Protection of Teleost Fish against Infectious Diseases through Oral Administration of Vaccines: Update 2021

Evaluation of protective immune response of immersion inactivated vaccine against Singapore grouper iridovirus

Singapore grouper iridovirus (SGIV) always causes high transmission efficiency and mortality in the larval and juvenile stages of grouper in aquaculture industry. Although inactivated virus and recombinant DNA vaccines administered via intraperitoneal injection have shown efficacy in protection against SGIV, their potential applications in field testing were limited due to the vaccine delivery methods. Here, we developed an immersion vaccine containing inactivated virus and Montanide IMS 1312 adjuvant (IMS 1312) and evaluated its protective efficacy against SGIV infection. Compared to the PBS group, fish vaccinated with immersion inactivated vaccine with or without IMS 1312 were significantly protected against SGIV, with a relative percent survival (RPS) of 57.69 % and 38.47 %, respectively. Furthermore, the transcripts of viral core genes were reduced, and the histopathological severity caused by SGIV were relatively mild in multiple tissues of the IMS + V group. The immersion vaccine activated the AKP and ACP activities and increased the mRNA levels of IFN and inflammation-associated genes. The transcriptome analysis showed that a total of 731 and 492 genes were significantly regulated in the spleen and kidney from the IMS + V group compared to the PBS group, respectively. Among them, 129 DEGs were co-regulated, and enriched in the KEGG pathways related to immune and cell proliferation, including MAPK signaling, JAK-STAT signaling and PI3K-Akt signaling pathways. Similarly, the DEGs specially regulated in the kidney and spleen upon vaccine immunization were significantly enriched in the KEGG pathways related to interferon and inflammation response. Together, our results elucidated that the immersion vaccine of inactivated SGIV with IMS 1312 induced a protective immune response of grouper against SGIV.

Immersion immunization with recombinant Saccharomyces cerevisiae displaying ORF25 induced protective immunity against cyprinid herpesvirus 2

Displaying antigens on yeast surface as an oral vaccine has been widely explored, while its potential as an immersion vaccine has not been evaluated. Here, an immersion vaccine was prepared by displaying ORF25 of Cyprinid herpesvirus 2 (CyHV-2) on the surface of Saccharomyces cerevisiae. Carassius auratus gibelio was immersion immunized by 2 × 107 CFU/mL yeast for 2 h, and reinforce the immunity using the same method 14 days after the first immunization. The results showed that ORF25 specific antibody in immunized crucian carp serum was detected at a high level, and the mRNA expression level of IgM, IgT, IL-1β, and IFN-1 in vaccinated head-kidney and spleen tissues were higher than the control group, indicating that innate and adaptive immunity were induced. Moreover, the immersion vaccination provided effective protection for fish against CyHV-2, leading to a relative percent survival of 50.2%. Meanwhile, immersion vaccination restrained virus replication and histological damage in CyHV-2 infected crucian carp. Our data suggested that immersion immunization of S. cerevisiae-displayed ORF25 could be served as a candidate vaccine to prevent CyHV-2 infection.

Evaluation of the efficacy of MONTANIDE™ GR01, a new adjuvant for feed-based vaccines, on the immune response and protection against Streptococcus agalactiae in oral vaccinated Nile tilapia (Oreochromis niloticus) under laboratory and on-farm conditions

Streptococcosis, an emerging infectious disease caused by Streptococcus agalactiae, has had adverse effects on farmed tilapia. Several vaccines have been developed to prevent this disease and induce a specific immune response against S. agalactiae infection. In this study the use of MONTANIDE™ GR01, a new adjuvant for oral vaccination, was optimized for use in tilapia under laboratory and field studies. In the laboratory trial the immune response and protective efficacy of two doses of MONTANIDE™ GR01, 20 % (w/w) and 2 % (w/w), included into the feed-based adjuvanted vaccines were assessed comparatively. Following immunization, the innate immune parameters studied in serum, including lysozyme, myeloperoxidase, catalase and glutathione peroxidase activity, were all increased significantly. Furthermore, specific IgM antibodies against S. agalactiae were induced significantly in serum post-vaccination, with higher levels observed in both groups that received the feed-based adjuvanted vaccine. Under both injection and immersion challenge conditions, the relative percent survival for the feed-based adjuvanted vaccine groups ranged from 78 % to 84 %. Following use of the low dose concentration of MONTANIDE™ GR01 for oral vaccination of tilapia in cage culture systems, several innate immune parameters were effectively enhanced in the immunized fish. Similarly, the levels of specific IgM antibodies in the serum of feed-based vaccinated fish were significantly enhanced, reaching their highest levels 2-5 months post-vaccination. Cytokines associated with innate and adaptive immunity were also examined, and the expression levels of several genes showed significant up-regulation. This indicates that both cellular and humoral immune responses were induced by the feed-based adjuvanted vaccine. The economic impact of a feed-based adjuvanted vaccine was examined following vaccination, considering the growth performance and feed utilization of the fish. It was found that the Economic Performance Index and Economic Conversion Ratio were unaffected by vaccination, further demonstrating that there are no negative impacts associated with administering a feed-based vaccine to fish. In conclusion, the data from this study indicate that MONTANIDE™ GR01 is a highly valuable adjuvant for oral vaccination, as demonstrated by its ability to induce a strong immune response and effectively prevent streptococcal disease in Nile tilapia.

An Introduction to Relevant Immunology Principles with Respect to Oral Vaccines in Aquaculture

Vaccines continue to play an enormous role in the progression of aquaculture industries worldwide. Though preventable diseases cause massive economic losses, injection-based vaccine delivery is cost-prohibitive or otherwise impractical for many producers. Most oral vaccines, which are much cheaper to administer, do not provide adequate protection relative to traditional injection or even immersion formulas. Research has focused on determining why there appears to be a lack of protection afforded by oral vaccines. Here, we review the basic immunological principles associated with oral vaccination before discussing the recent progress and current status of oral vaccine research. This knowledge is critical for the development and advancement of efficacious oral vaccines for the aquaculture industry.

High-Level Expression of Recombinant VHSV Glycoprotein Using Transformed C. vulgaris and Verification of Vaccine Efficacy

The demand for aquaculture is increasing, but production is declining due to high feed costs and disease outbreaks. Viral hemorrhagic septicemia (VHS) is a viral disease that seriously affects seawater and freshwater fish in aquaculture, including the olive flounder (Paralichthys olivaceus), a major aquaculture fish in Korea. However, very few vaccines are currently available for viral hemorrhagic septicemia virus (VHSV). The nutrient-rich microalga Chlorella vulgaris has been used as a feed additive in aquaculture and as a host for the industrial production of recombinant VHSV glycoprotein as an oral vaccine. In this study, VHSV glycoprotein was cloned with a salt-inducible promoter, and high levels of expression up to 41.1 mg/g wet C. vulgaris, representing 27.4% of total extracted soluble protein, were achieved by growing the transformed C. vulgaris for 5 days in the presence of 250 mM NaCl. The production of a neutralizing antibody was detected in the serum of fish given feed containing 9% VHSV glycoprotein-expressing C. vulgaris. Furthermore, relative survival rates of 100% and 81.9% were achieved following challenges of these fish with VHSV at 10⁶ and 10⁷ pfu/fish, respectively, indicating that C. vulgaris could be used as a platform for the production of recombinant proteins for use as oral vaccines in the control of viral diseases in aquaculture.

Exploring the Efficacy and Safety of Levamisole Hydrochloride against Microcotyle sebastis in Korean Rockfish (Sebastes schlegelii): An In Vitro and In Vivo Approach

Parasitic infections pose significant challenges in aquaculture, and the increasing resistance to conventional anthelmintics necessitates the exploration of alternative treatments. Levamisole hydrochloride (HCl) has demonstrated efficacy against monogenean infections in various fish species; however, research focused on Microcotyle sebastis infections in Korean rockfish (Sebastes schlegelii) remains limited. Therefore, this study aimed to evaluate the efficacy of levamisole HCl against M. sebastis infections in Korean rockfish with the goal of optimizing anthelmintic usage in aquaculture. In this study, we first assessed the susceptibility of M. sebastis to levamisole HCl in vitro. Subsequently, in vivo evaluations were conducted to assess the drug's efficacy, safety, and to identify optimal administration methods. In vitro experiments revealed concentration-dependent sensitivity of M. sebastis to levamisole HCl, with a minimum effective concentration (MEC) of 100 mg/L. In vivo experiments employed oral administration, intraperitoneal injection, and immersion treatments based on the MEC. Oral administration proved to be a safe method, yielding efficacy rates of 27.3% and 41.6% for 100 mg/kg and 200 mg/kg doses, respectively, in contrast to the immersion and injection methods, which induced symptoms of abnormal swimming, vomiting, and death. Biochemical analyses conducted to assess the safety of levamisole HCl revealed a transient, statistically significant elevation in the levels of glutamic oxaloacetic transaminase (GOT) and glutamic pyruvic transaminase (GPT) on day three post-administration at 20 °C. Following this, no substantial differences were observed. However, at 13 °C, the enzyme levels remained relatively consistent, emphasizing the role of water temperature conditions in influencing the action of levamisole HCl. Our research findings substantiate the efficacy of levamisole HCl against M. sebastis in Korean rockfish, underscoring its potential for safe oral administration. These results provide valuable insights for developing parasite control strategies involving levamisole HCl in Korean rockfish populations while minimizing adverse impacts on fish health and the environment. However, this study bears limitations due to its controlled setting and narrow focus. Future research should expand on these findings by testing levamisole HCl in diverse environments, exploring different administration protocols, and examining wider temperature ranges.

An improved oral vaccine with molecular adjuvant β-defensin protects grouper against nervous necrosis virus infection

Nervous necrosis virus (NNV) is one of the most important fish viral pathogens infecting more than 120 fish species worldwide. Due to the mass mortality rates often seen among larvae and juveniles, few effective vaccines against NNV were developed up to now. Here, the protective effect of recombinant coat protein (CP) from red-spotted grouper nervous necrosis virus (RGNNV) fused with grouper β-defensin (DEFB) as an oral vaccine was evaluated using Artemia as a biocarrier delivery system in pearl gentian grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀). Feeding with Artemia encapsulated with E. coli expressing control vector (control group), CP, or CP-DEFB showed no obvious side effects on the growth of groupers. ELISA and antibody neutralization assay showed that CP-DEFB oral vaccination group induced higher anti-RGNNV CP specific antibodies and exhibited higher neutralization potency than the CP and control group. Meanwhile, the expression levels of several immune and inflammatory factors in the spleen and kidney after feeding with CP-DEFB were also significantly increased compared with the CP group. Consistently, after challenge with RGNNV, groupers fed CP-DEFB and CP exhibited 100% and 88.23% relative percentage survival (RPS), respectively. Moreover, the lower transcription levels of viral genes and milder pathological changes in CP-DEFB group were detected compared with the CP and control group. Thus, we proposed that grouper β-defensin functioned as an efficient molecular adjuvant for an improved oral vaccine against nervous necrosis virus infection.

A new pathogenic isolate of Kocuria kristinae identified for the first time in the marine fish Larimichthys crocea

In recent years, new emerging pathogenic microorganisms have frequently appeared in animals, including marine fish, possibly due to climate change, anthropogenic activities, and even cross-species transmission of pathogenic microorganisms among animals or between animals and humans, which poses a serious issue for preventive medicine. In this study, a bacterium was clearly characterized among 64 isolates from the gills of diseased large yellow croaker Larimichthys crocea that were raised in marine aquaculture. This strain was identified as K. kristinae by biochemical tests with a VITEK 2.0 analysis system and 16S rRNA sequencing and named K. kristinae_LC. The potential genes that might encode virulence-factors were widely screened through sequence analysis of the whole genome of K. kristinae_LC. Many genes involved in the two-component system and drug-resistance were also annotated. In addition, 104 unique genes in K. kristinae_LC were identified by pan genome analysis with the genomes of this strain from five different origins (woodpecker, medical resource, environment, and marine sponge reef) and the analysis results demonstrated that their predicted functions might be associated with adaptation to living conditions such as higher salinity, complex marine biomes, and low temperature. A significant difference in genomic organization was found among the K. kristinae strains that might be related to their hosts living in different environments. The animal regression test for this new bacterial isolate was carried out using L. crocea, and the results showed that this bacterium could cause the death of L. crocea and that the fish mortality was dose-dependent within 5 days post infection, indicating the pathogenicity of K. kristinae_LC to marine fish. Since K. kristinae has been reported as a pathogen for humans and bovines, in our study, we revealed a new isolate of K. kristinae_LC from marine fish for the first time, suggesting the potentiality of cross-species transmission among animals or from marine animals to humans, from which we would gain insight to help in future public prevention strategies for new emerging pathogens.

An oral pH-responsive Streptococcus agalactiae vaccine formulation provides protective immunity to pathogen challenge in tilapia: A proof-of-concept study

Intensive tilapia farming has contributed significantly to food security as well as to the emergence of novel pathogens. This includes Streptococcus agalactiae or Group B Streptococcus (GBS) sequence type (ST) 283, which caused the first known outbreak of foodborne GBS illness in humans. An oral, easy-to-administer fish vaccine is needed to reduce losses in fish production and the risk of zoonotic transmission associated with GBS. We conducted a proof-of-concept study to develop an oral vaccine formulation that would only release its vaccine cargo at the site of action, i.e., in the fish gastrointestinal tract, and to evaluate whether it provided protection from experimental challenge with GBS. Formalin-inactivated S. agalactiae ST283, was entrapped within microparticles of Eudragit® E100 polymer using a double-emulsification solvent evaporation method. Exposure to an acidic medium simulating the environment in tilapia stomach showed that the size of the vaccine-loaded microparticles decreased rapidly, reflecting microparticle erosion and release of the vaccine cargo. In vivo studies in tilapia showed that oral administration of vaccine-loaded microparticles to fish provided significant protection from subsequent homologous pathogen challenge with GBS ST283 by immersion compared to the control groups which received blank microparticles or buffer, reducing mortality from 70% to 20%. The high efficacy shows the promise of the vaccine platform developed herein, which might be adapted for other bacterial pathogens and other fish species.

Characterization and evaluation of an oral vaccine via nano-carrier for surface immunogenic protein (Sip) delivery against Streptococcus agalactiae infection

Streptococcus agalactiae causes systemic disease in a variety of wild and farmed fish, resulting in high levels of morbidity and mortality, as well as serious economic losses to the Nile tilapia aquaculture industry. The development of economic and applicable oral vaccines is therefore urgently needed for the sustainable development of Nile tilapia aquaculture. In this study, mesoporous silica nanoparticles (MSNs) were fabricated using sol-gel synthesis technology, and the antigens of surface immunogenic protein (Sip) was loaded into MSNs to develop a nanovaccine MSNs-Sip@HP55. The results showed that the prepared nanovaccine exhibited pH-controlled release, which could survive in the simulated gastric environment (pH 1.5), and release antigens in the simulated intestinal environment at pH 7.4. The nanovaccine could induce innate and adaptive immune responses in Nile tilapia. When the challenge doses were 1.5 × 10⁶, 1.18 × 10⁶, and 0.88 × 10⁶ CFU/mL, the relative protection rates in immunized Nile tilapia were 63.33 %, 64.23 %, and 76.31 %, respectively. Taken together, the nanovaccine exhibited a high antigen utilization rate and was easily administered orally via feeding, which could protect Nile tilapia against challenge with S. agalactiae in large-scale farms. Oral vaccine based on MSNs carriers is a potentially promising strategy for the development of fish vaccines.

Bacterial Pathogenesis in Various Fish Diseases: Recent Advances and Specific Challenges in Vaccine Development

Aquaculture is a fast-growing food sector but is plagued by a plethora of bacterial pathogens that infect fish. The rearing of fish at high population densities in aquaculture facilities makes them highly susceptible to disease outbreaks, which can cause significant economic loss. Thus, immunity development in fish through vaccination against various pathogens of economically important aquaculture species has been extensively studied and has been largely accepted as a reliable method for preventing infections. Vaccination studies in aquaculture systems are strategically associated with the economically and environmentally sustainable management of aquaculture production worldwide. Historically, most licensed fish vaccines have been developed as inactivated pathogens combined with adjuvants and provided via immersion or injection. In comparison, live vaccines can simulate a whole pathogenic illness and elicit a strong immune response, making them better suited for oral or immersion-based therapy methods to control diseases. Advanced approaches in vaccine development involve targeting specific pathogenic components, including the use of recombinant genes and proteins. Vaccines produced using these techniques, some of which are currently commercially available, appear to elicit and promote higher levels of immunity than conventional fish vaccines. These technological advancements are promising for developing sustainable production processes for commercially important aquatic species. In this review, we explore the multitude of studies on fish bacterial pathogens undertaken in the last decade as well as the recent advances in vaccine development for aquaculture.

Oral Vaccination of Recombinant Saccharomyces cerevisiae Expressing ORF132 Induces Protective Immunity against Cyprinid Herpesvirus-2

Cyprinid herpesvirus 2 (CyHV-2) is the etiological agent of herpesviral hematopoietic necrosis (HVHN) disease, which causes serious economic losses in the crucian carp culture industry. In this study, by displaying ORF132 on the surface of Saccharomyces cerevisiae cells (named EBY100/pYD1-ORF132), we evaluated the protective efficacy of oral administration against CyHV-2 infection. Intense innate and adaptive immune responses were evoked in both mucosal and systemic tissues after oral vaccination with EBY100/pYD1-ORF132. Importantly, oral vaccination provided significant protection for crucian carp post CyHV-2 infection, resulting in a relative percent survival (RPS) of 64%. In addition, oral administration suppressed the virus load and relieved histological damage in selected tissues. Our results indicated that surface-displayed ORF132 on S. cerevisiae could be used as potential oral vaccine against CyHV-2 infection.

Why vaccines fail against Piscirickettsiosis in farmed salmon and trout and how to avoid it: A review

Piscirickettsiosis is the most severe, persistent, and damaging disease that has affected the Chilean salmon industry since its origins in the 1980s. As a preventive strategy for this disease, different vaccines have been developed and used over the last 30 years. However, vaccinated salmon and trout frequently die in the sea cages and the use of antibiotics is still high demonstrating the low efficiency of the available vaccines. The reasons why the vaccines fail so often are still debated, but it could involve different extrinsic and intrinsic factors. Among the extrinsic factors, mainly associated with chronic stress, we can distinguish: 1) biotic including coinfection with sea lice, sealions attacks or harmful algal blooms; 2) abiotic including low oxygen or high temperature; and 3) farm-management factors including overcrowding or chemical delousing treatments. Among the intrinsic factors, we can distinguish: 1) fish-related factors including host's genetic variability (species, population and individual), sex or age; 2) pathogen-related factors including their variability and ability to evade host immune responses; and 3) vaccine-related factors including low immunogenicity and poor matches with the circulating pathogen strain. Based on the available evidence, in order to improve the development and the efficacy of vaccines against P. salmonis we recommend: a) Do not perform efficacy evaluations by intraperitoneal injection of pathogens because they generate an artificial protective immune response, instead cohabitation or immersion challenges must be used; b) Evaluate the diversity of pathogen strains in the field and ensure a good antigenic match with the vaccines; c) Investigate whether host genetic diversity could be improved, e.g. through selection, in favor of better and longer responses to vaccination; d) To reduce the stressful effects at the cage level, controlling the co-infection of pathogens and avoiding fish overcrowding. To date, we do not know the immunological mechanisms by which the vaccines against P. salmonis may or may not generate protection. More studies are required to identify what type of response, cellular or molecular, is required to develop effective vaccines.

Oral vaccination for sustainable disease prevention in aquaculture-an encapsulation approach

The prevalence of infectious diseases in the aquaculture industry and a limited number of safe and effective oral vaccines has imposed a challenge not only for fish immunity but also a threat to human health. The availability of fish oral vaccines has expanded recently, but little is known about how well they work and how they affect the immune system. The unsatisfactory efficacy of existing oral vaccinations is partly attributable to the antigen degradation in the adverse gastrointestinal environment of fishes, the highly tolerogenic gut environment, and inferior vaccine formulation. To overcome such challenges in designing: an easier, cost-efficient, and effective vaccination method, several encapsulation methods are being adopted to safeguard antigens from the intestinal atmosphere for their immunogenic functions. Oral vaccination is easily degraded by gastric acids and enzymes before reaching the immunological site; however, this issue can be solved by encapsulating antigens in poly-biodegradable nanoparticles, transgenic designed bacteria, plant systems, and live feeds. To enhance the immunological impact, each antigen delivery method operates at a different level. Utilizing nanotechnology, it has been possible to regulate vaccination parameters, target particular cells, and lower the antigen dosage with potent nanomaterials such as chitosan, poly D,L-lactic-co-glycolic acid (PLGA) as vaccine carriers. Live feeds such as Artemia salina can be utilized as bio-carrier, owing to their appropriate size and non-filter feed system, through a process called bio-encapsulation. It ensures the protection of antigens over the fish intestine and ensures complete uptake by immune cells in the hindgut for increased immune response. This review comprises recent advances in oral vaccination in aquaculture in terms of an encapsulation approach that can aid in future research.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.

Oral vaccination of fish against vibriosis using spore-display technology

Oral vaccines are highly demanded by the aquaculture sector, to allow mass delivery of antigens without using the expensive and labor-intensive injectable vaccines. These later require individual handling of fish, provoking stress-related mortalities. One possible strategy to create injection-free vaccine delivery vehicles is the use of bacterial spores, extremely resistant structures with wide biotechnological applications, including as probiotics, display systems, or adjuvants. Bacterial spores, in particular those of Bacillus subtilis, have been shown to behave as mucosal vaccine adjuvants in mice models. However, such technology has not been extensively explored against fish bacterial disease. In this study, we used a laboratory strain of B. subtilis, for which a variety of genetic manipulation tools are available, to display at its spores surface either a Vibrio antigenic protein, OmpK, or the green fluorescence protein, GFP. When previously vaccinated by immersion with the OmpK- carrying spores, zebrafish survival upon a bacterial challenge with V. anguillarum and V. parahaemolyticus, increased up to 50 - 90% depending on the pathogen targeted. Further, we were able to detect anti-GFP-antibodies in the serum of European seabass juveniles fed diets containing the GFP-carrying spores and anti-V. anguillarum antibodies in the serum of European seabass juveniles fed the OmpK-carrying spores containing diet. More important, seabass survival was increased from 60 to 86% when previously orally vaccinated with in-feed OmpK- carrying spores. Our results indicate that B. subtilis spores can effectively be used as antigen-carriers for oral vaccine delivery in fish.