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A mucoadhesive chitosan polymer-based nanoplatform has been increasingly recognized as an effective mucosal vaccine delivery system for fish. The present study aimed to investigate the effectiveness of immersion vaccination with a chitosan polymer-based nanovaccine to elicit an immune response in serum and mucus of red tilapia and evaluate its protective efficacy after immersion challenge with a heterogenous strain of Aeromonas veronii UDRT09. Six hundred red tilapia (22 ± 1.8 g) were randomly allocated into four experimental groups: control, empty-polymeric nanoparticle (PC), formalin-killed vaccine (FKV), and chitosan polymer-based nanovaccine (CS-NV) in triplicate. The specific IgM antibody levels and their bactericidal activity were assessed in serum and mucus for 28 days after immersion vaccination and followed by immersion challenge with A. veronii. The immersion vaccine was found to be safe for red tilapia, with no mortalities occurring during the vaccination procedure. The specific IgM antibody levels and bactericidal activity against A. veronii in both serum and mucus were significantly higher in red tilapia vaccinated with CS-NV compared to the FKV and control groups at all time points. Furthermore, the serum lysozyme activity, ACH 50 , and total Ig levels demonstrated a significant elevation in the groups vaccinated with CS-NV compared to the FKV and control groups. Importantly, the Relative Percentage Survival (RPS) value of the CS-NV group (71 %) was significantly higher than that of the FKV (15.12 %) and PC (2.33 %) groups, respectively. This indicates that the chitosan polymer-based nanovaccine platform is an effective delivery system for the immersion vaccination of tilapia., (Copyright © 2024. Published by Elsevier Ltd.)

Streptococcus agalactiae is one of Thailand's most important pathogens in tilapia aquaculture. Vaccination is a very effective method for protecting fish against disease in aquaculture. Oral vaccination is an interesting route for vaccine delivery as it mimics the pathogenesis of S. agalactiae and provides convenient administration for mass vaccination of fish. Moreover, gut mucosal immunity is associated with a mucus layer on the gastrointestinal tract. Therefore, this study aimed to develop a novel cationic-based nanoemulsion vaccine containing bile salts (NEB) coated by chitosan (CS) and determined its physicochemical characterization, morphology, in vitro mucoadhesive property, permeability, and acid-base tolerance. In addition, the efficacy of NEB-CS as an oral vaccination for Nile tilapia was evaluated in order to investigate the innate immune response and protection against S. agalactiae. The groups of fish consisted of: (1) deionized water as a non-vaccinated control (Control); (2) an inactivated vaccine formulated from formalin-killed bacteria (IB); and (3) a novel cationic-based nanoemulsion vaccine containing bile salts (NEB) coated by chitosan (CS). The control, IB, and NEB-CS were incorporated into commercial feed pellets and fed to Nile tilapia. In addition, we evaluated the serum bactericidal activity (SBA) for 14 days post-vaccination (dpv) and protective efficacy for 10 days post-challenge, respectively. The mucoadhesiveness, permeability, and absorption within the tilapia intestine were also assessed in vivo. The NEB-CS vaccine appeared spherical, with the nanoparticles having a size of 454.37 nm and a positive charge (+47.6 mV). The NEB-CS vaccine had higher levels of mucoadhesiveness and permeability than the NEB (p < 0.05). The relative percent survival (RPS) of IB and NEB-CS, when administered orally to fish, was 48% and 96%, respectively. Enhanced SBA was noted in the NEB-CS and IB vaccine groups compared to the control group. The results demonstrate that a feed-based NEB-CS can improve the mucoadhesiveness, permeability, and protective efficacy of the vaccine, and appear to be a promising approach to protecting tilapia in aquaculture against streptococcosis., Competing Interests: Declaration of competing interest The authors declare that there are no conflicts of interest., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Tilapia lake virus (TiLV), an enveloped negative-sense single-stranded RNA virus, causes tilapia lake virus disease (TiLVD), which is associated with mass mortality and severe economic impacts in wild and farmed tilapia industries worldwide. In this study, we developed a chitosan nanoparticle TiLV immersion vaccine and assessed the efficacy of the vaccine in laboratory and field trials. Transmission electron microscopy showed that the inactivated vaccine had a particle size of 210.3 nm, while the nano inactivated vaccine had a spherical shape with a diameter of 120.4 nm. Further analysis using fluorescent staining and immunohistochemistry analysis revealed the mucoadhesive properties of the nanovaccine (CN-KV) through fish gills. We assessed the efficacy of an immersion-based TiLV nanovaccine using a cohabitation challenge model. The fish that received the nanovaccine showed better relative percent survival (RPS) at 68.17% compared with the RPS of the inactivated virus vaccine (KV) group at 25.01%. The CN-KV group also showed a higher TiLV-specific antibody response than the control and KV groups (p < 0.05). Importantly, under field conditions, the fish receiving the CN-KV nanovaccine had better RPS at 52.2% than the nonvaccinated control group. Taken together, the CN-KV nanovaccinated fish showed better survival and antibody response than the control and KV groups both under laboratory control challenge conditions and field trials. The newly developed immersion-based nanovaccine is easy to administer in small fish, is less labor-intensive, and allows for mass vaccination to protect fish from TiLV infection., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Red tilapia (Oreochromis sp.), one of the important freshwater fish species in fish farming in Thailand, has for long been suffering from a serious bacterial disease named epizootic ulcerative syndrome and hemorrhagic septicemia. The disease is mainly caused by Aeromonas veronii. Vaccine is proposed to be a major impact tool for sustainable control and prevention strategies. Vaccination by immersion has many benefits over injection. However, the conventional immersion method suffers from a low potency due to the inefficient uptake of antigens across mucosal tissue. Here, we developed a chitosan-polymer based nanovaccine together with an efficient delivery vehicle to enhance the immunogenicity of immersion vaccination, increasing bioavailability and inducing local immune responses during transit to mucosal inductive immune sites. The physiochemical properties of nanovaccine, which was modified on surface particle by using a mucoadhesive polymer, were assessed for size, zeta potential, and particle distribution. Our study demonstrated by SEM image and microscopic fluorescence image that nanovaccine greatly increased the binding and penetrating ability into gills when compared with formalin killed vaccine. The nano-sized particles were well dispersed in water and trapped in core nanoparticle as confirmed by TEM image. The efficacy of vaccine was performed by immersion challenge with virulent A.veronii after 30 days post vaccination in tilapia. The result revealed a high level of mortality in the control, empty-polymeric nanovaccine and formalin killed bacterin vaccine groups. A high relative percentage survival (RPS) of vaccinated fish was noted with chitosan-polymer based nanovaccine. Our studies indicated that this chitosan-polymer based nanovaccine derived from cell fragments and supernatant was the improved version of the conventional formalin killed vaccine. The chitosan polymer based particle could increase the efficacy of nanovaccine toward the target mucosal membrane and enhance protection against A. veronii infection in red tilapia., Competing Interests: Declaration of competing interest The authors declare that there are no conflicts of interest., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Francisella noatunensis subsp. orientalis (Fno) is one of the infectious diseases that causes economic losses associated with tilapia mortality. Even though direct immersion administration of vaccines is more practicable for small fish and fry compared with oral and injection vaccination in the fields, the efficacy is still insufficient due to lower potency of antigen uptake. Herein, we accomplished the development of a mucoadhesive nanovaccine platform using cetyltrimethylammonium bromide (CTAB), a cationic surfactant, to improve the efficiency of immersion vaccination against Fno in tilapia. Cationic Fno nanovaccine (CAT-Fno-NV) was prepared though emulsification using an ultrasonic method. In our investigation, the CAT-Fno-NV increased the opportunity of Fno vaccine uptake by extending the contact time between vaccine and mucosal surface of fish gills and enhancing the protective efficacy against Fno infection. Fish were vaccinated with the CAT-Fno-NV by a direct immersion protocol. The challenge trial by Fno injection revealed that CAT-Fno-NV at the concentration 1:100 ratio (approximately 1 × 10 6  cfu/mL) had the highest efficacy to protect fish from Fno infection at day 30 after post challenge period according to the total number of Fno detected in head kidney, spleen and liver. A significant upregulation of IgM gene was observed in gills, skin, head kidney, serum and peripheral blood lymphocytes (PBLs) and spleen tissues treated with WC and CAT-Fno-NV (1:100) vaccines, while IgT gene was highly expressed in only gills and skin tissues for treated WC and CAT-Fno-NV (1:100) groups. We anticipate that the cationic surfactant-based nanovaccine developed in this study could become an efficient alternative for direct immersion vaccination to induce humoral immune responses against Fno in vaccinated tilapia., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Red tilapia (Oreochromis sp.) has become one of the most important fish in aquaculture. Bacterial infection caused by Flavobacterium columnare, the causative agent of columnaris disease, has been now identified as one of the most serious infectious diseases in farmed red tilapia and cause major financial damage to the producers. Among the effective prevention and control strategies, vaccination is one of the most effective approach. As the surface of living fish is covered by mucus and directly associated with the mucosal immunity, we therefore hypothesized that better adsorption on mucosal surfaces and more efficient vaccine efficacy could be enhanced biomimetic nanoparticles mimicking the mucoadhesive characteristic of live F. columnare. In this work, we describe an effective approach to targeted antigen delivery by coating the surface of nanoparticles with mucoadhesive chitosan biopolymer to provide "pathogen-like" properties that ensure nanoparticles binding on fish mucosal membrane. The physiochemical properties of nanovaccines were analyzed, and their mucoadhesive characteristics and immune response against pathogens were also evaluated. The prepared vaccines were nano-sized and spherical as confirmed by scanning electron microscope (SEM). The analysis of hydrodynamic diameter and zeta-potential also suggested the successful modification of nanovaccines by chitosan as indicated by positively charged and the overall increased diameter of chitosan-modified nanovaccines. In vivo mucoadhesive study demonstrated the excellent affinity of the chitosan-modified nanovaccines toward fish gills as confirmed by bioluminescence imaging, fluorescent microscopy, and spectrophotometric quantitative measurement. Following vaccination with the prepared nanovaccines by immersion 30 min, the challenge test was then carried out 30 and 60 days post-vaccination and resulted in high mortalities in the control. The relative percent survival (RPS) of vaccinated fish was greater than 60% for mucoadhesive nanovaccine. Our results also suggested that whole-cell vaccines failed to protect fish from columnaris infection, which is consistent with the mucoadhesive assays showing that whole-cell bacteria were unable to bind to mucosal surfaces. In conclusion, we could use this system to deliver antigen preparation to the mucosal membrane of tilapia and obtained a significant increase in survival compared to controls, suggesting that targeting mucoadhesive nanovaccines to the mucosal surface could be exploited as an effective method for immersion vaccination., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Vaccination is the most effective approach for prevention of infectious diseases in aquaculture. Although immersion vaccination is more applicable compared to in-feed/oral administration and injection, this method suffers from low potency as the efficiency of uptake of antigens through mucosal membranes is limited. In this study, we have successfully developed a mucoadhesive vaccine delivery system to enhance the efficacy of direct immersion vaccination against Flavobacterium columnare, the causative agent of columnaris disease in red tilapia. A formalin-killed negatively charged, bacterial cell suspension was used to prepare a mucoadhesive vaccine by electrostatic coating with positively charged chitosan. Our results demonstrate that the chitosan-complexed vaccine greatly increases its mucoadhesiveness, thus increasing the chances of vaccine uptake by the gill mucosa and improving the protection obtained against columnaris infection. The surface charge of the chitosan-complexed vaccine was altered from anionic to cationic after chitosan modification. Tilapia were vaccinated with the prepared chitosan-complexed vaccine by immersion. The challenge test was then carried out 30 and 60 days post vaccination, which resulted in a high level of mortalities in the non-vaccinated and uncomplexed vaccine groups. A high relative percentage survival (RPS) of vaccinated fish was noted with the mucoadhesive vaccine. Our results indicated that the naked vaccine failed to protect the fish from columnaris infection, which is consistent with the mucoadhesive assays performed during the study showing that the naked vaccine was unable to bind to mucosal surfaces. This system is therefore an effective method for immersion vaccination in order to deliver the antigen preparation to the mucosal surface membrane of the fish., (Copyright © 2018 Elsevier Ltd. All rights reserved.)