function of MSA may be to mediate adherence of the bacteria to host tissues.…”

“…While this method can be very sensitive, it is time consuming due to the very slow growth of the bacterium. Immunological and molecular methods are available including the fluorescent antibody test (FAT), enzyme-linked immunosorbent assay (ELISA), and non-quantitative and quantitative procedures for the polymerase chain reaction (PCR) [ 2 ]. ELISA is the most widely used method, due to the low cost and ability to rapidly screen large numbers of fish.…”