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

Abnormalities in red blood cell production and pathogenesis of anemia in the progression of rock bream iridovirus (RBIV)

Rock bream iridovirus (RBIV), belonging to Megalocytivirus, causes severe mortality in rock bream. Almost all deaths associated with RBIV are accompanied by splenic enlargement and anemia. Although red blood cells (RBCs) are involved in the immune response against viral infections, their involvement in rock bream has not yet been studied in terms of the immune response against RBIV. In this study, the viral replication patterns, blood characteristics and anemia-related factors were evaluated in rock bream post RBIV infection. The virus-infected RBCs of rock bream demonstrated similarities in the expression levels of hemoglobins (HGB) (α and β), cytokine-dependent hematopoietic cell linker (CLNK) and hematopoietic transcription factor GATA (GATA), with significantly decreasing levels from 4 days post infection (dpi) to 17 (dpi), when the viral replication was at its peak. This suggests that the expression of blood-related genes is inadequate for HGB synthesis and RBC production, thereby causing anemia leading to death. Moreover, the levels of complete blood cell count (CBC) indicators, such as RBCs, HGB and hematocrit (HCT), significantly decreased from 10 to 17 dpi. This phenomenon suggests that blood-related gene expression and/or RBC-, HGB- and HCT-related levels are critical factors in RBIV-induced anemia and disease progression. These results highlight the significance of blood-mediated immune responses against RBIV infection in rock bream. Understanding blood-related gene levels to identify blood-related immune response interactions in rock bream will be useful for development of future strategies in controlling RBIV diseases in rock bream.

Riding the wave of innovation: immunoinformatics in fish disease control

The spread of infectious illnesses has been a significant factor restricting aquaculture production. To maximise aquatic animal health, vaccination tactics are very successful and cost-efficient for protecting fish and aquaculture animals against many disease pathogens. However, due to the increasing number of immunological cases and their complexity, it is impossible to manage, analyse, visualise, and interpret such data without the assistance of advanced computational techniques. Hence, the use of immunoinformatics tools is crucial, as they not only facilitate the management of massive amounts of data but also greatly contribute to the creation of fresh hypotheses regarding immune responses. In recent years, advances in biotechnology and immunoinformatics have opened up new research avenues for generating novel vaccines and enhancing existing vaccinations against outbreaks of infectious illnesses, thereby reducing aquaculture losses. This review focuses on understanding in silico epitope-based vaccine design, the creation of multi-epitope vaccines, the molecular interaction of immunogenic vaccines, and the application of immunoinformatics in fish disease based on the frequency of their application and reliable results. It is believed that it can bridge the gap between experimental and computational approaches and reduce the need for experimental research, so that only wet laboratory testing integrated with in silico techniques may yield highly promising results and be useful for the development of vaccines for fish.

Current risks of microbial infections in fish and their prevention methods: A review

A fast-expanding sector of agriculture worldwide is aquaculture. The production of fish internationally accounts for around 44 % of the total. Even though the aquaculture environment presents several difficulties, the current development in aquaculture production comes with an increase in infectious diseases, which significantly impacts the production, profitability, and sustainability of the worldwide aquaculture business. Many infectious agents, such as bacteria, viruses, fungi and parasites are causative agents for fish infections. Most infectious diseases found in all types of fish like marine water, freshwater and ornamental fishes are caused by bacteria, with many of them serving as secondary opportunistic invaders that attack sick animals by affecting their natural host immunity. To overcome this, addressing health issues based on methods that have been scientifically verified and advised will help lessen the effects of fish disease. This review aims to highlight some of the common microbial-infecting agents of fish in all types of aquatic systems and their effective methods for preventing infections in aquaculture.

ISOTHERMAL RECOMBINANT POLYMERASE AMPLIFICATION AND CRIPSR(CAS12A) ASSAY DETECTION OF RENIBACTERIUM SALMONINARUM AS AN EXAMPLE FOR WILDLIFE PATHOGEN DETECTION IN ENVIRONMENTAL DNA SAMPLES

Improving rapid detection methods for pathogens is important for research as we collectively aim to improve the health of ecosystems globally. In the northern hemisphere, the success of salmon (Oncorhynchus spp.) populations is vitally important to the larger marine, aquatic, and terrestrial ecosystems they inhabit. This has led to managers cultivating salmon in hatcheries and aquaculture to bolster their populations, but young salmon face many challenges, including diseases such as bacterial kidney disease (BKD). Early detection of the BKD causative agent, Renibacterium salmoninarum, is useful for managers to avoid outbreaks in hatcheries and aquaculture stocks to enable rapid treatment with targeted antibiotics. Isothermal amplification and CRIPSR-Cas12a systems may enable sensitive, relatively rapid, detection of target DNA molecules from environmental samples compared to quantitative PCR (qPCR) and culture methods. We used these technologies to develop a sensitive and specific rapid assay to detect R. salmoninarum from water samples using isothermal recombinase polymerase amplification (RPA) and an AsCas12a RNA-guided nuclease detection. The assay was specific to R. salmoninarum (0/10 co-occurring or closely related bacteria detected) and sensitive to 0.0128 pg/µL of DNA (approximately 20-40 copies/µL) within 10 min of Cas activity. This assay successfully detected R. salmoninarum environmental DNA in 14/20 water samples from hatcheries with known quantification for the pathogen via previous qPCR (70% of qPCR-positive samples). The RPA-CRISPR/AsCas12a assay had a limit of detection (LOD) of >10 copies/µL in the hatchery water samples and stochastic detection below 10 copies/µL, similar to but slightly higher than the qPCR assay. This LOD enables 37 C isothermal detection, potentially in the field, of biologically relevant levels of R. salmoninarum in water. Further research is needed to develop easy-to-use, cost-effective, sensitive RPA/CRISPR-AsCas12a assays for rapidly detecting low concentrations of wildlife pathogens in environmental samples.

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

Bacterial kidney disease (BKD) was assessed according to the criteria of the Animal Health Law (AHL), in particular the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as laid out in Article 9 and Article 8 for listing animal species related to BKD. The assessment was performed following the ad hoc method on data collection and assessment developed by AHAW Panel and already published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound ≥ 66%) or not (upper bound ≤ 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to this assessment, BKD can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (66-90% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that BKD does not meet the criteria in Sections 1, 2 and 3 (Categories A, B and C; 1-5%, 33-66% and 33-66% probability of meeting the criteria, respectively) but meets the criteria in Sections 4 and 5 (Categories D and E; 66-90% and 66-90% probability of meeting the criteria, respectively). The animal species to be listed for BKD according to Article 8 criteria are provided.

Chronic mandibular osteomyelitis associated with a novel probable Kocuria sp in a sugar glider (Petaurus breviceps)

Dental disease in sugar gliders (Petaurus breviceps) is understudied, with only limited clinical and pathological data available. An approximately 7-year-old female sugar glider presented to its primary care veterinarian for a decline in food intake, rapid weight loss and a mass involving the rostral mandible. At necropsy, the mandibular mass effaced most of the rostral mandible and adjacent musculature. Histologically, the mandible was disrupted by nodular infiltrates of variably degenerate neutrophils and macrophages encased in granulomatous inflammation and fibrous connective tissue. Within the neutrophilic cell population were segments of fragmented, necrotic bone and cloud-like colonies of gram-positive cocci. Aerobic culture yielded a heavy, pure growth of a gram-positive coccus morphologically consistent with those identified in the lesions, which was identified as a presumptively novel Kocuria sp by polymerase chain reaction and sequencing of the 16S rRNA gene. This is the first description of Kocuria infection in association with clinically significant pathology in an animal. Although isolated as a pure growth, Kocuria sp cannot be confirmed as the sole cause of lesion formation due to the case chronicity and potential for unculturable, polymicrobial infections. This report adds to our understanding of the clinical and pathological aspects of dental disease in sugar gliders.

Bacterial diseases in marine fish species: current trends and future prospects in disease management

The fisheries sub-sector of aquaculture-i.e., the pisciculture industry, contributes significantly to a country's economy, employing a sizable proportion of the population. It also makes important contributions to household food security because the current demand for animal protein cannot be fulfilled by harvesting wild fish from riverines, lakes, dams, and oceans. For good pond management techniques and sustaining fish health, the fisherfolk, and the industry require well-established regulatory structures, efficient disease management strategies, and other extended services. In rearing marine fish, infections resulting from disease outbreaks are a weighty concern because they can cause considerable economic loss due to morbidity and mortality. Consequently, to find effective solutions for the prevention and control of the major diseases limiting fish production in aquaculture, multidisciplinary studies on the traits of potential fish pathogens, the biology of the fish as hosts, and an adequate understanding of the global environmental factors are fundamental. This review highlights the various bacterial diseases and their causative pathogens prevalent in the pisciculture industry and the current solutions while emphasising marine fish species. Given that preexisting methods are known to have several disadvantages, other sustainable alternatives like antimicrobial peptides, synthetic peptides, probiotics, and medicinal treatments have emerged to be an enormous potential solution to these challenges.

Improved LC/MS/MS Quantification Using Dual Deuterated Isomers as the Surrogates: A Case Analysis of Enrofloxacin Residue in Aquatic Products

Extensive and high residue variations in enrofloxacin (ENR) exist in different aquatic products. A novel quantitative method for measuring ENR using high-performance liquid chromatography-tandem mass spectrometry was developed employing enrofloxacin-d₅ (ENR-d₅) and enrofloxacin-d₃ (ENR-d₃) as isotope surrogates. This reduced the deviation of detected values, which results from the overpass of the linear range and/or the large difference in the residue between the isotope standard and ENR, from the actual content. Furthermore, high residue levels of ENR can be directly diluted and re-calibrated by the corresponding curve with the addition of high levels of another internal surrogate without repeated sample preparation, avoiding the overflow of the instrument response. The validation results demonstrated that the method can simultaneously determine ENR residues from MQL (2 µg/kg) to 5000 × MQL (method quantification limit) with recoveries between 97.1 and 106%, and intra-precision of no more than 2.14%. This method realized a wide linear calibration range with dual deuterated isomers, which has not been previously reported in the literature. The developed method was successfully applied to the analysis of ENR in different aquatic products, with ENR residue levels varying from 108 to 4340 μg/kg and an interval of precision in the range of 0.175~6.72%. These results demonstrate that batch samples with a high variation in ENR residues (over the linear range with a single isotope standard) can be detected by the dual isotope surrogates method in a single sample preparation process.

Genomic and proteomic aspects of p57 protein from Renibacterium salmoninarum: Characteristics in virulence patterns

Renibacterium salmoninarum is one of the oldest known fish bacterial pathogens. This Gram-positive bacterium is the causative agent of Bacterial Kidney Disease (BKD), a chronic infection that primarily infects salmonids at low temperatures. Externally, infected fish may show exophthalmos, skin blisters, ulcerations, and hemorrhages at the base of the fins and along the lateral line. Internally, the kidney, heart, spleen, and liver may show signs of inflammation. The best characterized virulence factor of R. salmoninarum is p57, a 57 kDa protein located on the bacterial cell surface and secreted into surrounding fish tissue. The p57 protein in fish is the main mediator in suppressing the immune system, reducing antibody production, and intervening in cytokine activity. In this review, we will discuss aspects such as single nucleotide polymorphisms (SNPs) that modify the DNA sequence, variants in the number of copies of MSA genes, physical-chemical properties of the signal peptides, and the limited iron conditions that can modify p57 expression and increase the virulence of R. salmoninarum.

Nutritional immunomodulation of Atlantic salmon response to Renibacterium salmoninarum bacterin

We investigated the immunomodulatory effect of varying levels of dietary ω6/ω3 fatty acids (FA) on Atlantic salmon (Salmo salar) antibacterial response. Two groups were fed either high-18:3ω3 or high-18:2ω6 FA diets for 8 weeks, and a third group was fed for 4 weeks on the high-18:2ω6 diet followed by 4 weeks on the high-18:3ω3 diet and termed “switched-diet”. Following the second 4 weeks of feeding (i.e., at 8 weeks), head kidney tissues from all groups were sampled for FA analysis. Fish were then intraperitoneally injected with either a formalin-killed Renibacterium salmoninarum bacterin (5 × 10⁷ cells mL⁻¹) or phosphate-buffered saline (PBS control), and head kidney tissues for gene expression analysis were sampled at 24 h post-injection. FA analysis showed that the head kidney profile reflected the dietary FA, especially for C₁₈ FAs. The qPCR analyses of twenty-three genes showed that both the high-ω6 and high-ω3 groups had significant bacterin-dependent induction of some transcripts involved in lipid metabolism (ch25ha and lipe), pathogen recognition (clec12b and tlr5), and immune effectors (znrf1 and cish). In contrast, these transcripts did not significantly respond to the bacterin in the “switched-diet” group. Concurrently, biomarkers encoding proteins with putative roles in biotic inflammatory response (tnfrsf6b) and dendritic cell maturation (ccl13) were upregulated, and a chemokine receptor (cxcr1) was downregulated with the bacterin injection regardless of the experimental diets. On the other hand, an inflammatory regulator biomarker, bcl3, was only significantly upregulated in the high-ω3 fed group, and a C-type lectin family member (clec3a) was only significantly downregulated in the switched-diet group with the bacterin injection (compared with diet-matched PBS-injected controls). Transcript fold-change (FC: bacterin/PBS) showed that tlr5 was significantly over 2-fold higher in the high-18:2ω6 diet group compared with other diet groups. FC and FA associations highlighted the role of DGLA (20:3ω6; anti-inflammatory) and/or EPA (20:5ω3; anti-inflammatory) vs. ARA (20:4ω6; pro-inflammatory) as representative of the anti-inflammatory/pro-inflammatory balance between eicosanoid precursors. Also, the correlations revealed associations of FA proportions (% total FA) and FA ratios with several eicosanoid and immune receptor biomarkers (e.g., DGLA/ARA significant positive correlation with pgds, 5loxa, 5loxb, tlr5, and cxcr1). In summary, dietary FA profiles and/or regimens modulated the expression of some immune-relevant genes in Atlantic salmon injected with R. salmoninarum bacterin. The modulation of Atlantic salmon responses to bacterial pathogens and their associated antigens using high-ω6/high-ω3 diets warrants further investigation.

Prevalence and distribution of Renibacterium salmoninarum, causative agent of bacterial kidney disease, in wild trout fisheries in Colorado

Detections of Renibacterium salmoninarum in Colorado USA fish hatcheries have become more frequent in recent years, including one disease outbreak that originated with a wild broodstock. Our objectives were to document the prevalence and distribution of R. salmoninarum in Colorado's wild trout fisheries, investigate variables that influence that distribution, and evaluate the effectiveness of common testing methods on non-anadromous trout. We sampled wild trout across Colorado and tested kidney tissue with enzyme-linked immunosorbent assay (ELISA), quantitative polymerase chain reaction (qPCR), nested polymerase chain reaction (nPCR), and direct fluorescent antibody test (DFAT). Screening with ELISA showed high prevalence of R. salmoninarum among fish populations, but antigen levels were low. No clinical disease was observed in any of the fish sampled despite the antigen of R. salmoninarum being common. Antigen levels measured by ELISA increased in smaller streams with lower historic fish stocking rates. Brook trout Salvelinus fontinalis had the highest prevalence of the bacterium among fish species and highest ELISA antigen levels. The distribution of brook trout in the smallest streams may help explain the patterns of R. salmoninarum across the landscape. The most effective assays for screening wild trout were qPCR and ELISA; DFAT was inconsistent for bacterial levels encountered in wild trout and generally uninformative. Additionally, qPCR and ELISA can provide quantitative information about bacteria levels. The bacterium R. salmoninarum is ubiquitous in Colorado trout fisheries but is generally found at low levels. Active infections are rare and overt bacterial kidney disease appears more common in Colorado hatcheries than in wild fish.

Assessing the presence and spread of Renibacterium salmoninarum between farmed and wild fish in Sweden

Bacterial kidney disease (BKD) can be a devastating bacterial infection in salmonids, and it is present in aquaculture throughout the world. BKD is caused by the Gram-positive facultative intracellular bacterium Renibacterium salmoninarum (R. salmoninarum) that is spread both horizontally and vertically. Disease signs include external ulcerations and blisters and internal signs such as organ swelling, granulomas, petechiae and ascites. In Sweden, BKD accounts for a significant income loss in aquacultures due to expensive decontamination of the facility and increased disease susceptibility for the immunocompromised fish leading to higher mortality rates. In addition, uncontrolled spread in aquaculture may threaten the survival of wild fish populations. The aim of our study was to investigate the prevalence of R. salmoninarum in wild salmonids caught in Swedish waters where net pen farms with a recent history of BKD are present. Four rivers with at least one BKD-positive or recently BKD-positive farm were selected. In addition, we evaluated the use of environmental DNA (eDNA) for surveillance and monitoring of ongoing infections at these locations. In total, 1058 fish were sampled from four different river systems, and of them 52 (4.9%) were positive for R. salmoninarum by antigen ELISA. Surprisingly, these fish were not evenly distributed between the four river systems, but 50 were caught in the same river (Ljungan). This accounts for an alarmingly high rate of 17% R. salmoninarum-positive samples in wild salmonids in this area. This number is far above what was expected and clearly shows the risk with an open farming system as well as the importance of effective health monitoring programmes to avoid an uncontrolled spread of the disease. The use of eDNA for monitoring BKD is somewhat difficult to evaluate. Few of the water samples analysed were PCR positive for R. salmoninarum (2 of 38) and those were collected where no ELISA positive fish were identified. In addition to water, sediment samples were collected under a net pen farm that had recently slaughtered all fish due to ongoing R. salmoninarum infections. Sediment samples are more promising than water as 4 of 5 samples at one farming facility where positive for R. salmoninarum. Thus, sediment samples may be valuable for monitoring potential ongoing BKD in farms, without the need to sacrifice valuable fish.

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

Bacterial kidney disease (BKD) is a major health problem of salmonids, affecting both wild and cultured salmon. The disease is caused by Renibacterium salmoninarum (Rs), a fastidious, slow-growing and strongly Gram-positive diplobacillus that produces chronic, systemic infection characterized by granulomatous lesions in the kidney and other organs, often resulting in death. Fast detection of the pathogen is important to limit the spread of the disease, particularly in hatcheries or aquaculture facilities. Aptamers are increasingly replacing conventional antibodies as platforms for the development of rapid diagnostic tools. In this work, we describe the first instance of isolating and characterizing a ssDNA aptamer that binds with high affinity to p57 or major soluble antigen (MSA), the principal antigen found on the cell wall surface of Rs. Specifically, in this study a construct of the full-length protein containing a DNA binding domain (MSA-R2c) was utilized as target. Aptamers were isolated from a pool of random sequences using GO-SELEX (graphene oxide-systematic evolution of ligands by exponential enrichment) protocol. The selection generated multiple aptamers with conserved motifs in the random region. One aptamer with high frequency of occurrence in different clones was characterized and found to display a strong binding affinity to MSA-R2c with a Kd of 3.0 ± 0.6 nM. The aptamer could be potentially utilized for the future development of a sensor for rapid and onsite detection of Rs in water or in infected salmonids, replacing time-consuming and costly lab analyses.

Anemia in salmon aquaculture: Scotland as a case study

Anemia in salmonid aquaculture is a recognized blood disorder resulting from the reduction of hemoglobin concentration and/or erythrocyte count. Because of sub-optimal oxygen supply to the tissues, as a negative impact of anemia fish will experience reduced growth and poor health. This health challenge may be linked with several factors including anthropogenic changes in the marine environment, infectious etiology (viral, bacterial, and parasitic), nutritional deficiencies, or hemorrhaging. From the mid-late summer of 2017 to 2019, Scottish salmon farming companies began to report the occurrence of anemic events in open-net marine sites. At that time, the industry had little understanding of the pathogenesis and possible mechanisms of anemia and limited the ability to formulate effective mitigation strategies. Clinical examination of fish raised suspicion of anemia and this was confirmed by generating a packed cell volume value by centrifugation of a microhematocrit tube of whole anticoagulated blood. Company health team members, including vets and biologists, reported discoloration of gills and local hemorrhages. This paper reviews various commercially significant cases and lesser-known cases of anemia in cultured salmonid species induced by various biological factors. The current methods available to assess hematology are addressed and some future methods that could be adopted in modern day fish farming are identified. An account of the most recent anemic event in Scottish farmed Atlantic salmon (Salmo salar) is presented and discussed as a case study from information provided by two major Scottish salmon producers. The percent of total marine sites (n = 80) included in this case study, that reported with suspected or clinical anemia covering the period mid-late summer 2017 to 2019, was between 1 and 13%. The findings from this case study suggest that anemia experienced in most cases was regenerative and most likely linked to blood loss from the gills.

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

Globally, aquaculture has faced serious economic problems due to bacterial, viral, and various other infectious diseases of different origins. Even though such diseases are being detected and simultaneously treated with several therapeutic and prophylactic methods, the broad-spectrum activity of vaccines plays a vital role as a preventive measure in aquaculture. However, treatments like use of antibiotics and probiotics seem to be less effective when new mutant strains develop and disease causing pathogens become resistant to commonly used antibiotics. Therefore, vaccines developed by using recent advanced molecular techniques can be considered as an effective way of treating disease causing pathogens in aquatic organisms. The present review emphasizes on the current advances in technology and future outlook with reference to different types of vaccines used in the aquaculture industries. Beginning with traditional killed/inactivated and live attenuated vaccines, this work culminates in the review of modern new generation ones including recombinant, synthetic peptides, mucosal and DNA, subunit, nanoparticle-based and plant-based edible vaccines, reverse vaccinology, and monovalent and polyvalent vaccines.

Factors associated with baseline mortality in Norwegian Atlantic salmon farming

In 2019, it was estimated that more than 50 million captive Atlantic salmon in Norway died in the final stage of their production in marine cages. This mortality represents a significant economic loss for producers and a need to improve welfare for farmed salmon. Single adverse events, such as algal blooms or infectious disease outbreaks, can explain mass mortality in salmon cages. However, little is known about the production, health, or environmental factors that contribute to their baseline mortality during the sea phase. Here we conducted a retrospective study including 1627 Atlantic salmon cohorts put to sea in 2014-2019. We found that sea lice treatments were associated with Atlantic salmon mortality. In particular, the trend towards non-medicinal sea lice treatments, including thermal delousing, increases Atlantic salmon mortality in the same month the treatment is applied. There were differences in mortality among production zones. Stocking month and weight were other important factors, with the lowest mortality in smaller salmon stocked in August-October. Sea surface temperature and salinity also influenced Atlantic salmon mortality. Knowledge of what affects baseline mortality in Norwegian aquaculture can be used as part of syndromic surveillance and to inform salmon producers on farming practices that can reduce mortality.