Effect of Bivalent Vaccines against Vibrio anguillarum and Aeromonas salmonicida Subspecie achromogenes on Health and Survival of Turbot

Current Challenges of Vaccination in Fish Health Management

Vaccination is an essential method of immunological preventive care required for the health management of all animals, including fish. More particularly, immunization is necessary for in-land aquaculture to manage diseases in fish broodstocks and healthy seed production. According to the latest statistics in 2020, 90.3 million tons of capture fishery production was achieved from the aquaculture sector. Out of the above, 78.8 million tons were from marine water aquaculture sectors, and 11.5 million tons were from inland water aquaculture sectors. About a 4% decline in fish production was achieved in 2020 in comparison to 2018 from inland aquaculture sectors. On the other hand, the digestive protein content, healthy fats, and nutritional values of fish products are comparatively more affordable than in other meat sources. In 2014, about 10% of aquatic cultured animals were lost (costing global annual losses > USD 10 billion) due to infectious diseases. Therefore, vaccination in fish, especially in broodstocks, is one of the essential approaches to stop such losses in the aquaculture sector. Fish vaccines consist of whole-killed pathogens, protein subunits, recombinant proteins, DNA, or live-attenuated vaccines. Challenges persist in the adaption of vaccination in the aquaculture sector, the route of administration, the use of effective adjuvants, and, most importantly, the lack of effective results. The use of autogenous vaccines; vaccination via intramuscular, intraperitoneal, or oral routes; and, most importantly, adding vaccines in feed using top dressing methods or as a constituent in fish feed are now emerging. These methods will lower the risk of using antibiotics in cultured water by reducing environmental contamination.

Efficacy of bivalent vaccine against Aeromonas salmonicida and Edwardsiella tarda infections in turbot

In recent years, more than one pathogenic organism has usually been isolated from diseased turbot Scophthalmus maximus, creating a pressing need for the development of combination vaccines to prevent fish diseases brought on simultaneously by various infections. In this study, the inactivated bivalent vaccine of Aeromonas salmonicida and Edwardsiella tarda was prepared by the formalin inactivation method. After challenge with A. salmonicida and E. tarda at 4 weeks post-vaccination in turbot, the relative percentage survival (RPS) of the inactivated bivalent vaccine was 77.1%. In addition, we assessed the effects of the inactivated bivalent vaccine and evaluated the immunological processes after immunization in a turbot model. Serum antibody titer and lysozyme activity of the vaccinated group were both upregulated and higher than that in control group after vaccination. The expression levels of genes (TLR2, IL-1β, CD4, MHCI, MHCⅡ) that related to antigen recognition, processing and presentation were also studied in the liver, spleen and kidney tissues of vaccinated turbot. All the detected genes in the vaccinated group had a significant upward trend, and most of them reached the maximum value at 3-4 weeks, which had significant differences from the control group, suggesting that antigen recognition, processing and presentation pathway was activated by the inactivated bivalent vaccine. Our study provides a basis for further application of the killed bivalent vaccine against A. salmonicida and E. tarda in turbot, making it good potential that can be applied in aquaculture.

Immune response of European sea bass (Dicentrarchus labrax L.) against combination of antigens from three different pathogens

Three of the most important diseases of Mediterranean intensive European sea bass farming are, viral nervous necrosis (VNN) caused by the red grouper nervous necrosis virus (RGNNV) genotype of b-nodavirus, photobacteriosis caused by Photobacterium damselae subsp. piscicida (Phdp) and vibriosis caused mainly by the O1 serotype of Vibrio anguillarum (VaO1). Prevention against these diseases is performed through vaccination with a monovalent vaccine against the viral disease and, usually, with bivalent vaccines against the bacterial diseases. However, it is very difficult to program two vaccinations during the same season for the same fish stock and producers are forced to either vaccinate for the viral or the bacterial diseases or to perform double vaccination with both vaccines, without any prior knowledge on any interactions that may occur due to the plethora of antigens (Ag) injected. Ideally, therefore, a trivalent vaccine should be developed against all three diseases. The objective of this work was to analyse the immune response of sea bass against combinations of Ags from all three pathogens, namely viral particles, Phdp whole cells (WC), lipopolysaccharide (LPS), capsular polysaccharide (CPS) and extracellular products (ECPs) and VaO1 WC and ECPs in respect to the identification of any phenomena of immunodominance/immunosuppression between Ags with a view to select candidate Ags for inclusion in a trivalent vaccine formulation. Eight triplicate groups of fish were immunized with different combinations of the aforementioned Ags and another triplicate group served as negative control. Blood serum was isolated at various time-points post-immunization for the measurement of specific antibodies against each Ag and, in addition, leucocytes were isolated at day 29 post-immunization for analysis of various cellular activities. Results indicated that best levels of specific a-NNV virus antibodies (Abs) were produced when VaO1 ECPs were not included in the Ag combinations, in contrast to the leucocytes proliferation assay where best stimulation against NNV Ags was measured when VaO1 ECPs were present in Ag combinations. VaO1 ECPs apparently is a strong immunogen for both humoral and cellular responses but suppresses immunological reactions against the other Ags.VaO1 WC, Phdp LPS and ECPs raised good humoral immune responses in the groups with best responses against VNN Ags, but only VaO1 WC and Phdp ECPs provided good stimulation of leucocytes, with Phdp WC and CPS effecting either similar stimulation with untrained leucocytes (control groups) or down-stimulation. Results are discussed with a view to select Ags from all three pathogens for inclusion in trivalent vaccine against all three pathogens.

Comparative transcriptome analysis reveals the immune response of turbot (Scophthalmus maximus) induced by inactivated bivalent vaccine

Vibrio species are important pathogens that affect a wide range of farmed fish. Vaccination is regarded as the most efficacious strategy for fighting bacterial infections. However, the underlying mechanisms remain to be elucidated. In the present study, a comparative transcriptome analysis was performed on the spleens from turbot (Scophthalmus maximus) induced by an inactivated bivalent vaccine (Vibrio anguillarum and Vibrio harveyi, IVVah1) at 4 week and 1 day post further challenge. Strong immune responses were induced by the bivalent vaccine, besides differentially expressed genes (DEGs) associated with adaptive immunity, more innate immunity-related DEGs were detected. At the late stage of vaccination, immune-related molecules associated with pattern recognition receptors, inflammatory factors, complement and coagulation cascade-related components, and antigen processing and presentation were significantly regulated, and some of them were even further up-regulated after the bacterial challenge, indicating the cooperation of multiple immune processes during the vaccine immunization process. In addition to the terms or pathways associated with the immune response, enrichment analysis revealed multiple significantly enriched terms/pathways associated with the response to stimulus/stress, homeostasis, metabolism, and biosynthesis, suggesting that a defensive status was established by the bivalent vaccine. This study furnishes new insights into the internal mechanism of immunity upon a combined vaccine administrating in turbot and lays a foundation for developing highly immunogenic vaccines in teleost.

Strategies for Prevention and Control of Vibriosis in Asian Fish Culture

It is estimated that vibriosis account for about half of the economic losses in Asian fish culture. Consequently, the prevention and control of vibriosis is one of the priority research topics in the field of Asian fish culture disease. Relevant measures have been proposed to control some Vibrios that pose a threat to Asian fish culture, but there are currently only a few effective vaccines available to combat these Vibrios. The purpose of our review is to sum up the main prevention methods and the latest control strategies of seven Vibrio species that cause great harm to Asian aquaculture, including Vibrio harveyi, Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio mimicus, Vibrio anguillarum, Vibrio alginolyticus and Vibrio cholerae. Strategies such as antibiotics, probiotics, bacteriophages, antimicrobials from plants and other natural sources, as well as vaccines, are compared and discussed here. We expect this review will provide some new views and recommendations for the future better prevention and control of vibriosis in Asian fish culture.

Different antigen ratio in bivalent vaccine can affect immunological activation and protection against Aeromonas salmonicida and Vibrio anguillarum in Atlantic salmon

To effectively prevent the outbreak of furunculosis and vibriosis in Atlantic salmon culture in northern China, the immunological properties and efficacies of bivalent inactivated vaccines composed of two local pathogenic strains, Aeromonas salmonicida C4 and Vibrio anguillarum M3, was analyzed in salmon. The effect of formalin-inactivated A. salmonicida C4, V. anguillarum M3, and their combined vaccines with different antigen ratio of 1:1, 1:2 and 2:1 on relative percentage of survival (RPS), specific antibody production and expression of immune-related genes of fish were investigated. It was found that both C4 and M3 antigen had adjuvant effects on each other's vaccines. Furthermore, the proportion of antigens in the combined vaccines had significantly different effects on immune response and protection against A. salmonicida or V. anguillarum infection. Compared with other vaccinated groups, the C4:M3 = 1:2 vaccine group had the highest immunological activation ability and best immune protection against A. salmonicida and V. anguillarum. Our studies suggest that the development of new multivalent vaccines should carefully design the ratio of different antigens in according to their immunological properties to achieve efficient protection and avoid inhibitory effects of antigens on specific pathogen.

A compound ginseng stem leaf saponins and aluminium adjuvant enhances the potency of inactivated Aeromonas salmonicida vaccine in turbot

Furunculosis caused by Aeromonas salmonicida in turbot farming is increasingly leading to huge economic losses. In this study, an inactivated vaccine containing a compound adjuvant of ginseng stem leaf saponins and aluminum hydroxide gel (GSLS/Alum) was developed against A. salmonicida and evaluated on turbot. As a result, serum antibody titer in vaccinated group was significantly higher than that in control group and the relative percentage survival (RPS) was up to 75.7%. Comparatively, the RPS of groups that vaccinated with only inactivated vaccine and vaccine containing Alum adjuvant or an oil emulsion Montanide™ ISA 763A were 32.4%, 48.6% and 64.9%, respectively. Although the vaccine containing oil adjuvant elicited comparable IgM level as that containing the compound GSLS/Alum adjuvant, the latter had no obvious side effects. Moreover, the inactivated vaccine containing the compound adjuvant was more likely to induce a higher cellular immune response according to the expressions of some immune related genes. Most importantly, an excellent protection of the vaccine containing GSLS/Alum adjuvant was obtained when turbots were naturally infected under clinical condition. In summary, our study demonstrated that the formulation of GSLS and Alum is a potential compound adjuvant in turbot vaccine development.

A novel portable label-free electrochemical immunosensor for ultrasensitive detection of Aeromonas salmonicida in aquaculture seawater

Infectious diseases caused by Aeromonas salmonicida (A. salmonicida) have a huge impact and produce significant losses in aquaculture and fish farming. Fish pathogen early detection is a critical step for the rapid identification and prevention of these problems. This work presents a novel portable label-free ultrasensitive electrochemical immunosensor for A. salmonicida detection in seawater. It consists of a fluidic integrated electrochemical-cell-chip (ECC) with independent chambers enclosing three electrochemical cells (ECs). Anti-A. salmonicida (AbSalm) antibodies were covalently attached to the gold surface of the microfabricated electrodes and were used for the sensitive detection of A. salmonicida. The antibody-antigen immunoreaction was studied by enzyme-linked immunosorbent assay (ELISA), and the surface functionalization was characterized by using quartz crystal microbalance (QCM), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). The performance of the developed immunosensor, in terms of sensitivity, repeatability, and specificity, was also studied. The linear working range varied between 1 and 10⁷ CFU mL^-1, with a limit of detection (LOD) as low as 1 CFU mL^-1. The suitability of the immunosensor for real sample detection was successfully demonstrated via recovery studies performed in spiked seawater samples. The proposed technology supports the use of low-cost and portable instrumentation that concedes the ultrasensitive, simple, and fast quantification of the A. salmonicida. To the best of our knowledge, this is the first portable sensing system for the detection of A. salmonicida in seawater samples, which provides a promising online monitoring platform for the detection of this bacterium in aquaculture facilities.

Vibriosis Outbreaks in Aquaculture: Addressing Environmental and Public Health Concerns and Preventive Therapies Using Gilthead Seabream Farming as a Model System

Bacterial and viral diseases in aquaculture result in severe production and economic losses. Among pathogenic bacteria, species belonging to the Vibrio genus are one of the most common and widespread disease-causing agents. Vibrio infections play a leading role in constraining the sustainable growth of the aquaculture sector worldwide and, consequently, are the target of manifold disease prevention strategies. During the early, larval stages of development, Vibrio species are a common cause of high mortality rates in reared fish and shellfish, circumstances under which the host organisms might be highly susceptible to disease preventive or treatment strategies such as vaccines and antibiotics use, respectively. Regardless of host developmental stage, Vibrio infections may occur suddenly and can lead to the loss of the entire population reared in a given aquaculture system. Furthermore, the frequency of Vibrio-associated diseases in humans is increasing globally and has been linked to anthropic activities, in particular human-driven climate change and intensive livestock production. In this context, here we cover the current knowledge of Vibrio infections in fish aquaculture, with a focus on the model species gilthead seabream (Sparus aurata), a highly valuable reared fish in the Mediterranean climatic zone. Molecular methods currently used for fast detection and identification of Vibrio pathogens and their antibiotic resistance profiles are addressed. Targeted therapeutic approaches are critically examined. They include vaccination, phage therapy and probiotics supplementation, which bear promise in supressing vibriosis in land-based fish rearing and in mitigating possible threats to human health and the environment. This literature review suggests that antibiotic resistance is increasing among Vibrio species, with the use of probiotics constituting a promising, sustainable approach to prevent Vibrio infections in aquaculture.

Improving the Activity of Antimicrobial Peptides Against Aquatic Pathogen Bacteria by Amino Acid Substitutions and Changing the Ratio of Hydrophobic Residues

With the increasing number of drug-resistant bacteria, there is an urgent need for new antimicrobial agents, and antimicrobial peptides (AMPs), which exist in the human non-specific immune system, are one of the most promising candidates. It is an effective optimization strategy to modify antimicrobial peptides (AMPs) according to the distribution of amino acids and hydrophobic characteristics. The addition of bacterial pheromones to the N short peptide can increase the ability to recognize bacteria. In this study, we designed and synthesized AMP1–6 by amino acid substitution of mBjAMP1. Additionally, P-6, S-6, and L-6 were designed and synthesized by adding bacterial pheromones based on 1–6. Functional tests showed that the four AMPs had the ability to kill Gram-negative Vibrio anguillarum, Pseudomonas mendocina, and Vibrio parahaemolyticus, and Gram-positive Micrococcus luteus and Listeria monocytogenes. Additionally, all four AMPs induced permeabilization and depolarization of bacterial cell membranes and increased intracellular reactive oxygen species (ROS) levels. Importantly, they had little or no mammalian cytotoxicity. At the same time, 1–6 and L-6 protected the stability of intestinal flora in Sebastes schlegelii and increased the relative abundance of Lactobacillaceae. In summary, our results indicate that the designed AMPs have broad application prospects as a new type of polypeptide antimicrobial agent.

Transcriptome Analysis in the Head Kidney of Rainbow Trout (Oncorhynchus mykiss) Immunized with a Combined Vaccine of Formalin-Inactivated Aeromonas salmonicida and Vibrio anguillarum

This study aimed to identify the molecular mechanisms regulated by a combined vaccine against Aeromonas salmonicida and Vibrio anguillarum (O1 serotype). These bacteria cause furunculosis and vibriosis, respectively, and are associated with a high mortality in rainbow trout in Korea. The vaccine upregulated gene expression of TCRα, T-bet, sIgM, and mIgM, markers of an activated adaptive immune response. On days 1, 3, and 5, transcriptome analysis revealed 862 (430 up- and 432 downregulated), 492 (204 up- and 288 downregulated), and 741 (270 up- and 471 downregulated) differentially expressed genes (DEGs), respectively. Gene ontology (GO) enrichment analysis identified 377 (108 MF, 132 CC, 137 BP), 302 (60 MF, 180 CC, 62 BP), and 314 (115 MF, 129 CC, 70 BP) GOs at days 1, 3, and 5, respectively. Kyoto Encyclopedia of Genetic and Genomic enrichment analysis identified eight immune system-related pathways like cytokine-cytokine receptor interaction, NF-kappaB signaling pathway, TNF signaling pathway, NOD-like receptor signaling pathway, cytosolic DNA sensing pathway, cell adhesion molecule, complement and coagulation cascade, and antigen processing and presentation. In the analysis of the protein–protein interaction of immune-related DEGs, a total of 59, 21, and 21 interactional relationships were identified at days 1, 3, and 5, respectively, with TNF having the highest centrality at all three time points.