New attenuated vaccine against columnaris disease in fish: choosing the right parental strain is critical for vaccine efficacy

Gut dysbiosis induced by florfenicol increases susceptibility to Aeromonas hydrophila infection in Zebrafish Danio rerio after the recommended withdrawal period

OBJECTIVE

Florfenicol (FFC) is a broad-spectrum antibiotic approved by the U.S. Food and Drug Administration to treat both systemic and external bacterial infections in food fish. The objective of this study was to evaluate the effect of FFC-medicated feed on the gut microbiota of Zebrafish Danio danio to determine (1) if the therapeutic dose of FFC-medicated feed induces dysbiosis and (2) if fish with altered gut microbiota were more susceptible to subsequent infection by the common opportunistic fish pathogen Aeromonas hydrophila.

METHODS

Zebrafish that were treated with regular and FFC-medicated feeds were artificially challenged with A. hydrophila at the end of the recommended 15-day antibiotic withdrawal period. The gut microbiota of the Zebrafish at different stages was analyzed using 16S ribosomal RNA gene sequencing.

RESULT

Our results found that FFC-medicated feed induced disruption of the gut microbiota. Dysbiosis was observed in all treated groups, with a significant increase in bacterial diversity, and was characterized by a remarkable bloom of Proteobacteria and a drastic decline of Mycoplasma and Cetobacterium in treated animals but without noticeable clinical signs or mortalities. In addition, the increase of Proteobacteria was not significantly reduced after the recommended 15-day withdrawal period, and the Zebrafish treated with FFC-medicated feed exhibited a significantly higher mortality rate when they were subsequently challenged with A. hydrophila compared to the control (regular feed) groups. Interestingly, the most dramatic changes in the gut microbiome composition occurred at the transition time between the late stage of the medicated treatment and the beginning of the withdrawal period instead of the time during the Aeromonas infection.

CONCLUSION

The administration of FFC-medicated feed at the recommended dose induced gut dysbiosis in Zebrafish, and fish did not recover to the baseline after the recommended withdrawal period. Our findings suggest that the use of antibiotics in fish elicits a response similar to those previously described in mammals and possibly makes the host more susceptible to subsequent infections of opportunistic pathogens. This study using a controlled model system suggests that antibiotics in aquaculture may have long-term effects on the general well-being of the fish.

Early divergent responses to virulent and attenuated vaccine isolates of Flavobacterium covae sp. nov. In channel catfish, Ictalurus punctatus

Columnaris disease continues to inflict substantial losses among freshwater cultured species since its first description one hundred years ago. The experimental and anecdotal evidence suggests an expanded range and rising virulence of columnaris worldwide due to the warming global climate. The channel catfish (Ictalurus punctatus) are particularly vulnerable to columnaris. A recently developed live attenuated vaccine (17-23) for Flavobacterium columnare (now Flavobacterium covae sp. nov.) demonstrated superior protection for vaccinated catfish against genetically diverse columnaris isolates. In this study, we aimed to elucidate the molecular mechanisms and patterns of immune evasion and host manipulation linked to virulence by comparing gene expression changes in the host after the challenge with a virulent (BGSF-27) or live attenuated F. covae sp. nov. vaccine (17-23). Thirty-day-old fry were accordingly challenged with either virulent or vaccine isolates. Gill tissues were collected at 0 h (control), 1 h, and 2 h post-infection, which are two critical time points in early host-pathogen interactions. Transcriptome profiling of the gill tissues revealed a larger number (518) of differentially expressed genes (DEGs) in vaccine-exposed fish than those exposed to the virulent pathogen (321). Pathway analyses suggested potent suppression of early host immune responses by the virulent isolate through a higher expression of nuclear receptor corepressors (NCoR) responsible for antagonizing macrophage and T-cell signaling. Conversely, in vaccinated fry, we observed induction of Ca2+/calmodulin-dependent protein kinase II (CAMKII), responsible for clearing NCoR, and commensurate up-regulation of transcription factor AP-1 subunits, c-Fos, and c-Jun. As in mammalian systems, AP-1 expression was connected with a broad immune activation in vaccinated fry, including induction of CC chemokines, proteinases, iNOS, and IL-12b. Relatedly, divergent expression patterns of Src tyrosine kinase Lck, CD44, and CD28 indicated a delay or suppression of T-cell adhesion and activation in fry exposed to the virulent isolate. Broader implications of these findings will be discussed. The transcriptomic differences between virulent and attenuated bacteria may offer insights into how the host responds to the vaccination or infection and provide valuable knowledge to understand the early immune mechanisms of columnaris disease in aquaculture.

The protective effect of inactivated Flavobacterium columnare vaccine in grass carp (Ctenopharyngodon idellus)

Flavobacterium columnare, which causes columnaris disease, is responsible for significant mortality in grass carp. Vaccination is a safe and effective measure to combat this disease, and this study aimed to investigate the immune protective effects of different treatments using an inactivated F. columnare vaccine. The vaccine was prepared by inactivating the bacteria with 0.05% formaldehyde at 4°C for 24 hours. The experiments involving grass carp were divided into two parts. In Experiment 1, the immune effects of two isolates, JX-01 (genomovar I) and MU-04 (genomovar II), were compared, along with the impact of white oil adjuvant and the number of immunizations. The results showed that when the white oil adjuvant was used as a booster, the relative percent survival (RPS) of the JW2 group and MW2 group after 8 weeks of the first immunization was 34% and 61%, respectively. In Experiment 2, only the MU-04 (genomovar II) isolate was used as an antigen, with the white oil adjuvant as a booster. The effects of different doses (CFU=10⁸, 10⁷, and 10⁶ bacteria/mL) on immune responses were compared, and the RPS values in the MW6, MW7, and MW8 groups after 4 weeks of the first immunization were found to be 38%, 57%, and 71%, respectively. Furthermore, in the cross-antigen protection experiment, the MW2 group exhibited an RPS of 55% against the JX-01 isolate, which was significantly higher than the control group (33%). These findings suggest that an inactivated vaccine comprising an appropriate antigen isolate when administered with a white oil adjuvant as a booster, can provide effective protection in grass carp.

Secreted Extracellular Products of Flavobacterium covae as Potential Immunogenic Factors for Protection against Columnaris Disease in Channel Catfish (Ictalurus punctatus)

Columnaris disease caused by Flavobacterium covae leads to substantial economic losses in commercially important fish species worldwide. The US channel catfish (Ictalurus punctatus) industry is particularly vulnerable to this disease. Therefore, there is an urgent need to develop a vaccine to reduce the economic losses caused by this disease. Secreted extracellular products (SEPs) are considered to be essential bacterial virulence factors that often provide immunogenicity and protection. The current study sought to identify the main SEPs of F. covae and to evaluate their potential to provide protection in channel catfish against columnaris disease. SDS-PAGE analysis of SEPs revealed five protein bands with molecular weights ranging from 13 to 99 kDa. Mass spectrometry analysis showed that these SEPs were hypothetical protein (AWN65_11950), zinc-dependent metalloprotease (AWN65_10205), DNA/RNA endonuclease G (AWN65_02330), outer membrane protein beta-barrel domain (AWN65_12620), and chondroitin-sulfate-ABC endolyase/exolyase (AWN65_08505). Catfish fingerlings were vaccinated with SEPs, SEPs emulsified with mineral oil adjuvant, or heat-inactivated SEPs, or they were sham-immunized through intraperitoneal (IP) injection. After 21 days, an F. covae challenge showed 58.77% and 46.17% survival in the catfish vaccinated with the SEPs and the SEPs emulsified with adjuvant compared to the sham-vaccinated control (100% mortality within 120 h post-infection). However, the heat-inactivated SEPs failed to provide significant protection (23.15% survival). In conclusion, although SEPs contain potentially important immunogenic proteins, further work is needed to optimize their use for long-lasting protection against columnaris disease in fish. These results are significant given the economic impact of columnaris disease on fish farming worldwide.

Safety and immunogenicity of a TK/ gI/gE gene-deleted feline herpesvirus-1 mutant constructed via CRISPR/Cas9 in feline

Feline herpesvirus-1 (FHV-1) is the aetiological agent of feline viral rhinotracheitis, which accounts for approximately 50 % of all viral upper respiratory diseases in cats. Commercially available modified live vaccines containing FHV-1 are generally safe and effective, but these FHV-1 vaccines retain full virulence genes and can establish latency and reactivate to cause infectious rhinotracheitis in vaccine recipients, raising safety concerns. To address this shortcoming, we constructed a novel TK/gI/gE -gene-deleted recombinant FHV-1 (WH2020-ΔTK/gI/gE) through CRISPR/Cas9-mediated homologous recombination. The growth kinetics of WH2020-ΔTK/gI/gE were slightly delayed compared to those of the parent strain WH2020. Recombinant FHV-1 had severely impaired pathogenicity in cats. Felines immunized with WH2020-ΔTK/gI/gE produced high levels of gB-specific antibodies, neutralizing antibodies and IFN-β. Additionally, WH2020-ΔTK/gI/gE provided greater protection against challenge with FHV-1 field strain WH2020 than did the commercial modified live vaccine. After challenge, the cats vaccinated with WH2020-ΔTK/gI/gE showed significantly fewer clinical signs, pathological changes, viral shedding, and viral loads in the lung and trigeminal ganglia than those vaccinated with the commercial vaccine or unvaccinated. Our results suggest that WH2020-ΔTK/gI/gE is a promising candidate as a safer and more efficacious live FHV-1 vaccine, with a decreased risk of vaccine-related complications, and could inform the design of other herpesvirus vaccines.

Development of a bivalent mucoadhesive nanovaccine to prevent francisellosis and columnaris diseases in Nile tilapia (Oreochromis niloticus)

The occurrence of francisellosis caused by Francisella orientalis sp. nov. (Fo) and columnaris disease caused by Flavobacterium oreochromis (For) is negatively impacting Nile tilapia (Oreochromis niloticus) production, especially when high stocking densities are used. A new and innovative bivalent mucoadhesive nanovaccine was developed in this study for immersion vaccination of tilapia against francisellosis and columnaris disease. It was shown to have the potential to improve both innate and adaptive immunity in vaccinated Nile tilapia. It increased innate immune parameters, such as lysozyme activity, bactericidal activity, phagocytosis, phagocytic index, and total serum IgM antibody levels. Additionally, the vaccine was effective in elevating specific adaptive immune responses, including IgM antibody levels against Fo and For vaccine antigens and upregulating immune-related genes IgM, IgT, CD4⁺, MHCIIα, and TCRβ in the head kidney, spleen, peripheral blood leukocytes, and gills of vaccinated fish. Furthermore, fish vaccinated with the mucoadhesive nanovaccine showed higher survival rates and relative percent survival after being challenged with either single or combined infections of Fo and For. This vaccine is anticipated to be beneficial for large-scale immersion vaccination of tilapia and may be a strategy for shortening vaccination times and increasing immune protection against francisellosis and columnaris diseases in tilapia aquaculture.

Effects of Various Doses of Saccharomyces cerevisiae on the Growth, Survival Rate, and Blood Profile of Saline Red Tilapia (Oreochromisspp.) in the Semi-Intensive Culture Conditions

The availability of good feeding on fish performance is needed to improve the poor aquaculture condition. The study examines the effects of Saccharomyces cerevisiae supplementation on the growth, survival rate, and blood profile of saline red tilapia in the semi-intensive culture. Sixty saline red tilapia (4.58 ± 0.09 g) were divided into five groups and received standard feed with supplementation of S. cerevisiae at different doses: (A-E) 0, 5, 10, 15, and 20 g/kg feed. S. cerevisiae was sprayed onto the feed and given to fish three times/day for 49 days. The efficiency of feed utilization (EFU), feed conversion ratio (FCR), the protein efficiency ratio (PER), relative growth rate (RGR), survival rate (SR), and blood profiles were examined. The water quality parameters were also evaluated, including temperature, pH, dissolved oxygen, NH₃, and salinity. Data were analyzed using ANOVA followed by the Duncan test. The supplementation of 10 g/kg S. cerevisiae could significantly (p<0.05) increase EFU (85.13%), PER (3.76), RGR (4.53%), and survival rate (97.53%), and decrease FCR (1.42) of the saline red tilapia. All doses of S. cerevisiae significantly (p<0.05) increased the number of hematocytes (34.68, 35.96, 34.14, and 34.52%, respectively) and hemoglobin (12.98, 13.58, 12.73, and 12.67%, respectively) of fish. The highest number of erythrocytes and leucocytes was found in dose 10 g/kg feed. All water parameters were stable during the study. Therefore, S. cerevisiae can be used as a feed supplement for red tilapia fish to increase the growth, survival rate, and blood profile of the saline red tilapia.

A large-scale, multi-year microbial community survey of a freshwater trout aquaculture facility

Aquaculture is an important tool for solving the growing worldwide food demand, but infectious diseases of farmed animals represent a serious roadblock to continued industry growth. Therefore, it is essential to understand the microbial communities that reside within the built environments of aquaculture facilities to identify reservoirs of bacterial pathogens and potential correlations between commensal species and specific disease agents. Here, we present the results from 3 years of sampling a commercial rainbow trout aquaculture facility. We observed that the microbial communities residing on the abiotic surfaces within the hatchery were distinct from those residing on the surfaces at the facility's water source as well as the production raceways, despite similar communities in the water column at each location. Also, a subset of the water community seeds the biofilm communities. Lastly, we detected a common fish pathogen, Flavobacterium columnare, within the hatchery, including at the source water inlet. Importantly, the relative abundance of this pathogen was correlated with clinical disease. Our results characterized the microbial communities in an aquaculture facility, established that the hatchery environment contains a unique community composition and demonstrated that a specific fish pathogen resides within abiotic surface biofilms and is seeded from the natural water source.

Mucoadhesive cationic lipid-based Flavobacterium oreochromis nanoencapsulation enhanced the efficacy of mucoadhesive immersion vaccination against columnaris disease and strengthened immunity in Asian sea bass (Lates calcarifer)

Columnaris is a bacterial disease, found in freshwater fish, caused by Flavobacterium oreochromis. The disease has a devastating impact on a range of cultured and wild freshwater fish species e.g. Lates calcarifer (Asian sea bass), which is a serious economic losses to the freshwater aquaculture in Thailand. The disease can be prevented by an efficacious vaccine, however, no licensed effective vaccine is available to date. Current study was based on the development of a novel mucoadhesive nano-encapsulated vaccine (EncapFlavoNP⁺⁺), where, cationic lipid-based nanoparticles were combined with an antigen obtained from F. oreochromis. Various parameters including transmission electron microscopy (TEM), physiochemical properties; zeta potential, and polydispersity index were determined. The TEM results depicted well-formed circular-shaped nano-encapsulates complexed with cationic lipid surfactants. The average diameter of the molecules was 200 nm, having a zeta potential of 31.82 mV, while, the polydispersity index (PDI) was 0.31. The in vivo study lasted for 8 weeks, the immunologic and protective potentials of the prepared molecules were determined by challenging the fish for 8 weeks. The most effective dilutions of EncapFlavoNP⁺⁺ solution were 1:100 and 1:200, which significantly improved the efficacy of the immunity by increasing the level of antibody specific to F. oreochromis. A trend of upregulation was found in the immune-related genes including immunoglobulin M heavy chain (IgM), major histocompatibility complex class IIα molecules (MHC-IIα), and dendritic cell specific transcript (DCs) in gills, skin, liver, peripheral blood lymphocytes (PBLs), head kidneys, and spleen as compared to the control group (P < 0.05 and P < 0.01). Upon immunization with EncapFlavoNP⁺⁺ solution at the dilution of 1:100 and 1:200, the significant increase in survival rate (SR) and relative percent survival (RPS) were found in fish challenged with virulent F. oreochromis bacterium (SR 72.50% and RPS 62.07) and (SR 65.83% and RPS 52.87), respectively as compared to the control group (P < 0.05). It can be concluded that immunization with EncapFlavoNP⁺⁺ solution has significant immunologic and protective effects against Columnaris disease. Furthermore, the prepared vaccine candidate has more potential as compared to whole-cell immersion vaccination (FK-WC). It can be used on a large scale in the freshwater aquaculture industry to boost immunity against Columnaris disease.

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.

Deciphering the Molecular Basis for Attenuation of Flavobacterium columnare Strain Fc1723 Used as Modified Live Vaccine against Columnaris Disease

Vaccines are widely employed in aquaculture to prevent bacterial infections, but their use by the U.S. catfish industry is very limited. One of the main diseases affecting catfish aquaculture is columnaris disease, caused by the bacterial pathogen Flavobacterium columnare. In 2011, a modified-live vaccine against columnaris disease was developed by selecting mutants that were resistant to rifampin. The previous study has suggested that this vaccine is stable, safe, and effective, but the mechanisms that resulted in attenuation remained uncharacterized. To understand the molecular basis for attenuation, a comparative genomic analysis was conducted to identify specific point mutations. The PacBio RS long-read sequencing platform was used to obtain draft genomes of the mutant attenuated strain (Fc1723) and the parent virulent strain (FcB27). Sequence-based genome comparison identified 16 single nucleotide polymorphisms (SNP) unique to the mutant. Genes that contained mutations were involved in rifampin resistance, gliding motility, DNA transcription, toxin secretion, and extracellular protease synthesis. The results also found that the vaccine strain formed biofilm at a significantly lower rate than the parent strain. These observations suggested that the rifampin-resistant phenotype and the associated attenuation of the vaccine strain result from the altered activity of RNA polymerase (RpoB) and possible disrupted protein secretion systems.

Immersion Vaccination by a Biomimetic-Mucoadhesive Nanovaccine Induces Humoral Immune Response of Red Tilapia (Oreochromis sp.) against Flavobacterium columnare Challenge

Immersion vaccination with a biomimetic mucoadhesive nanovaccine has been shown to induce a strong mucosal immune response against columnaris disease, a serious bacterial disease in farmed red tilapia caused by Flavobacterium columnare. However, the induction of a systemic immune response by the vaccine is yet to be investigated. Here, we examine if a specific humoral immune response is stimulated in tilapia by a biomimetic-mucoadhesive nanovaccine against Flavobacterium columnare using an indirect-enzyme-linked immunosorbent assay (ELISA), serum bactericidal activity (SBA) and the expression of immune-related genes within the head-kidney and spleen, together with assessing the relative percent survival of vaccinated fish after experimentally infecting them with F. columnare. The anti-IgM antibody titer of fish at 14 and 21 days post-vaccination was significantly higher in chitosan complex nanoemulsion (CS-NE) vaccinated fish compared to fish vaccinated with the formalin-killed vaccine or control fish, supporting the serum bactericidal activity results at these time points. The cumulative mortality of the unvaccinated control fish was 87% after challenging fish with the pathogen, while the cumulative mortality of the CS-NE vaccinated group was 24%, which was significantly lower than the formalin-killed vaccinated and control fish. There was a significant upregulation of IgM, IgT, TNF α, and IL1-β genes in the spleen and kidney of vaccinated fish. Significant upregulation of IgM and IgT genes was observed in the spleen of CS-NE vaccinated fish. The study confirmed the charged-chitosan-based mucoadhesive nanovaccine to be an effective platform for immersion vaccination of tilapia, with fish generating a humoral systemic immune response against columnaris disease in vaccinated fish.

Ghonaim A, Wu H, Sun Q, Fan S, He Q. Recombinant Pseudorabies Virus with TK/gE Gene Deletion and Flt3L Co-Expression Enhances the Innate and Adaptive Immune Response via Activating Dendritic Cells

Owing to viral evolution and recombination, emerging pseudorabies virus (PRV) strains have caused unprecedented outbreaks in swine farms even when the pigs were previously vaccinated, which might indicate that traditional vaccines were unable to provide effective protection. The development of safe and efficacious vaccines presents prospects to minimize the clinical signs and eventually eradicate the infection. In this study, we used an emerging PRV strain, HNX, as the parental strain to construct a recombinant PRV with TK/gE gene deletion and Fms-related tyrosine kinase 3 ligand (Flt3L) expression, named HNX-TK⁻/gE⁻-Flt3L. HNX-TK⁻/gE⁻-Flt3L enhanced the maturation of bone marrow derived dendritic cells (DCs) in vitro. Significantly more activated DCs were detected in HNX-TK⁻/gE⁻-Flt3L-immunized mice compared with those immunized with HNX-TK⁻/gE⁻. Subsequently, a remarkable increase of neutralizing antibodies, gB-specific IgG antibodies, and interferon-gamma (IFN-γ) was observed in mice vaccinated with HNX-TK⁻/gE⁻-Flt3L. In addition, a lower mortality and less histopathological damage were observed in HNX-TK⁻/gE⁻-Flt3L vaccinated mice with upon PRV lethal challenge infection. Taken together, our results revealed the potential of Flt3L as an ideal adjuvant that can activate DCs and enhance protective immune responses and support the further evaluation of HNX-TK⁻/gE⁻-Flt3L as a promising PRV vaccine candidate.

State-of-the-Art Vaccine Research for Aquaculture Use: The Case of Three Economically Relevant Fish Species

In the last three decades, the aquaculture sector has experienced a 527% growth, producing 82 million tons for a first sale value estimated at 250 billion USD. Infectious diseases caused by bacteria, viruses, or parasites are the major causes of mortality and economic losses in commercial aquaculture. Some pathologies, especially those of bacterial origin, can be treated with commercially available drugs, while others are poorly managed. In fact, despite having been recognized as a useful preventive measure, no effective vaccination against many economically relevant diseases exist yet, such as for viral and parasitic infections. The objective of the present review is to provide the reader with an updated perspective on the most significant and innovative vaccine research on three key aquaculture commodities. European sea bass (Dicentrarchus labrax), Nile tilapia (Oreochromis niloticus), and Atlantic salmon (Salmo salar) were chosen because of their economic relevance, geographical distinctiveness, and representativeness of different culture systems. Scientific papers about vaccines against bacterial, viral, and parasitic diseases will be objectively presented; their results critically discussed and compared; and suggestions for future directions given.

An effective live attenuated vaccine against Aeromonas veronii infection in the loach (Misgurnus anguillicaudatus)

Aeromonas veronii is a major pathogenic bacterium in humans and animals. When it causes outbreaks, there are enormous economic losses to the aquaculture industry. An effective live attenuated vaccine strain, ΔhisJ, was obtained in our previous studies by gene knockout in Aeromonas veronii TH0426 using the suicide vector pRE112. Here, we evaluated whether the live attenuated vaccine ΔhisJ was suitable for prevention of Aeromonas veronii infection by injection and immersion in loaches. Compared with that of the TH0426 wild-type strain, the virulence of the live vaccine was significantly weakened. Vaccine safety assessment results also indicated that 1 × 10⁷ CFU/mL live vaccine was safe and did not induce clinical symptoms or obvious pathological changes. Additionally, after challenging loaches with Aeromonas veronii TH0426, the relative percent survival of the IN3 injection group was 65.66%, and that of the IM group was 50.78%. Our data show that the live attenuated vaccine ΔhisJ can improve the immune protection rate of loaches. Furthermore, increased enzyme activity parameters (SOD, LZM, ACP, and AKP) in the skin mucus, increased enzyme activity parameters (SOD, LZM, ACP, AKP, and GPx) in the serum, increased specific IgM antibodies and cytokine IL-1β contents in the serum, and increased cytokine (IL-15, pIgR, IL-1β, and TNF-α) expression in the liver and spleen were observed. These data are the first to indicate that the live attenuated vaccine ΔhisJ is suitable for the development of a safe and effective vaccine against Aeromonas veronii infection in loach aquaculture.

Development of fish vaccine in Southeast Asia: A challenge for the sustainability of SE Asia aquaculture

Southeast (SE) Asia plays an important role in global food security as this region has been regarded as one of the major producers of aquaculture product and, to date, freshwater fish accounted for one-third of the total aquaculture in SE Asia. The intensification of freshwater farming corresponding to increase of consumer demands has inevitably led to the emergence and re-emergence of diseases causing tremendous economic loss in the region. Nile tilapia (Oreochromis niloticus) and striped catfish (Pangasianodon hypophthalmus), the major freshwater fish species of SE Asia, have been reported susceptible to several bacterial pathogens, e.g. Streptococcus agalactiae, Edwardsiella ictalurid and Flavobacterium columnare. Since only a limited number of vaccines being registered and marketed, these pathogenic organisms still represent a severe threat to aquaculture industry in SE Asia. However, there is profound advancement in the understanding of disease epidemiology, pathogenic mechanisms, teleost mucosal immunity and vaccine delivery system over the last few years. This review aimed to summarize those recent findings which hopefully can provide novel insight into the future development of suitable vaccine and vaccination regime against bacterial infection in SE Asia region.

Genomic Prediction of Columnaris Disease Resistance in Catfish

Catfish is an important aquaculture species in the USA. Columnaris disease is distributed worldwide, affecting a wide variety of fish species including catfish . It leads to huge economic losses each year to the US catfish industry. Channel catfish in general is highly resistant to the disease, while blue catfish is highly susceptible. Genomic selection is an effective and accurate way to predict the breeding values and thus was expected to improve the prediction veracity of columnaris disease resistance in catfish effectively. In this study, two different methods, elastic net genomic best linear unbiased prediction (ENGBLUP) and genomic best linear unbiased prediction (GBLUP), were used to predict the columnaris disease resistance evaluated by binary survival status. Cross-validation showed that the prediction accuracy of ENGBLUP and GBLUP was 0.7347 and 0.4868, respectively, showing that ENGBLUP had a high prediction accuracy. It was shown that fitting QTL and polygenic effect with different distribution will improve genomic prediction accuracy for binary traits. In this study, an accurate and effective genomic selection method was proposed to predict the columnaris resistance in catfish, and its application should be beneficial to catfish breeding.

A Review of Fish Vaccine Development Strategies: Conventional Methods and Modern Biotechnological Approaches

Fish immunization has been carried out for over 50 years and is generally accepted as an effective method for preventing a wide range of bacterial and viral diseases. Vaccination efforts contribute to environmental, social, and economic sustainability in global aquaculture. Most licensed fish vaccines have traditionally been inactivated microorganisms that were formulated with adjuvants and delivered through immersion or injection routes. Live vaccines are more efficacious, as they mimic natural pathogen infection and generate a strong antibody response, thus having a greater potential to be administered via oral or immersion routes. Modern vaccine technology has targeted specific pathogen components, and vaccines developed using such approaches may include subunit, or recombinant, DNA/RNA particle vaccines. These advanced technologies have been developed globally and appear to induce greater levels of immunity than traditional fish vaccines. Advanced technologies have shown great promise for the future of aquaculture vaccines and will provide health benefits and enhanced economic potential for producers. This review describes the use of conventional aquaculture vaccines and provides an overview of current molecular approaches and strategies that are promising for new aquaculture vaccine development.

Draft Genome Sequences of Flavobacterium columnare Strains ARS1 and BGFS27, Isolated from Channel Catfish (Ictalurus punctatus)

Flavobacterium columnare strain BGFS27 was isolated from an apparently healthy wild channel catfish (Ictalurus punctatus) collected from the Mobile River in 2005. F. columnare strain ARS1 was isolated from a channel catfish suffering from columnaris disease in a commercial farm in 1996. BGFS27 belongs to genomovar II (genetic group 2), while ARS1 belongs to genomovar III (genetic group 3). Here, we report the draft genome sequences of F. columnare BGFS27 and ARS1, obtained by PacBio sequencing.

IgM and IgD heavy chains of yellow catfish (Pelteobagrus fulvidraco): Molecular cloning, characterization and expression analysis in response to bacterial infection

Three different immunoglobulin (Ig) isotypes, namely IgM, IgD, and IgT/IgZ have been described in most teleost, among which IgM and IgT are considered crucial in systematic and mucosal immunity, respectively. However, some teleost have no IgT/IgZ and it is unclear how other Ig isotypes interact to perform immune-protective roles in both systematic and mucosal sites. In this study, the complete cDNA sequences of IgM and IgD heavy chains were cloned and analyzed from yellow catfish (Pelteobagrus fulvidraco). The full-length cDNA of Pf-IgM and Pf-IgD heavy chains contained an open reading frame (ORF) of 1710 and 2991 bp encoding a predicted protein of 570 and 997 amino acids, respectively. Tissue-specific expression analysis indicated that both IgM and IgD were highly expressed in kidney and spleen, and higher expression levels were found at zygote and 13th day post hatching during early development. Multiple sequence alignment and phylogenetic analysis showed IgM and IgD of yellow catfish are closely related to other fish of Siluriformes. Moreover, we also constructed the infection model of yellow catfish with bacteria (Flavobacterium columnare G₄) for the first time to study the function of Pf-IgM and Pf-IgD heavy chain genes in immune response. Quantitative real-time PCR (qRT-PCR) showed that significantly up-regulated expression of Pf-IgM was not only detected in liver and spleen, but also in mucosal tissues including skin and intestine, while Pf-IgD was just significantly increased in liver and spleen, which might suggest the main immune-protecting roles of IgM in mucosal tissues of yellow catfish.

A Review of Molecular Responses of Catfish to Bacterial Diseases and Abiotic Stresses

Catfish is one of the major aquaculture species in the United States. However, the catfish industry is threatened by several bacterial diseases such as enteric septicemia of catfish (ESC), columnaris disease and Aeromonas disease, as well as by abiotic stresses such as high temperature and low oxygen. Research has been conducted for several decades to understand the host responses to these diseases and abiotic stresses. With the development of sequencing technologies, and the application of genome-wide association studies in aquaculture species, significant progress has been made. This review article summarizes recent progress in understanding the molecular responses of catfish after bacterial infection and stress challenges, and in understanding of genomic and genetic basis for disease resistance and stress tolerance.

Immunogenicity study of an expressed outer membrane protein U of Vibrio vulnificus in Japanese eel (Anguilla japonica)

AIMS

Vibrio vulnificus is a common bacterial pathogen causing haemorrhagic septicaemia in eel farming. This study investigates the immunogenicity of an outer membrane protein U (OmpU) of V. vulnificus and the feasibility of the protein as a new subunit vaccine against V. vulnificus.

METHODS AND RESULTS

Partial gene sequence of the OmpU of V. vulnificus was cloned, and then the OmpU was expressed and purified. Three groups of Japanese eels (Anguilla japonica) were intraperitoneally (i.p) injected with bovine serum albumin (BSA group), formalin-killed whole cell of V. vulnificus (FKC group) or the expressed OmpU of V. vulnificus (OMP group). On 14, 21, 28 and 42 days postimmunization (dpi), the whole blood cells were collected to evaluate the stimulation index (SI) and bactericidal activity. The serum was obtained to assess the titres of specific antibody, lysozyme activity, complement activity and bactericidal activity. The lysozyme activities in the suspension of kidney, skin mucus and liver in eels were also ascertained. The results showed that the SI and the titres of anti-V. vulnificus antibody in the OMP group was significantly increased on 28 dpi; lysozyme activity in the kidney and skin mucus of OMP group on 42 and 14 dpi were both significantly higher than BSA group; eels in OMP group showed strong bactericidal capacity on 21 and 28 days; and the relative percent survival of OMP vs BSA group after challenged by V. vulnificus on 28 dpi was 80%.

CONCLUSIONS

These results showed that the expressed OmpU of V. vulnificus could significantly improve the immune function of Japanese eel and the resistance of eels to the infection of V. vulnificus.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study offered an alternative preliminary strategy of making aquaculture vaccines against V. vulnificus for eel farming.

More than just antibodies: Protective mechanisms of a mucosal vaccine against fish pathogen Flavobacterium columnare

A recently developed attenuated vaccine for Flavobacterium columnare has been demonstrated to provide superior protection for channel catfish, Ictalurus punctatus, against genetically diverse columnaris isolates. We were interested in examining the mechanisms of this protection by comparing transcriptional responses to F. columnare challenge in vaccinated and unvaccinated juvenile catfish. Accordingly, 58 day old fingerling catfish (28 days post-vaccination or unvaccinated control) were challenged with a highly virulent F. columnare isolate (BGSF-27) and gill tissues collected pre-challenge (0 h), and 1 h and 2 h post infection, time points previously demonstrated to be critical in early host-pathogen interactions. Following RNA-sequencing and transcriptome assembly, differential expression (DE) analysis within and between treatments revealed several patterns and pathways potentially underlying improved survival of vaccinated fish. Most striking was a pattern of dramatically higher basal expression of an array of neuropeptides (e.g. somatostatin), hormones, complement factors, and proteases at 0 h in vaccinated fish. Previous studies indicate these are likely the preformed mediators of neuroendocrine cells and/or eosinophilic granular (mast-like) cells within the fish gill. Following challenge, these elements fell to almost undetectable levels (>100-fold downregulated) by 1 h in vaccinated fish, suggesting their rapid release and/or cessation of synthesis following degranulation. Concomitantly, levels of pro-inflammatory cytokines (IL-1b, IL-8, IL-17) were induced in unvaccinated fish. In contrast, in vaccinated catfish, we observed widespread induction of genes needed for collagen deposition and tissue remodeling. Taken together, our results indicate an important component of vaccine protection in fish mucosal tissues may be the sensitization, proliferation and arming of resident secretory cells in the period between primary and secondary challenge.

The effect of temperature on the mucosal IgM antibody response to DNP-KLH in channel catfish (Ictalurus punctatus)

Bath immersion remains a practical route for immunizing against disease in channel catfish; however research efforts in this area have revealed variable results when activating mucosal Ab responses with different antigens. This is likely due to a number of factors including the individual species, age of the fish, preparation of the immunogens, and differences in the overall dosage and the duration of exposure to vaccines. The current study sought to evaluate the effect of water temperature on the in vivo mucosal adaptive immune response in channel catfish to a protein-hapten antigen, DNP-KLH. Fish were bath immersed at different water temperatures and periodically evaluated over an eighteen week period for the development of serum and mucosal IgM antibodies to DNP-KLH using an indirect enzyme-linked immunosorbent assay. None of the temperature groups produced a serum antibody response; however there were detectable DNP-KLH specific IgM antibodies in the mucus starting at week eight. The extent of the mucosal antibody response and duration differed between the treatments. Our results show that there are intrinsic differences in the capacity to generate in vivo mucosal Ab responses in the skin at different water temperatures and the implications of these findings to channel catfish farming will be discussed.

Comparative Genomics and Transcriptional Analysis of Flavobacterium columnare Strain ATCC 49512

Flavobacterium columnare is a Gram-negative fish pathogen causing columnaris disease in wild and cultured fish species. Although the pathogen is widespread in aquatic environments and fish worldwide, little is known about biology of F. columnare and mechanisms of columnaris disease pathogenesis. Previously we presented the complete genome sequence of F. columnare strain ATCC 49512. Here we present a comparison of the strain ATCC 49512 genome to four other Flavobacterium genomes. In this analysis, we identified predicted proteins whose functions indicate F. columnare is capable of denitrification, which would enable anaerobic growth in aquatic pond sediments. Anaerobic growth of F. columnare ATCC 49512 with nitrate supplementation was detected experimentally. F. columnare ATCC 49512 had a relatively high number of insertion sequences and genomic islands compared to the other Flavobacterium species, suggesting a larger degree of horizontal gene exchange and genome plasticity. A type VI subtype III secretion system was encoded in F. columnare along with F. johnsoniae and F. branchiophilum. RNA sequencing proved to be a valuable technique to improve annotation quality; 41 novel protein coding regions were identified, 16 of which had a non-traditional start site (TTG, GTG, and CTT). Candidate small noncoding RNAs were also identified. Our results improve our understanding of F. columnare ATCC 49512 biology, and our results support the use of RNA sequencing to improve annotation of bacterial genomes, particularly for type strains.

Protective efficacy of Nigella sativa seeds and oil against columnaris disease in fishes

Columnaris disease, caused by the bacterium Flavobacterium columnare, is currently the most frequently reported bacterial disease affecting farm-raised channel catfish in the USA. Common treatments against the disease include the use of medicated feed that has led to emergent antibiotic resistant strains of F. columnare. Nigella sativa (Black cumin) is a medicinal herb commonly used by many cultures as a natural remedy for numerous disorders. Recently, we have discovered the antibacterial activity of N. sativa and its oil extract against F. columnare. In this study, we showed N. sativa oil (NSO) strongly inhibited the growth of all of the strains of F. columnare tested and yielded significantly larger zones of inhibition than those produced by oxytetracyclin. We tested the protective effect against columnaris disease in vivo by incorporating NSO (5%) or N. sativa seeds (NSS) (5%) into fish feeds. Fishes (Ictalurus punctatus and Danio rerio) fed amended diets displayed significantly lower mortality than those fed control diets. Per cent mortalities in control groups ranged from 77% to 44% and from 70% to 18% in zebrafish and channel catfish, respectively. A dose study using different NSS concentrations showed that 5% NSS offered the most protection against columnaris disease in channel catfish.

Missing the target: DNAk is a dominant epitope in the humoral immune response of channel catfish (Ictalurus punctatus) to Flavobacterium columnare

Vaccination remains a viable alternative for bacterial disease protection in fish; however additional work is required to understand the mechanisms of adaptive immunity in the channel catfish. To assess the humoral immune response to Flavobacterium columnare; a group of channel catfish were first immunized with F. columnare LV-359-01 cultured in iron-depleted media, before being challenged with wild type F. columnare LV-359-01. The immunization protocol did not confer increased protection against F. columnare; however both control and immunized responders generated serum and skin IgM antibodies against F. columnare proteins. Western blot analyses of individuals from both groups showed that IgM antibodies were generated to the same 70 kDa extracellular protein, which was identified to be the bacterial chaperonin protein DNAk. Antibodies generated were cross reactive to DNAk proteins found in other gram negative bacteria. Our data suggests that DNAk is the dominant epitope in the channel catfish B-cell response to F. columnare.

Rainbow trout (Oncorhynchus mykiss) resistance to columnaris disease is heritable and favorably correlated with bacterial cold water disease resistance

Columnaris disease (CD), caused by Flavobacterium columnare, is an emerging disease affecting rainbow trout aquaculture. Objectives of this study were to 1) estimate heritability of CD resistance in a rainbow trout line (ARS-Fp-R) previously selected 4 generations for improved bacterial cold water disease (BCWD) resistance; 2) estimate genetic correlations among CD resistance, BCWD resistance, and growth to market BW; and 3) compare CD resistance among the ARS-Fp-R, ARS-Fp-S (selected 1 generation for increased BCWD susceptibility), and ARS-Fp-C (selection control) lines. Heritability of CD resistance was estimated using data from a waterborne challenge of 44 full-sib ARS-Fp-R families produced using a paternal half-sib mating design, and genetic correlations were estimated using these data and 5 generations of BCWD resistance, 9-mo BW (approximately 0.5 kg), and 12-mo BW (approximately 1.0 kg) data from 405 ARS-Fp-R full-sib families. The CD and BCWD challenges were initiated at approximately 52 and 84 d posthatch, or approximately 650 and 1,050 degree days (°C × d), respectively. Survival of ARS-Fp-R families ranged from 0 to 48% following CD challenge and heritability estimates were similar between CD (0.17 ± 0.09) and BCWD (0.18 ± 0.03) resistance, and the genetic correlation between these 2 traits was favorable (0.35 ± 0.25). Genetic correlations were small and antagonistic (-0.15 ± 0.08 to -0.19 ± 0.24) between the 2 resistance traits and 9- and 12-mo BW. Two challenges were conducted in consecutive years to compare CD resistance among ARS-Fp-R, ARS-Fp-C, and ARS-Fp-S families. In the first challenge, ARS-Fp-R families (83% survival) had greater CD resistance than ARS-Fp-C (73.5%; P = 0.02) and ARS-Fp-S (68%; P < 0.001) families, which did not differ (P = 0.16). In the second challenge, using an approximately 2.5-fold greater challenge dose, ARS-Fp-R families exhibited greater CD resistance (56% survival) than ARS-Fp-S (38% survival; P = 0.02) families. The favorable genetic correlation between CD and BCWD resistance is supported by greater CD resistance of the ARS-Fp-R line compared to the ARS-Fp-C and ARS-Fp-S lines and suggests that both traits will be improved simultaneously when selection is practiced on only 1 trait. In summary, these data indicate the feasibility of further selective breeding of the BCWD-resistant ARS-Fp-R line for increased CD resistance to produce a double pathogen-resistant line of rainbow trout.

Potassium permanganate elicits a shift of the external fish microbiome and increases host susceptibility to columnaris disease

The external microbiome of fish is thought to benefit the host by hindering the invasion of opportunistic pathogens and/or stimulating the immune system. Disruption of those microbial communities could increase susceptibility to diseases. Traditional aquaculture practices include the use of potent surface-acting disinfectants such as potassium permanganate (PP, KMnO₄) to treat external infections. This study evaluated the effect of PP on the external microbiome of channel catfish and investigated if dysbiosis leads to an increase in disease susceptibility. Columnaris disease, caused by Flavobacterium columnare, was used as disease model. Four treatments were compared in the study: (I) negative control (not treated with PP nor challenged with F. columnare), (II) treated but not challenged, (III) not treated but challenged, and (IV) treated and challenged. Ribosomal intergenic spacer analysis (RISA) and pyrosequencing were used to analyze changes in the external microbiome during the experiment. Exposure to PP significantly disturbed the external microbiomes and increased catfish mortality following the experimental challenge. Analysis of similarities of RISA profiles showed statistically significant changes in the skin and gill microbiomes based on treatment and sampling time. Characterization of the microbiomes using 16S rRNA gene pyrosequencing confirmed the disruption of the skin microbiome by PP at different phylogenetic levels. Loss of diversity occurred during the study, even in the control group, but was more noticeable in fish subjected to PP than in those challenged with F. columnare. Fish treated with PP and challenged with the pathogen exhibited the least diverse microbiome at the end of the study.

Lack of association between Flavobacterium columnare genomovar and virulence in hybrid tilapia Oreochromis niloticus (L.)×Oreochromis aureus (Steindachner)

Columnaris disease can be problematic in tilapia (Oreochromis spp.) production. An understanding of the pathogenesis and virulence of Flavobacterium columnare is needed to develop prevention strategies. The objective of this study was to determine the virulence of genetically defined isolates of F. columnare in sex-reversed hybrid tilapia, Oreochromis niloticus (L.)×O. aureus (Steindachner). A series of immersion challenge trials were performed using isolates of the five established genomovars of F. columnare: I, II, II-B, III and I/II. The mean per cent mortality of fish challenged with genomovar I, II and III isolates ranged from 0 to 100, 3.3-78 and 3.3-75%, respectively. The mean per cent mortality of fish challenged with genomovar II-B ranged from 35 to 96.7%, and the only genomovar I/II isolate tested caused no mortality. Contrary to previous work in other fish species, there did not appear to be an association between F. columnare genomovar and virulence in tilapia. The challenge model used resulted in acute mortality. An alternative challenge model was tested by cohabitating healthy fish with dead fish infected with F. columnare. This method resulted in rapid appearance of clinical signs and mortality, suggesting the potential for F. columnare to increase in virulence upon growth on/in a fish host.

Effect of dietary supplementation with Echinacea purpurea on vaccine efficacy against infection with Flavobacterium columnare in zebrafish (Danio rerio)

The effect of dietary Echinacea purpurea (EP) on the response of zebrafish (Danio rerio) to a Flavobacterium columnare vaccine was investigated. Two hundred D. rerio with an average weight of 290 ± 40 g were selected and fed different levels of E. purpurea (5 g kg-1 diet - group 1, 10 g kg-1 diet - group 2, 20 g kg-1 diet - group 3, 30 g kg-1 diet - group 4, and 0 g kg-1 diet - group 5). Experimental feeding was begun 3 weeks prior to bath immunization and continued until the end of the experiment. Twenty-eight days after immunization the fish were challenged by bath immersion with F. columnare at a concentration of 1x106 CFU/ml. The relative percent survival of the experimental groups (1, 2, 3, 4 and 5) was 5.0, 6.0, 30.0, 36.0 and 5.0, respectively. In conclusion, diet supplementation with E. purpurea may effectively enhance the response of zebrafish to a F. columnare vaccine.