Alternative therapies recently applied in controlling farmed fish diseases: mechanisms, challenges, and prospects

Abstract

The aquaculture industry is suffering from significant financial setbacks due to an increasing frequency of disease outbreaks, posing a threat to the sector’s sustainability. Various bacterial, viral, parasitic, and fungal pathogens have led to massive mortalities in farmed fish worldwide. Throughout the years, the management of fish diseases has predominantly centered around the utilization of conventional antibiotics and chemicals. Nevertheless, their indiscriminate use has given rise to serious implications, including an increase in resistant pathogens, disruptions in the metabolic processes of fish, degradation of the aquatic environment, the presence of drug residues in aquatic products, and a potential threat to human health. Various effective bio-based and immunoprophylaxis alternative therapies have been developed to overcome these impediments. Recent alternative therapeutic approaches to fish diseases encompass a range of strategies, including phytotherapeutics, nanotherapeutics, probiotics, prebiotics, synbiotics, phage therapy, vaccination, quorum quenching, antimicrobial peptides, biosurfactants, bacteriocins, stem cells, and diagnostic-based therapy. Advancements in biotechnology have significantly enhanced the efficacy of these therapies. However, additional research is essential to refine the utilization of these therapeutic approaches. Critical concerns, such as efficacy, cost, risks, availability, and adverse effects on fish and the ecosystem, need to be addressed to establish guidelines for their sustainable application in aquaculture. This review will increase aquaculturists’ awareness of recent therapies used in fish farming, their mechanisms, challenges, and impacts while promoting the sustainability of commercial aquaculture.

Introduction

Aquaculture stands out as one of the fastest-growing food-producing sectors globally. Its significant contributions to food security, income generation, poverty alleviation, and cultural identity cannot be overstated (Elgendy et al. 2023a). The key to achieving higher fish production, in order to meet the growing demand for fish and address the stagnation of capture fisheries, lies in the intensification of fish farming practices (Araujo et al. 2022). The overstocking of fish farming operations is often accompanied by the adoption of unfavorable management measures, neglecting environmental care and compromising the health of farmed fish (Elgendy et al. 2022a, b, c). Fish farmed under such conditions experience stress, making them more susceptible to attacks from disease-causing agents (Ali et al. 2020; 2022). Currently, infectious disease outbreaks pose significant constraints on the productivity and sustainability of the aquaculture industry (Elgendy et al. 2023b). The intensification of aquaculture practices has resulted in the frequent occurrence of dangerous pathogens, leading to massive fish mortality disease epizootics (Elgendy et al. 2015a, b; Moustafa et al. 2015). Numerous bacterial, viral, parasitic, and fungal diseases break out frequently in intensive and semi-intensive fish culture systems, leading to partial or complete loss in production and colossal economic losses (Moustafa et al. 2014; Radwan et al. 2022a; Elgendy et al. 2023a, b). The potential economic impact of tilapia summer mortality in Egyptian farms was estimated at around 100 million USD (Fathi et al. 2017). Furthermore, in a comprehensive 7-year study conducted by Abdelrahman et al. (2023) in Alabama, USA, the annual financial losses attributed to diseases in commercial catfish farms in West Alabama amounted to a total of 11.1 million USD. Bacterial diseases were identified as the predominant cause of such epizootics, accounting for 83% of disease-related losses.

Aquaculturists have to use substantial quantities of chemotherapeutics, such as antibiotics, disinfectants, and pesticides, to tackle invading pathogens (Salma et al. 2022). The indiscriminate use of these chemotherapeutics has led to the emergence of resistant pathogens and detrimental effects on the ecosystem, posing a threat to human health (Elgendy et al. 2023a, b). The accumulation of antibiotic residues, whether in the environment or within fish tissues, and the emergence of resistant microbial strains serve as discouraging factors for their continued use (Bhat et al. 2022). Moreover, the horizontal transfer of genetic elements, which enables the transmission of drug-resistant genes to bacteria infecting humans, further intensifies antibiotics-associated risks (Jeon et al. 2023). Numerous antiparasitic and antifungal chemotherapeutics are extensively used in aquaculture, including drugs like formalin, formaldehyde, and malachite green. However, their irresponsible application is accompanied by a well-known issue of high toxicity to aquatic animals (UEMS 2016). These substances not only contribute to significant environmental problems but also pose substantial public health hazards (Sudova et al. 2007). The Food and Drug Administration (FDA) recently imposed a ban on formalin and malachite green in food fish production, citing concerns about their lack of consumer-friendliness and, in the case of malachite green, its carcinogenic, mutagenic, and teratogenic properties (Niska et al. 2009; Elgendy et al. 2023b). Therefore, developing effective alternative therapies to conventional chemotherapeutics for controlling pathogenic organisms affecting farmed fish is a global critical concern (Wang et al. 2022; Yilmaz et al. 2022; Bondad-Reantaso et al. 2023). This review, which focused on the alternative therapies recently applied in controlling/mitigating farmed fish diseases, including phytotherapeutics, nanotherapeutics, probiotics, prebiotics, synbiotics, phage therapy, vaccination, quorum quenching, antimicrobial peptides, biosurfactants, bacteriocins, stem cells, and diagnostic-based therapies, is discussed. The therapeutic and health benefits of these therapies against different pathogens affecting farmed fish are highlighted. Moreover, challenges confronting the application of these therapies in aquaculture and their impacts are briefly discussed.

Recent alternative therapies for controlling fish diseases in aquaculture

Numerous alternative disease management strategies have been developed to address the limitations of conventional chemotherapeutics in aquaculture (Fig. 1). These measures include the use of various bio-based chemotherapeutics and immunoprophylaxis approaches (Soliman et al. 2019; Wang et al. 2022). The bio-based alternative therapies include probiotics, bacteriophages, vaccination, quorum quenching, antimicrobial peptides, biosurfactants, and bacteriocins (Soliman et al. 2019; Yilmaz et al. 2022; Bondad-Reantaso et al. 2023). Furthermore, active immunoprophylaxis measures, such as using vaccines, and passive approaches, including phytotherapeutics, probiotics, and prebiotics, have been established to counteract attacking pathogens (Hussein et al. 2023; Subasinghe et al. 2023). Developing effective disease diagnostic technologies is essential as it enables accurate pathogen detection, helps prevent disease spread, and facilitates the implementation of targeted therapies (Abdelsalam et al. 2023; Dong et al. 2023).

Fig. 1

Recent alternative therapies for treating fish diseases

Phytotherapeutics(medicinal plants and their derivatives)

The application of phytotherapeutics to enhance aquatic animal health involves utilizing medicinal products derived from plants or their derivatives for preventive or therapeutic purposes (Elgendy et al. 2023a; Silva et al. 2024). The use of phytotherapeutics aligns with the growing demand for sustainable and environmentally friendly practices in aquaculture (Ali et al. 2021). Unlike some conventional treatments that may pose adverse impacts on fish, humans, and the ecosystem, plant-based therapeutics offer a natural and eco-friendly approach for controlling diseases affecting farmed fish (Elgendy et al. 2021a).

Medicinal plants and their derivatives possess a diverse range of biologically active components that exhibit growth-promoting, immunostimulatory, antioxidant, and antimicrobial properties (Bondad-Reantaso et al. 2023). The plant bioactive compounds include essential oils, alkaloids, steroids, phenolics, tannins, organosulfur, terpenoids, saponins, glycosides, and flavonoids (Citarasu 2010; Cabello-Gómez et al. 2022; Elgendy et al. 2023a). These compounds often exhibit a broad spectrum of activities (Fig. 2), targeting multiple pathogens simultaneously and providing a holistic approach to disease management (Ergena et al. 2023).

Fig. 2

Health benefits and antimicrobial mechanisms of phytotherapeutics

Immunostimulatory outcomes of phytotherapeutics

The administration of medicinal plants or their derivatives to farmed fish has been shown to enhance their immunological responses and disease resistance (Fig. 2). These improvements include increased phagocytic activity, elevated levels of immunoglobulins, enhanced respiratory burst activity, heightened nitrogen oxide production, elevated myeloperoxidase content, boosted complement activity, higher levels of total proteins, as well as enhanced antiprotease activity (Elgendy et al. 2016; 2021a; 2023b; Abbas et al. 2019). Medicinal plants and their derivatives can be employed in various forms, including crude preparations, extracts, or distinct active compounds derived from the plants (Elgendy et al. 2021a). Phytotherapeutics can be used either independently or in combination with other therapeutic approaches (Zhang et al. 2009). Many herbs, spices, and their extracts have proven to be effective against a variety of dangerous fish infections (bacterial, parasitic, viral, and fungal pathogens), as shown in Tables 1, 2, and 3 and Fig. 2.

Table 1 Phytotherapeutics exhibiting promising antibacterial properties in aquaculture

Table 2 Phytotherapeutics exhibiting promising antiparasitic properties in aquaculture

Table 3 Phytotherapeutics with antifungal and antiviral effects against fish pathogens

Antimicrobial efficacy of phytotherapeutics in aquaculture

The potential use of phytotherapeutics exhibiting antibacterial activity in aquaculture has gained considerable attention as an alternative to conventional antibiotics (Elgendy et al. 2021a). Phytotherapeutics, derived from various plant sources, possess diverse bioactive compounds that can efficiently combat bacterial infections in aquaculture (Ali et al. 2021). Several plant extracts and essential oils have demonstrated promising antibacterial properties, making them potential candidates for sustainable disease management in aquaculture, as seen in Fig. 2 and Table 1. Turmeric (Curcuma longa) (Elgendy et al. 2016), lemongrass (Cymbopogon citratus) (Al-Sagheer et al. 2018), moringa (Moringa oleifera) (Elgendy et al. 2021a), rosemary (Rosmarinus officinalis) (Naiel et al. 2019), garlic (Allium sativum) (Guo et al. 2012), and leek extract (Allium ampeloprasum L.) (Ali et al. 2021) are among the promising medicinal plants with significant antibacterial activities.

Moreover, there has been a growing trend in the use of medicinal plants for managing parasitic diseases affecting farmed fish worldwide (Radwan et al. 2022b). Numerous medicinal herbs and their extracts demonstrate strong antagonistic potentials against several detrimental fish parasitic infestations (Table 2). Biological active compounds in medicinal herbs, such as essential oils and diallyl tetrasulfide (allicin), have demonstrated a broad spectrum of antiparasitic activity in aquaculture (Yao et al. 2010; Pérez et al. 2016; Nizio et al. 2018; Adeshina et al. 2021). Bathing the fish in water containing phytotherapeutic agents is the primary method for treating parasitized fish (Radwan et al. 2022b). However, these chemotherapeutics can also be administered to fish through dietary supplementation (Aboud 2010).

Plant-based compounds also have demonstrated significant anti-fungal properties against numerous fish pathogens (Table 3). Essential oils extracted from plants such as Thymus vulgaris and Mentha spicata have shown inhibitory effects against a variety of fungal pathogens (Soković et al. 2009). Furthermore, both crude and extracted forms of Allium cepa dietary supplementation demonstrate antagonistic activity against the highly fatal Saprolegnia parasitica infections in tilapia (Elgendy et al. 2023a). The active components in these medicinal herbs cross the fungal cell membranes, blocking vital enzymes, destroying cell membrane, and suppressing their biochemical process, ultimately causing fungal cell death (Madhuri et al. 2012). Therefore, these compounds can be employed to prevent and mitigate fungal infections of farmed fish. In addition to their anti-fungal properties, certain phytotherapeutics exhibit anti-viral activity (Table 3), making them valuable therapies in controlling viral infections in aquaculture settings. Green tea (Camellia sinensis) extracts and its active compounds such as tea polyphenols, epicatechin gallate, and epigallocatechin displayed excellent antiviral activities against Iridovirus infection in grouper fish, Epinephelus spp. (Li et al. 2022b). Furthermore, a myriad of plant species, including neem (Azadirachta indica), Passiflora spp., Piper longum, and Zanthoxylum bungeanum harbor a diverse array of biologically active compounds, notably alkaloids, renowned for their robust antiviral outcomes (Pal 2023). These compounds hold significant promise in effectively combating viral infections affecting farmed fish in aquaculture operations, as demonstrated in Table 3.

Challenges of phytotherapeutic application in aquaculture

There are some shortcomings facing the widespread application of phytotherapy in aquaculture, as discussed by Barbosa et al. (2023), including the absence of standardized measures for herbal medications and their reliance on practical effectiveness rather than quantified ingredient content. Moreover, the concentrations of bioactive compounds in medicinal herbs are variable and influenced by factors like plant species and extraction methods. Additionally, identifying the specific active compounds responsible for therapeutic effects in plant extracts is a complex process. The bioavailability and absorption of phytochemicals in aquatic environments are influenced by water quality parameters. Lastly, the unavailability of certain plant extracts in adequate quantities also poses an obstacle.

Nanotherapeutics

The application of nanotechnology in aquaculture holds immense potential to bring transformative changes in this industry, including the detection and control of pathogens, water treatment, sterilization of fish ponds, and the efficient delivery of nutrients and drugs (Elgendy et al. 2021b; El-Adawy et al. 2023a, b). The distinctive features of nanotherapeutics, which encompass size-dependent reactivity and precisely targeted delivery mechanisms, make it a potent tool for tackling health issues in farmed fish (Doszpoly et al. 2023). The sizes of nanoparticles range between 1 and 100 nm and are sensitized by numerous physical, chemical, and biological methods (Doszpoly et al. 2023).

Biogenic synthesis of metal nanoparticles, known as green nanotechnology, has paved the way for discovering a broad spectrum of innovative therapies in aquaculture (Khursheed et al. 2023; Salem 2023a, b). This method of nanoparticle synthesis involves the use of various natural biological agents, ranging from simple unicellular organisms to complex multicellular ones, such as bacteria, fungi, actinomycetes, yeast, algae, and plant materials (Moitra et al. 2020; Salem and Fouda 2021; Salem 2023a, b). Extracellular and intracellular filtrates from various microbial cultures can serve as effective reducing agents for nanoparticle production (Li et al. 2011). The biomolecules released by organisms, including enzymes, proteins, polysaccharides, and carbohydrates, oxidize or reduce metallic ions to produce nanoparticles (Salem 2023a, b). Various phytocompounds, such as those from Azadirachta indica (neem) (Rather et al. 2017), Cinnamomum zeylanicum bark (Sathishkumar et al. 2009), and black tea leaf extract (Ara et al. 2009), have been involved in the biosynthesis of numerous nanoparticles (Table 4).

Table 4 Nanotherapeutics with antimicrobial properties against fish pathogens

Incorporating selenium nanoparticles (Se NPs) synthesized using the green microalga Pediastrum boryanum into the diets of Nile tilapia, Oreochromis niloticus, improved their immune defense and intestinal integrity compared to control basal diets containing an inorganic Se source (Zahran et al. 2024). Moreover, Gum Arabic-based silver nanoparticles (Ag NPs-GA) displayed significant antibacterial and antibiofilm activities in vitro against Aeromonas hydrophila and Pseudomonas aeruginosa, with minimum inhibitory concentrations of 1.625 µg/ml and 3.25 µg/ml, respectively. These findings make AgNPs-GA suitable for use in commercial antibacterial products for the aquaculture industry (El-Adawy et al. 2021). Biogenic synthesis of metal nanoparticles has garnered significant attention worldwide due to its eco-friendly nature, low cost, and straightforward synthesis process (Singh et al. 2018). Additionally, microbes can be genetically engineered to synthesize nanoparticles with greater control over shape and size (Mabey et al. 2019).

Polymeric nanoparticles also play a prominent role in recent therapeutic applications in aquaculture and are recognized as efficient drug delivery systems (Ahmed et al. 2019a, b; 2020). They are organic-based nanoparticles composed of natural polymers such as chitosan, alginate, and gelatin, as well as synthetic polymers like polyalkylcyanoacrylate (PACA) (Begines et al. 2020). Various methods are used to synthesize polymeric nanoparticles, including solvent evaporation, dialysis, nanoprecipitation, and salting out (Krishnamoorthy and Mahalingam 2015). Nevertheless, biodegradable biopolymer nanoparticles prepared using simple methods without organic solvents are preferred for biological applications (Agarwal et al. 2018). Their unique physical and chemical properties, including large surface areas, enable them to serve a broad spectrum of therapeutic applications (Du et al. 2009).

Polymeric nanoparticles, such as those made from chitosan, can encapsulate active ingredients within their core or adsorb them onto their surfaces (Mohammed et al. 2017; Ahmed et al. 2019a, b). These nanoparticles serve as efficient drug delivery systems by encapsulating drugs, protecting them from degradation, and releasing them in a controlled manner (Ahmed et al. 2019a, b). Polymeric nanoparticles, such as chitosan, are characterized by low toxicity, good biocompatibility, and biodegradation at specific sites, making them suitable for numerous valuable therapeutic applications (Agarwal et al. 2018; Ahmed et al. 2019a, b). Polymeric nanoparticles can be utilized in gene therapy, imaging, diagnostics, and vaccine development (Elsabahy et al. 2015).

Earlier studies have demonstrated the health benefits of chitosan nanoparticles (CSNPs) and their potential therapeutic applications in aquaculture, including immunomodulation against infections (Nikapitiya et al. 2018), as well as antibacterial and antifungal activities (Abdel-Razek 2019a, b). Ahmed et al. (2021) suggested that feeding rainbow trout (Oncorhynchus mykiss) with marine bio-sourced chitosan nanoparticles (CSNPs) enhanced their intestinal immunity and antimicrobial defense mechanisms against Yersinia ruckeri infection. Moreover, the authors observed that CSNPs seemed to be more effective in therapeutic applications rather than for short-term prophylactic use. Additionally, in vitro studies by Ahmed et al. (2020) suggested that CSNPs could be incorporated into disease management strategies in aquaculture. These studies demonstrated that CSNP suspensions (105 nm) possess antimicrobial properties against common bacterial and oomycete pathogens affecting fish. CSNPs demonstrated significant antibacterial efficacy against Pseudomonas fluorescens, Aeromonas hydrophila, and Yersinia ruckeri. Higher doses of CSNPs were effective against Pseudomonas aeruginosa, Pseudomonas putida, Edwardsiella tarda, and Aeromonas salmonicida. Furthermore, CSNPs were capable of reducing the viability of Edwardsiella ictaluri, Francisella noatunensis subsp. orientalis, Aeromonas caviae, Aeromonas veronii, Aphanomyces invadans, and Saprolegnia parasitica.

Several nanomaterials have garnered significant attention and shown potential applications in aquaculture including silver nanoparticles (Ag NPs), zinc oxide nanoparticles (ZnO NPs), copper-based nanoparticles (Cu NPs), gold nanoparticles (Au NPs), and titanium dioxide nanoparticles (TiO2 NPs), as seen in Table 4.

Health benefits and antimicrobial activity of nanotherapeutics against fish pathogens

Nanotherapeutics derived from various metals exhibit powerful antimicrobial properties, positioning them as compelling alternatives to conventional therapies in aquaculture (Doszpoly et al. 2023). These innovative therapies effectively target a wide range of detrimental fish pathogens, including bacteria, parasites, fungi, and viruses (Saleh et al. 2017; Elgendy et al. 2021b; Doszpoly et al. 2023). Nanotherapeutics can be applied in aquaculture through various pathways including injection, bath treatment, or dietary supplementation (Das et al. 2020; Elgendy et al. 2021b). The mechanisms underlying the antimicrobial activities of nanotherapeutics are demonstrated in Fig. 3. These mechanisms include damaging the microbial cell membrane/cell wall, disrupting protein transport, inhibiting vital enzymes, and increasing the generation of reactive oxygen species (Lee et al. 2019). The attachment of nanoparticles to the microbial cell membrane induces damaging changes in membrane potential, disrupts permeability, and leads to a reduction in ATP levels, ultimately culminating in cell death (Saleh et al. 2017). Nanoparticles also attach to nucleic acids, inhibit tRNA binding to the ribosome, and prevent cell division (Huang et al. 2012). Various nanoparticles have been reported to have bactericidal effects against a range of devastating fish bacterial infections, including Aeromonas spp. (Elgendy et al. 2021b), Vibrio spp. (Swain et al. 2014), Streptococcus spp. (Ghetas et al. 2022), Pseudomonas spp. (El-Adawy et al. 2021), Flavobacterium spp. (Shaalan et al. 2020), Edwardsiella spp. (Luis et al. 2020), Ph. damselae subsp. damselae (Cheng et al. 2011), and Yersinia spp. (Demirbas 2021), as seen in Table 4. Moreover, some serious fish parasitic infestations like Ichthyophthirius multifiliis (Saleh et al. 2017), Monogenean parasitic spp. (Malheiros et al. 2020), and Cichlidogyrus spp. (Pimentel-Acosta et al. 2019) have been successfully inhibited by nanotherapeutics, as shown in Table 4. Moreover, certain nanotherapeutics have exhibited inhibitory effects against some fungal fish diseases, like saprolegniasis (AL-Shammari et al. 2022), as well as specific viral diseases, like spring viraemia of carp virus and white spot syndrome virus (WSSV), as seen in Table 4.

Fig. 3

Antimicrobial mechanisms of nanotherapeutics

The aquatic animal health sector can also leverage the immunostimulatory effects of nanotherapeutics for the prophylaxis and prevention of diseases in farmed fish (Abd Elshafy et al. 2023). Several nanoparticles have shown immunostimulatory effects, enhancing the resistance of fish against detrimental microbial pathogens (Elgendy et al. 2021b). Earlier studies have shown that dietary supplementation of nano-selenium and selenium nanoparticles synthesized using microalgae can significantly enhance the immunological responses and resistance of fish against invading bacterial pathogens (Saffari et al. 2018; Neamat-Allah et al. 2019; Zahran et al. 2024).

Nano-based drugs and vaccines are part of the recent therapeutic approaches employed to address disease challenges in aquaculture. The small size of nanoparticles and their capability to traverse biological barriers have sparked interest in using them as drug and vaccine delivery vehicles (Fenaroli et al. 2014). Moreover, nanomaterials can function as vaccine adjuvants (Myhr and Myskja 2011). Incorporating nanomaterials, such as chitosan nanoparticles, into vaccine formulations enhances the stability of antigens, shields them from enzymatic degradation, preserves immunogenicity, and facilitates the sustained release of the vaccine over the long term (Wang et al. 2011; Zhao et al. 2014). Nano-based vaccines demonstrated promising results in countering certain harmful fish pathogens like Vibrio anguillarum (Vimal et al. 2014) and lymphocystis disease virus (LCDV) (Tian and Yu 2011). Moreover, the combination of nano-probiotics and prebiotics also is an effective approach to deliver these components efficiently to living organisms, enhancing the immune competence of farmed fish against attacking pathogens (Abd Elshafy et al. 2023). The integration of nanoparticles into aquatic nano-feed formulations, as well as the use of nano-derived natural products (green-synthesized nanoparticles), offers powerful health benefits in aquaculture (Ghetas et al. 2022). These nanoparticles enhance the immune responses of farmed fish, thereby serving as a disease-preventive strategy (Elgendy et al. 2021b).

Nanotechnology holds numerous significant applications in aquaculture, spanning from disease diagnosis to the mitigation of antibiotic pollution in the aquatic environment. Numerous nanoscale materials are employed in immune and molecular-based disease diagnostic approaches, with nano-based assays exhibiting superior specificity, sensitivity, and accuracy compared to traditional techniques in detecting fish pathogens (El-Adawy et al. 2023a, b). The distinctive characteristics of nanoparticles, notably their expansive surface area, enable interactions with a greater number of conjugates and compounds (Doszpoly et al. 2023). Consequently, this enhances their sensitivity and specificity in identifying and quantifying disease markers (Wang et al. 2011). Nanobiosensors can detect extremely low levels of pathogens, as well as pollutants in the aquatic ecosystem (Chen et al. 2015). Moreover, the application of nano-based remediation technologies, such as nanofiltration (Ren et al. 2022), nanoadsorbents (Vu et al. 2020), nanocatalytic oxidation processes (Chen et al. 2021), and nanosensors monitoring, proves effective in tackling antibiotic pollution in the aquatic environment (Kumar and Guleria 2020).

Challenges of nanotherapeutics application in aquaculture

The potential toxicity of nanoparticles to aquatic organisms and humans is a significant concern (Naguib et al. 2020). These nanoparticles can pose harm to non-target species or the ecosystem. Therefore, ensuring that the nanoparticles reach the target organisms in a controlled and sustained manner without damaging the surrounding environment is crucial (Desai 2012). The high cost of some nanotherapeutics as well as the persistence of certain nanoparticles in aquatic environments are also critical challenges (Bello and Leong 2017). There is a need for effective standardized testing guidelines to assess the safety of nanomaterials in aquatic environments (Desai 2012).

Probiotics, prebiotics, synbiotics, parabiotics, and postbiotics

Probiotics, prebiotics, and synbiotics represent powerful bio-based alternative approaches to traditional chemotherapeutic methods in fish farming (Yilmaz et al. 2022). These innovative therapies possess significant potential for successfully managing diseases in aquaculture (Hardi et al. 2022; Bondad-Reantaso et al. 2023). Probiotics comprise live microorganisms that exert positive influences on the health and overall well-being of their host organisms when administered in the appropriate amounts (Dawood et al. 2020). Probiotics include bacterial microorganisms, such as Lactobacillus spp., Bacillus spp., and Phaeobacter spp. as well as yeasts like Saccharomyces cerevisiae (Dawood et al. 2021). On the other hand, prebiotics are non-digestible food ingredients usually oligosaccharides, such as inulin, fructooligosaccharides, and galactooligosaccharides, that foster the growth of intestinal beneficial microorganisms of aquatic animals (Dawood et al. 2020; Yilmaz et al. 2022). The modification of the microbiome by these beneficial microorganisms stimulates local and systemic fish immune responses, thereby increasing fish resistance to pathogenic bacteria (Manning et al. 2004). Synbiotic is a combination of prebiotics and probiotics (Hardi et al. 2022). Parabiotics, comprising the deceased cells of probiotics and postbiotics such as supernatants from probiotic cultures (metabolic by-products secreted by bacteria), can also play a crucial role in enhancing fish resistance to serious diseases (Goh et al. 2022).

Live, inactivated, or dead microbial cultures are being used in these bio-based therapeutic formulations; however, the administration of live cells induces more health benefits in fish farming (Abbass et al. 2010). Probiotics can be administered in aquaculture, either as a single-strain (mono) probiotic or as a combination of multiple strains (multi-strains). The delivery methods of probiotics encompass bathing the host in a bacterial suspension, supplementing through artificial inert diets, or employing bioencapsulation (Dawood et al. 2020). These bio-based therapeutics offer numerous advantageous applications in aquaculture, such as disease control in fish (Chen et al. 2016), improving feed utilization, and mitigating the adverse effects of antibiotics and other antimicrobials on fish (El-Saadony et al. 2021). Furthermore, probiotics play a role in modulating the transcription of immune-related genes, providing robust immunity and protection against challenging pathogens (Aly et al. 2008). They also improve water quality via increasing the abundance of beneficial bacteria in water (Mohammadi et al. 2021). Probiotics can also be employed to enhance antioxidant capacity (Abdel-Tawwab et al. 2020) and mitigate the toxicity of other chemotherapeutics (El Euony et al. 2020).

Health benefits and antimicrobial mechanisms

The mechanisms of these bio-based therapeutics are intricate (Fig. 4). Key pathways include competitive exclusion of pathogens and competition for binding sites and nutrients (Brunt et al. 2007; Yilmaz et al. 2022). These mechanisms also involve the production of bactericidal and bacteriostatic antimicrobial compounds. Bacteriocins, proteases, hydrogen peroxide, organic acids, and volatile fatty acids are examples of these antimicrobial substances (Vine et al. 2006). Immunostimulation is another significant mode of action of probiotics and prebiotics (Newaj-Fyzul and Austin 2015). Numerous probiotics exhibited immunostimulatory outcomes in aquaculture such as Spirulina platensis green algae (Ibrahem et al. 2013), Bacillus subtilis, Lactobacillus acidophilus (Aly et al. 2008), and Brewer’s yeast (Gokulakrishnan et al. 2022). Probiotics also may serve as a live vaccine carrier. Recombinant live vector vaccines made with Lactobacillus lactis and B. subtilis stimulate potent protective immune responses in fish (Jiang et al. 2019; Sun et al. 2020). These bio-based therapies have demonstrated antagonistic activities against diverse fish pathogens including bacterial, viral, fungal, and parasitic infections as shown in Table 5.

Fig. 4

Health benefits and antimicrobial mechanisms of probiotics

Table 5 Probiotic therapies exhibiting antimicrobial properties against fish pathogens

Challenges of probiotic application in aquaculture

Wang et al. (2008) and Jankovic et al. (2010) discussed the challenges facing the applications of these bio-based therapies. The occurrence of genetic exchange between the viable strains in the probiotics and other microbiota makes the therapy less effective. Furthermore, some probiotic strains could act as potential opportunistic fish pathogens. Therefore, the safety profile of a potential probiotic strain is of critical importance. Ensuring the stability of probiotic strains in the aquatic environment is a significant hurdle. Factors such as water quality, temperature fluctuations, and feed interactions can impact the viability of these therapies. The indigenous microbial community in aquaculture systems may also compete with introduced strains. Regular application of probiotic products, which is required to allow probiotic strains to build up in the system, increases their cost.

Phage therapy as a biocontrol approach for fish bacterial diseases

Therapeutic bacteriophages are viruses that infect bacterial cells. Bacteriophages represent a promising potential alternative to antibiotics for effectively controlling bacterial infections that impact farmed fish (Bondad-Reantaso et al. 2023; Ngoc et al. 2023). These phages are abundant in aquatic environments and persist for extended periods (Cao et al. 2020). Phages replicate through the lytic, lysogenic, pseudo-lysogenic, or chronic cycles, relying entirely on the metabolism of their host for reproduction (Li et al. 2020). Most phages exhaust the resources of their bacterial host, disrupting their metabolism and subsequently destroying them upon releasing their progeny (Rørbo et al. 2018). Phage therapy commonly employs lytic phages, which lyse bacteria, as opposed to lysogenic phages that integrate their genome into the bacterial chromosome (Li et al. 2020).

Health benefits and antimicrobial mechanisms of bacteriophages

Phages possess numerous lytic enzymes with a tremendous structural diversity (Fig. 5). The bacteriophage-encoded enzymes are categorized into three types: endolysins, exolysins, and depolymerases (Latka et al. 2017). These enzymatic proteins hold significant potentials for medicinal applications in aquaculture as novel antimicrobial agents (Katharios et al. 2017; Matamp and Bhat 2019). Phage-derived lysins effectively break down the cell walls and destroy biofilms produced by bacterial fish pathogens (Latka et al. 2017). Bacteriophage also stimulates the fish’s immune responses (Van Belleghem et al. 2017). Bacteriophage therapies possess distinct advantages compared to other therapeutic agents such as (1) they exhibit specific and narrow host ranges, ensuring they do not impact the normal intestinal microbiota of fish; (2) they can self-replicate within the bacterial target, eliminating the requirement for repeated applications; (3) they are safe and have no reported adverse effects on the fish or the environment; (4) phage survival is not affected by aquatic environmental conditions such as temperature, pH, salinity, or organic matter concentration (Mathur et al. 2003; Madsen et al. 2013).

Fig. 5

Antimicrobial mechanisms of bacteriophages

The effectiveness of bacteriophage therapy in aquaculture depends critically on the ability of bacteriophages to reach their target bacterial host (Matamp and Bhat 2019). Bacteriophages can be administered through various routes, including oral administration with feed, injection, topical application on the skin, or via water (Onarinde and Dixon 2018; Akmal et al. 2020). Bath administration of phages is more effective for fish bacterial infection initiated on the skin and gills. On the other hand, phage administration through feed will be more valuable for infections established through the oral route (Nakai and Park 2002).

Phage therapy comes in two forms: active and passive. In active therapy, bacteriophages are administered at a dosage capable of reducing the target host population through multiple replication cycles. In passive therapy, the extensive quantity of administered bacteriophages is sufficient to lyse the entire host population without the necessity for reproduction cycles (Li et al. 2020; Kunttu et al. 2021). Monophage therapy “one single type of bacteriophage” and polyphage therapy “combination of several phages” protocols have been tested in aquaculture (Kunttu et al. 2021). Bacteriophage cocktails, comprising multiple bacteriophages, enable the treatment of a broad spectrum of pathogenic bacterial strains, resulting in more rapid and effective outcomes (Chen et al. 2019). Moreover, the utilization of bacteriophage cocktails targeting diverse receptors of a single bacterium may assist in mitigating the development of resistance to these therapies (Culot et al. 2019). Phage therapy has been effectively used to control numerous destructive bacterial infections affecting fish including, Vibrio spp., Flavobacterium spp., Aeromonas spp., Edwardsiella spp., Streptococcus spp., and Lactococcus garvieae (Table 6).

Table 6 Phages used as potential alternative therapies against bacterial fish pathogens

Challenges of bacteriophage therapy in aquaculture

Earlier studies by Mathur et al. (2003), Laanto et al. (2012), and Middelboe et al. (2009) discussed the limitations of bacteriophage therapy in aquaculture. The occurrence of genetic exchange and mutation resulting from the interaction between phages and their hosts makes phage therapy less effective unless new phage/bacteria combinations are identified. Furthermore, broad-spectrum phages and phage cocktail are required to ensure effective therapy against coinfections with multiple pathogens. Moreover, pathogenic bacteria can develop resistance mechanisms to evade phage infection, potentially reducing the therapy’s efficacy over time. Furthermore, the production of antibodies against phages can result in a decline in their effectiveness. Phage therapy also can distribute antibiotic resistance between strains. Some phages also can enhance the virulence and pathogenicity of bacterial pathogens. This alternative therapeutic approach also encounters limitations against intracellular bacteria. The cost and propagation of phage-based products also may act as an additional challenge in certain aquaculture facilities.

Vaccines as alternative preventive therapies to conventional treatments

Aquatic vaccines are therapeutic formulations composed of intentionally modified pathogenic microorganisms and their by-products, designed to prevent the occurrence of fish infectious diseases (Bondad-Reantaso et al. 2023; Yang et al. 2023). Vaccination stands out as one of the most effective methods for combating disease outbreaks in aquaculture (Abotaleb et al. 2023). The global aquaculture sector is expected to embrace vaccines as a routine preventive measure against a wide spectrum of diseases in the future, as seen in Table 7. This shift is propelled by advancements in aquatic vaccine technology and the decreasing costs associated with their development (Wan et al. 2023).

Table 7 Vaccine therapies targeting major pathogens affecting farmed fish

Types of vaccines and their health benefits in aquaculture

Monovalent and multivalent vaccines are existing. Moreover, live, inactivated, or genetically engineered vaccines are also available (Youssef et al. 2023), as seen in (Fig. 6). Injection, immersion, and oral immunization are the three widely used vaccination methods in aquaculture (Abotaleb et al. 2023). Administering vaccines before pathogen exposure is crucial to allow sufficient time for the development of immunity in fish (Thorarinsson et al. 2021).

Fig. 6

Types of vaccines used in aquaculture

Live-attenuated vaccine

Live attenuated aquatic vaccines are composed of pathogenic microbial strains that have been weakened or mutated. The intended microorganism can replicate within the host, generating antigenic stimuli that mimic those found in natural infections (Zhao et al. 2022). These vaccines usually exhibit robust prolonged immunogenicity, often eliminating the necessity for booster shots or the incorporation of adjuvants (Liu et al. 2016a, b; Ahangarzadeh et al. 2023). Various methods exist for generating live attenuated vaccines, such as chemical/physical mutagenesis, genetic engineering, attenuated culture, and antibiotic-induced attenuation (Nguyen et al. 2018a, b; Laith et al. 2019). The advancements in genetic engineering technologies enabled the production of safe and effective live attenuated vaccine strains by knocking out virulence-related genes in virulent strains (Zhao et al. 2022). Attenuated vaccines effectively protected fish against major pathogens like A. hydrophila (Abdel-Hadi et al. 2009), Ed. tarda (Yamasaki et al. 2015b), S. agalactiae (Liu et al. 2019), and Vibrio alginolyticus (Zhou et al. 2020), as seen in Table 7.

Inactivated vaccines

These vaccines employ physical (ultraviolet, high-temperature heating, ultrasound, and gamma irradiation) or chemical techniques (formalin, formaldehyde, hydrogen peroxide, and polyoxylene ethers) to deactivate highly virulent pathogenic microorganisms (Ramos-Espinoza et al. 2020). Despite their deactivation, they retain their immunogenicity and induce specific resistance in vaccinated aquatic animals (Gu et al. 2021). Chemical inactivation is the most frequently employed method for the preparation of these vaccines. Inactivated vaccines have certain drawbacks, notably the vulnerability of the vaccine’s protective efficacy to the preparation process (Mohamad et al. 2021). Additionally, these vaccines necessitate higher doses and result in a shorter duration of immunity. Incorporating suitable adjuvants and creating multivalent or combination vaccines are essential to address these issues (Cimica et al. 2017). Nevertheless, inactivated vaccines offer advantages such as safety, the absence of virulence issues, stable storage, and low cost. They currently stand as the most widely reported commercial vaccines in aquaculture (Thorarinsson et al. 2021). Various deactivated vaccines have been developed to combat major bacterial fish diseases such as motile Aeromonas septicemia (Osman et al. 2009), vibriosis (Abotaleb et al. 2023), enteric red mouth disease, lactococcosis (Abu-Elala et al. 2019), and streptococcosis (Abotaleb et al. 2023). Inactivated vaccines demonstrate notable efficacy against viral fish diseases such as tilapia lake virus (TiLV) (Mai et al. 2022) a us (IHNV) (Anderson et al. 2008), as shown in Table 7.

Genetically engineered vaccines

Genetically engineered vaccines primarily employ genetic engineering technology to extract immune antigen genes from the targeted pathogen and introduce them into recipient organisms (Zhao et al. 2022). This technology enables the organism to generate a substantial quantity of protective antigens, enhancing disease resistance (Thorarinsson et al. 2021). Recombinant subunit vaccines, nucleic acid vaccines, recombinant live vector vaccines, gene deletion/mutation-based vaccines, and plant-based vaccines are the main representatives of these vaccines.

Subunit vaccine

These vaccines target immune responses to specific microbial determinants. They are generated by incorporating a pathogen antigen into a recombinant expression vector. This involves expressing a gene product, such as a recombinant peptide or protein, utilizing either a prokaryotic or eukaryotic expression system (Wan et al. 2023). The antigen is purified and employed in the vaccine products. Subunit vaccines are useful for microorganisms that are challenging to culture. They pose no risk of inducing disease in the host, as they can not replicate within the host (Lin et al. 2023).

The outer bacterial membrane proteins are considered a promising subunit vaccine candidate against numerous dangerous fish pathogens such as Ed. tarda (Liu et al. 2017b), A. hydrophila (Sharma and Dixit 2016), Vibrio mimicus (Fu et al. 2021b), Vibrio harveyi (Nguyen et al. 2018a), and V. anguillarum (Xing et al. 2020). Flagellin is also regarded as a favorable candidate for a subunit vaccine (Gonzalez-Stegmaier et al. 2021). Viral capsid proteins are also considered noteworthy potential antigens for subunit viral vaccine formulations (Rout et al. 2022). Yeast, bacteria, transgenic plants, cyanobacteria, and microalgae have been reported as potential systems to produce recombinant subunit vaccines (Kis et al. 2019). The subunit vaccine, prepared from heat shock protein and outer membrane proteins of F. orientalis and expressed in diatoms, significantly enhanced Nile tilapia resistance against F. orientalis infection (Shahin et al. 2020).

These vaccines can offer protection against multiple infections via combining multiple antigenic genes of different pathogens. However, the expression of recombinant viral and protozoan membrane antigens in their natural structural state poses a challenge for subunit vaccines (Rigano et al. 2009; Cueva et al. 2021). The production of microbial misfolded proteins is a concern. Moreover, subunit vaccines are predominantly administered through injection, and the requirement for adjuvants contributes to an increase in their overall cost (Dhar et al. 2014).

Nucleic acid vaccine

Nucleic acid vaccine technology refers to the introduction of plasmids carrying a specific antigen of pathogens into the host to achieve immune protective effects. These vaccines have gained wide attention for promoting protective immunity against various fish pathogens (Chang 2022). DNA vaccines show high efficacy in combating numerous bacterial infections in fish, such as Ed. tarda (Liu et al. 2016a, b), V. anguillarum (Xing et al. 2021), and V. harveyi (Sun et al. 2020). Nucleic acid vaccines are also effective against various viral pathogens in aquaculture, such as cyprinid herpesviruses (Huo et al. 2020); viral hemorrhagic septicemia (VHSV), and infectious hematopoietic necrosis virus infections (IHNV) (Marsella et al. 2022). Currently, some commercial DNA vaccines are authorized in aquaculture against numerous viral fish diseases (Chang 2022). Studies also reported the efficacy of DNA vaccines against parasitic fish diseases like I. multifiliis (Xu et al. 2020).

DNA vaccines also show promise in protecting fish against some dangerous fungal fish diseases like saprolegniasis, caused by S. parasitica oomycete. Ongoing research in this area indicates that the serine protease from S. parasitica (SpSsp1) holds the potential to protect against this disease, suggesting its possible application as a vaccine candidate against saprolegniasis in aquaculture (Minor et al. 2014).

DNA vaccines are largely free of toxicity. However, it is not possible to determine whether the host genome will successfully integrate with exogenous nucleic acid. Furthermore, immune tolerance of the organism to the expressed antigen is a concern of DNA vaccines (Xu et al. 2020). There is no potential risk of mutagenesis with RNA vaccines as it can be degraded by normal cellular processes (Parenrengi et al. 2022).

Recombinant live vector-based vaccine

These vaccines use a non-pathogenic virus or bacteria as a vector or carrier to deliver genetic material from a pathogen and express immune-related antigens that confer protection in the vaccinated fish (Travieso et al. 2022; Swain et al. 2023). This vaccine can confer protection against multiple infections via inserting the antigen genes of different pathogens into the vector (Parenrengi et al. 2022). Lactobacillus spp. and B. subtilis are suitable as live bacterial vector carriers, while baculovirus and IHNV can serve as potential live viral vectors (Zhao et al. 2019). Zhu et al. (2023) successfully developed a recombinant baculovirus vector vaccine to counter the highly lethal infectious spleen and kidney necrosis virus. The vaccine demonstrated an outstanding level of protection, reaching nearly 100% efficacy in small bass and an impressive 85.7% in larger bass (Micropterus salmoides). Despite the fact that live vector-based vaccines offer numerous advantages, they also come with drawbacks, as certain bacteria with reduced virulence may still pose risks to aquatic animal health. Hence, additional optimization of both vector selection and immunization strategy is essential to enhance the efficacy of live vector vaccines (Travieso et al. 2022).

Revers vaccination

This vaccination approach employs bioinformatics and genomics to predict immunogenic sequences to be used as potential vaccine candidates (Zhang et al. 2023). The predicted antigens are then produced in the laboratory as recombinant proteins and screened in vitro for immunogenicity. Reverse vaccination has been implemented against certain fish pathogens, including Ph. damsella piscicida (Andreoni et al. 2016), Ed. tarda, and Flavobacterium columnare (Mahendran et al. 2016).

Mucosal vaccines

Mucosal vaccines represent innovative vaccination technology in aquaculture. These vaccines stimulate protective responses at mucosal surfaces, thus preventing pathogens from replicating at their initial site of invasion (Muñoz-Atienza et al. 2021). Mucosal vaccines confer a longer period of immunity in vaccinated fishes and can be administered through a variety of routes, such as oral, immersion, and nasal vaccination. Determining the necessary dose of the protective antigen to confer immunity is among the notorious challenges in designing fish mucosal vaccines (Dadar et al. 2016).

Nano-based vaccines

Nano-based vaccines incorporate various nanomaterials, such as alginate, and chitosan in vaccine formulations. Nanoparticles enhance both vaccine delivery and the intensity of immune responses. In the study conducted by Kitiyodom et al. (2019), a nanoparticle-based vaccine targeting the mucosal surface of F. columnare exhibited promising results with a protective efficacy of 60% in Oreochromis spp.

Challenges of aquatic vaccines application in aquaculture

The safety concerns of fish vaccines related to their limited immunogenicity are among their critical limitations (Thorarinsson et al. 2021). This drawback could potentially lead to the development of severe diseases and reduced production in vaccinated fish. Furthermore, live attenuated microorganisms are unstable and may revert to a virulent state. Additionally, the insufficient availability of authorized vaccines and their high cost limit their field application (Mondal and Thomas 2022).

Quorum quenching as a biocontrol method for managing bacterial fish pathogens

Quorum quenching (QQ) serves as a strategic innovative biocontrol approach for fish diseases (Bondad-Reantaso et al. 2023). QQ refers to the disruption or interference with the quorum-sensing systems (QS) of bacterial pathogens (Shaheer et al. 2021). QS is a process through which bacteria communicate with each other by detecting and producing signaling molecules known as autoinducers. These molecules allow bacteria to synchronize their behavior including microbe-microbe and host-microbe interactions (Shaheer et al. 2021).

QQ mode of action

Pathogenic bacteria employ QS to regulate the expression of genes controlling several pathogenic mechanisms such as invasion, biofilm formation, defense pathways, and virulence factors in a manner dependent on cell density (Chen et al. 2020) (Table 8) (Fig. 7). Certain compounds impede QS by inhibiting the synthesis of autoinducers, degrading QS signaling, or disrupting the interaction between autoinducers and their receptors (Santhakumari et al. 2018; Paopradit et al. 2022).

Table 8 Quorum sensing inhibitors tested against bacterial fish pathogens

Fig. 7

Quorum quenching inhibition of bacterial pathogens

Interfering with QS not only halts the expression of the genes that regulate bacterial virulence and disease mechanisms but also effectively reduces the pathogenicity of bacteria (Shaheer et al. 2021). QQ agents may take the form of microbial strains, enzymes, or quorum-sensing activity inhibitors (QSIs) (Sun et al. 2022b).

Bacillus spp. has emerged as a prominent producer of QQ enzymes (Chen et al. 2020). Moreover, numerous QSIs like sodium houttuyfonate, curcumin, thiazolidine, coumarin, and indole derivatives, are promising in fighting infectious agents (Chaverra Daza et al. 2021). Chen et al. (2020) concluded that Bacillus licheniformis T-1 has promising QQ capabilities against A. hydrophila virulent fish pathogen. The QSI activity of B. licheniformis improved the survival rate of fish infected with A. hydrophila to up to 70%. Moreover, Sun et al. (2022b) investigated the antimicrobial and QQ effects of Bacillus velezensis DH82 against V. parahaemolyticus 17SZ. In vitro studies demonstrated a reduction in the growth and biofilm formation of V. parahaemolyticus. Moreover, the DH82 bacteria not only inhibited QS regulation of V. parahaemolyticus but also mitigated its pathogenicity by downregulating the expression of its virulence genes. Dietary supplementation of B. velezensis to shrimp increased the survival rate of shrimp challenged with V. parahaemolyticus to up to (95%). Furthermore, earlier studies conducted by Shaheer et al. (2021) underscored the robust QQ capabilities of Bacillus spp. strains against V. harveyi, B. subtilis, B. lentus, and B. firmus strains disrupted QS signaling, inhibited biofilm formation, and downregulated the expression of V. harveyi virulence genes. Bacillus spp. strains demonstrated protective effects against V. harveyi challenge in P. monodon larvae, offering a notable 90.66% survival rate.

Challenges of quorum quenching application in aquaculture

There are some challenges associated with QS application in aquaculture as discussed by García-Contreras et al. (2013) and Krzyzek (2019) including the complexity of targeting bacterial QS mechanisms. Ensuring that QQ agents disrupt only the harmful bacteria’s communication without affecting beneficial ones is a multifaceted task. Furthermore, inhibition of the genes responsible for regulating quorum sensing QS can lead to an augmentation of pathogenic traits. Bacteria also could develop resistance to QSI over time. This could reduce the efficacy of these therapies and potentially lead to the evolution of more resilient bacterial strains (García-Contreras et al. 2013).

Antimicrobial peptides

Antimicrobial peptides (AMPs) are one of the best alternative therapies for tackling fish pathogens (Bhat et al. 2022). AMPs are small proteins (< 40 amino acids) encoded in the genomes of both vertebrates and invertebrates, displaying potent antibacterial, antifungal, antiviral, and antiparasitic properties (Colorni et al. 2008; Falco et al. 2008). There are five major classes of AMPs including histone-derived, piscidins, hepcidines, cathelicidins, and defensins (Simora et al. 2021). AMPs’ mode of action is either via direct killing of pathogens or modulation of the fish immune system (Ruangsri et al. 2013; Haney and Hancock 2013) (as seen in Fig. 8). AMPs are effective at low concentrations, and pathogens encounter difficulty in developing resistance against them (Bhat et al. 2022).

Fig. 8

Antimicrobial peptide mechanisms of action

In vitro and in vivo studies on natural and synthetic AMPs have demonstrated their antagonistic activity against a variety of fish pathogens such as Y. ruckeri (Chettri et al. 2017); Vibrio spp., Aeromonas spp., Ps. aeruginosa (Bhat et al. 2022); Ed. tarda, and Streptococcus spp. (Peng et al. 2012). The dietary supplementation of O. niloticus with tilapia piscidin 4 (TP4) enhanced fish immune responses and their resistance against Streptococcus iniae challenge. Fish treated with piscidin 4 (TP4) exhibited an enhanced survival rate of up to 70% compared to 25% in non-treated fish (Zahran et al. 2019). AMPs also have potent antifungal, antiviral, and antiparasitic properties. The sensitized KK16 peptide completely inhibited the highly fatal S. parasitica infection in embryonated fish eggs (Bhat et al. 2022). β-defensin 1 derived from Oncorhynchus mykiss (O. mykiss) showed in vitro antiviral activity against (VHSV) infection (Falco et al. 2008). Moreover, Pisicidin 2, isolated from hybrid striped bass, Morone saxatilis L., showed potent anti-parasitic effects against Cryptocaryon irritans and Trichodina spp., A. ocellatum, and I. multifiliis ectoparasites (Colorni et al. 2008). AMP therapy has some drawbacks such as elevated cost, limited stability, and heightened host toxicity (Anunthawan et al. 2015). Additionally, AMPs are susceptible to degradation by both endogenous and microbial proteases and some exhibiting cytotoxic properties (Yang et al. 2018).

Biosurfactants

Biosurfactants (BS) are surface-active amphipathic biomolecules produced by microorganisms, with multifunctional properties. They include acids, peptides, polysaccharides, and fatty acids (Rodriguez-Lopez et al. 2019). BS have valuable antimicrobial (antibacterial, antifungal, and antiviral) activities (Fig. 9). BS disrupt bacterial cell membrane and inhibit biofilm formation (Hamza et al. 2017). BS can also be employed as immunostimulants strengthening farmed fish defense mechanisms against different infections (Rajeswari et al. 2016). Hamza et al. (2017) extracted a glycolipid biosurfactant from Staphylococcus lentus obtained from a marine aquatic environment. This biosurfactant inhibited V. harveyi and Ps. aeruginosa biofilms. The glycolipid biosurfactant also protected Artemia salina nauplii against V. harveyi and Ps. Aeruginosa experimental infections, exhibiting post-challenge survival rates of 85 and 67.5%, respectively.

Fig. 9

Antimicrobial activity of biosurfactants

Moreover, Giri et al. (2017) investigated the therapeutic effects of Bacillus licheniformis VS16-derived biosurfactant against A. hydrophila challenge in Labeo rohita fingerlings. This biosurfactant improved fish defense mechanisms. Fish treated (I/P) with the VS16-derived biosurfactant showed enhanced immune responses such as lysozyme, phagocytic activities, and immunoglobulin levels. Fish I/P injected with BS exhibited the highest post-challenge survival rate (72.7%). Application of biosurfactants in aquaculture faces some challenges including the high cost of substrates, purifying and screening process (Gaur et al. 2022).

Bacteriocins

Bacteriocins are bioactive compounds synthesized by ribosomes and produced by numerous bacteria like Bacillus spp. and Lactococcus spp. (Kumariya et al. 2019; Bondad-Reantaso et al. 2023). Bacteriocins are considered promising eco-friendly substitutes for antibiotics in aquaculture (Fig. 10). Bacteriocins can function in various capacities, acting as colonizing peptides, killing peptides, or signaling peptides activating the fish immune system (Pereira et al. 2022). Bacteriocins kill pathogens through various mechanisms, including the disruption of the bacterial cell wall, inhibition of peptidoglycan synthesis, and the formation of pores in the cell wall, ultimately resulting in rapid cell death (Cotter et al. 2013). Bacteriocins also inhibit Gram-negative bacterial pathogens by interfering with DNA, RNA, and protein metabolism (Prabhakar et al. 2013). Class II peptides, bacteriocins, insert themselves into the bacterial cell membrane, inducing depolarization and ultimately resulting in the lysis of the target cell (Cotter et al. 2005). Moreover, bacteriocins inhibit pathogens through their RNase and phospholipase toxic enzymatic activities (Prabhakar et al. 2013).

Fig. 10

Antimicrobial mechanisms of bacteriocins

Numerous bacteriocins with broad bacteriostatic and bactericidal effects have been identified. Bacteriocins derived from L. lactis effectively inhibit the highly pathogenic L. garvieae (Sequeiros et al. 2015). Additionally, bacteriocins extracted from B. subtilis, isolated from roho fish (Labeo rohita), exhibit antagonistic activity against A. hydrophila, A. salmonicida, and Ps. fluorescens (Banerjee et al. 2017). Moreover, Nisin, produced by L. lactis and isolated from the tilapia gut, inhibits Ps. aeruginosa and Vibrio spp. fish pathogens (Kaktcham et al. 2019). Application of bacteriocins in aquaculture faces constraints.

Toxicity of some bacteriocins crude extracts to the natural fish gut flora, potentially causing alterations in fish bioactive compounds and cytokines, is among the challenges facing their application (Malaczewska al. 2021). It can disrupt the bioremediation process via inhibiting the microbiota. Moreover, the direct use of bacteriocin-producing bacteria poses a risk of disease, as certain strains can revert to pathogens (Feliatra et al. 2018).

Stem cell and aquatic animal health: possibilities and prospective

Stem cell technology has opened new avenues for enhancing all sectors of animal production, including aquaculture, and holds the potential to revolutionize therapies for numerous diseases. Stem cell-assisted therapy affords a new expectation for the treatment of incurable diseases, whereas the conventional therapeutic approaches did not afford complete recovery or treatment of the primary cause (Abdal Dayem et al. 2019). The ultimate goal of stem cell-based therapy, an essential component of regenerative medicine, is to strengthen the body’s ability to recover from the injury itself by stimulating, regulating, and changing the population of endogenous stem cells and/or replenishing the cell pool that is in favor of tissue homeostasis and regeneration (O’Brien and Barry 2009; Hoang et al. 2022).

The unique stem cell properties, such as self-renewal and multi-lineage capacities are key factors in stem cell-mediated damaged tissue repair or replacing damaged cells with new functional cells (Biehl and Russell 2009). Apart from their capacity to regulate inflammation, MSCs exhibit antibacterial properties by impeding microbial growth and mitigating antibiotic resistance in Pseudomonas aeruginosa infections (Ren et al. 2020). Previous reports displayed the potent antimicrobial activity of MSCs, which is attributed to their direct and indirect activity (Alcayaga-Miranda et al. 2017). It has been demonstrated that MSC releases antimicrobial peptides including cathelicidins, beta-defensins, and lipocalin (Sung et al. 2016). Interesting study evidenced the enhanced secretion of the antimicrobial cathelicidin LL-37 by MSC upon the secretion of the bacterial products (Krasnodembskaya et al. 2010).

Another mode of action for the antimicrobial activity of MSCs is their modulatory activity. Studies have shown that exposure to MSC-secreted factors boosts the phagocytosis process and the killing capacity of monocytes and neutrophils, and MSC possesses potent anti-inflammatory activity in sepsis (Brandau et al. 2014; Németh et al. 2009). In addition, the synergistic action of MSCs with antibiotics is reported to promote the survival rate after the systemic infection in the mice sepsis model (Alcayaga-Miranda et al. 2015). The wound-healing capacity of MSC is also one of the key mechanisms of MSC-related regeneration. MSC-assisted wound healing capacity is ascribed to angiogenesis activation, activation of the endogenous stem cell population, alleviation of inflammation, and immune cell recruitment (Motegi and Ishikawa 2017). Therefore, stem cells’ potent anti-microbial activity and wound-healing capacity should be investigated and applied to fish diseases. Then, for the application of stem cells in fish diseases, stem cells could be injected into fish muscles, and topically applied using cream base or biomaterials. Similar to the common routes of drug administration in fish, stem cells can be administered to the fish via topical application using scaffolds or cream base, intramuscular injection, intravenous injection, and supplementation in food pellets. Of note, the suitable cell number that exerts an efficient therapeutic activity should be taken into consideration. In summary, stem cell application in fish diseases is still in its infancy and needs further in-depth research. The application of stem cells in the control of fish diseases offers innovative approaches for managing health in aquaculture. This burgeoning field explores three main applications: developing fish cell lines, disease modeling, and drug screening, as well as germ cell transplantation for treating fish diseases. Stem cells are utilized to create cell lines that model fish diseases, providing a crucial platform for understanding pathogenesis and conducting preliminary drug tests. Germ cell transplantation emerges as a promising method for directly treating diseases or restoring fertility in fish affected by various diseases. By replacing or supplementing diseased cells with healthy germ cells derived from stem cells, researchers aim to produce fish breeds that are resistant to diseases and carry a robust trait. Overall, the integration of stem cell technology in these areas represents a significant advancement in aquatic veterinary medicine, opening up new pathways for disease management and therapeutic innovation.

Diagnostic therapy

The advancement of immunological and molecular diagnostic tools for fish health management is dynamic and constantly evolving, offering substantial potential for the aquatic animal health sector (Abdelsalam et al. 2023). Effective vaccinations and disease management strategies in aquaculture have greatly benefited from early identification and precise diagnosis of fish diseases, facilitated by advancements in disease diagnostic approaches (Jaies et al. 2024). Accurate and early diagnosis of invading pathogens is crucial for the effectiveness of therapy and allows for timely intervention and treatment (Dong et al. 2023). Effective diagnostic technologies (Fig. 11) provide fish farmers with precise information on the health of farmed fish, enabling them to optimize overall health, prevent disease spread, and implement targeted management plans (Li et al. 2022a, b, c).

Fig. 11

Diagnostic therapy

Collecting case history and conducting clinical and postmortem examinations constitute the initial critical steps in the pathway toward correct diagnosis (Abdelsalam et al. 2023). While wet mounts, microscopic examinations, and isolation and culturing on agar may be helpful, they are not sufficient for accurately identifying the species of pathogens (Abdelsalam et al. 2023). Various diagnostic techniques based on phenotypic, morphological, and serological characteristics of pathogens are available, but some of them have shortcomings that can lead to misdiagnosis (Dong et al. 2023; El-Adawy et al. 2023a, b). Molecular diagnostic methods, relying on PCR-based assays and genome sequencing, offer high accuracy (Austin and Austin 2012). Combining immunological and molecular methodologies (Fig. 11) can lead to a more comprehensive understanding of fish health, immune responses, and the factors influencing farmed fish (Jaies et al. 2024).

Immunological diagnostic techniques

Immunological techniques encompass a broad array of laboratory methods used to diagnose various fish diseases by detecting specific antigens, antibodies, or immune cells in biological specimens (Im et al. 2019). Advancements in immunological techniques are essential for developing, optimizing, and evaluating effective aquatic vaccines in aquaculture. These techniques contribute significantly to vaccination by enabling vaccine efficacy testing, ensuring safety, and conducting quality control measures, thereby enhancing disease prevention and management in aquaculture (Jaies et al. 2024).

The enzyme-linked immunosorbent assay (ELISA)

The ELISA assay is designed to detect and measure specific antigens (such as proteins, peptides, or other compounds) or antibodies with precision and accuracy. The technique is based on enzymatic reactions and the binding of antigens and antibodies (Jaies et al. 2024). The most commonly utilized types of ELISA in immunological assays include Direct ELISA, Indirect ELISA, Sandwich/Capture ELISA, Competitive ELISA, and Multiplex ELISA (El-Adawy et al. 2023a, b). ELISA plays a crucial role in the aquatic health sector, with applications ranging from disease diagnosis and vaccine development to monitoring fish immune responses and environmental conditions (Munangandu et al. 2019; El-Adawy et al. 2023a, b; Jaies et al. 2024). The main limitations of the ELISA assay include cross-reactivity, challenges with specificity, and restricted multiplexing capabilities, which hinder its ability to simultaneously investigate multiple aspects of fish health (Jaies et al. 2024).

Immunofluorescence

Immunofluorescence is an immunohistochemistry technique that utilizes fluorophores to visualize various cellular antigens. The assay is based on the visualization of antigen–antibody interactions using fluorescent labeling and fluorescence microscopy (Jaies et al. 2024). Immunofluorescence has diverse applications, such as diagnosing infections affecting fish, examining the distribution of antigens in fish tissue, studying fish immune responses, and evaluating the effects of environmental pollutants on fish health (Brehm-Stecher and Johnson 2004; Austin 2019). One of the primary limitations of immunofluorescence is its dependence on the permeability of fish tissues, such as scales or skin, to antibody penetration, which significantly influences labeling efficiency (Jaies et al. 2024). Moreover, cross-reactivity and non-specific binding of antibodies to similar antigens can lead to false-positive results, while non-specific background fluorescence can interfere with signal detection (Pina et al. 2022).

Immunohistochemistry

Immunohistochemistry is a powerful immunological diagnostic approach based on antigen–antibody interactions (Jaies et al. 2024). This assay is commonly used to detect and localize specific antigens within tissues (Magaki et al. 2019). Immunohistochemistry has diverse applications across various fields, including disease diagnosis, analysis of histopathological tissue changes associated with diseases, and the study of tissue-specific protein expression. It also plays a crucial role in assessing the effectiveness of medications and vaccines in fish (Ezyaguirre et al. 2011; Zhang et al. 2017; Su and Chen 2021). Primary hurdles in immunohistochemistry methods include cross-reactivity with fish tissues, challenges in fixing fish-specific tissues, and tissue autofluorescence (Jaies et al. 2024).

Molecular diagnostic techniques

Polymerase chain reaction (PCR)

PCR is one of the most widely used techniques in molecular biology and is applied across various sectors. The assay works by amplifying specific DNA sequences from complex mixtures using specific primers and enzymes within a thermal cycler (Abdelsalam et al. 2023). PCR encompasses several types, each serving distinct purposes: Conventional PCR, Reverse transcription PCR (RT-PCR), Quantitative PCR (qPCR), nested PCR, multiplex PCR, multiplex ligation-dependent probe amplification (MLPA), digital PCR, and overlap extension PCR (OE PCR) (Abdelsalam et al. 2023; Jaies et al. 2024). PCR assays have several important applications, including disease diagnosis, drug development, cloning, and recombinant DNA technology. Challenges facing PCR diagnostic techniques include the occurrence of false-positive or false-negative results and interference from various PCR inhibitors (Jaies et al. 2024)

Loop-mediated isothermal amplification (LAMP)

Loop-mediated isothermal amplification (LAMP) is an extremely precise molecular method of nucleic acid amplification (Notomi et al. 2000). The target DNA is recognized by six different regions using a series of four (or six) distinct primers that bind to specific sites on the target gene. The LAMP reaction involves both strand displacement and amplification phases. It amplifies targeted DNA at a constant temperature without the need for a thermal cycler (Abdelsalam et al. 2023). LAMP has numerous applications due to its specificity and simplicity, including genotyping, pathogen detection, and environmental monitoring. However, primer design complexity and cross-reactivity are important constraints facing this diagnostic approach (Jaies et al. 2024).

Next-generation sequencing (NGS)

Next-generation sequencing technology represents a significant advancement in molecular biology, enabling detailed and large-scale genetic analyses that were previously impossible (Satam et al. 2023). NGS can process millions of fragments simultaneously, providing a high-throughput approach to DNA and RNA sequencing, thereby enabling the generation of enormous volumes of valuable data (Qin 2019). This method combines numerous sequencing matrices and bioinformatics technologies. NGS has various applications, including precise pathogen diagnosis, targeted treatment plans, whole genome sequencing, identifying genetic mutations, analyzing epigenetic modifications, and sequencing DNA from environmental samples to study microbial communities (Satam et al. 2023). Challenges facing next-generation sequencing technology include high costs, the need for advanced bioinformatics techniques and knowledge, and the difficulty and time-consuming nature of analyzing raw sequencing data (Jaies et al. 2024).

Recent advancements in fish disease diagnostic approaches

Metagenomics and multi-omics Metagenomics studies genetic material obtained from microbial communities in environmental samples, without the need for culturing individual species (Zhang et al. 2021). Metagenomics enables comprehensive exploration of microbial populations associated with fish ecosystems, including bacteria, viruses, and fungi, using high-throughput DNA sequencing methods (Petrosino et al. 2009; Pérez-Cobas et al. 2020). This approach provides significant new insights into the complex interactions between the microbiome and the fish host, revealing the role of microbial communities in regulating immune responses, preventing disease, and maintaining health (Jaies et al. 2024). Metagenomics approaches have emphasized the role of beneficial microbes in promoting fish health and improving the identification of potential fish pathogens (Yukgehnaish et al. 2020).

A comprehensive understanding of data from proteomics, metabolomics, transcriptomics, and genomics enables the identification of significant biomarkers for diagnostics (Natnan et al. 2021). Proteomic analyses play a crucial role in elucidating pathogen survival strategies and host defense responses, thereby enhancing our understanding of interactions between fish and pathogens (Ahmed et al. 2019a, b). The integration of multi-omics data supports the development of predictive models that reveal how genetic and environmental factors impact fish health and performance (Jaies et al. 2024).

Biosensors and nanotechnology

The aquatic animal health sector benefits from biosensors and nanotechnology in disease diagnosis through enhanced sensitivity, rapid detection, and improved accuracy in identifying pathogens (Bohara et al. 2023). This allows for prompt intervention techniques to mitigate their effects. Biosensors utilize biological molecules to detect specific compounds or biomarkers associated with diseases in fish, offering real-time monitoring and early pathogen detection capabilities (Jaies et al. 2024). Nanotechnology enhances diagnostic tools by miniaturizing sensors, thereby improving their efficiency and enabling on-site testing (El-Adawy et al. 2023a, b). These advancements contribute to proactive disease management, reducing economic losses, and promoting sustainable aquaculture practices. Fernandes et al. (2015) developed a highly sensitive genosensor incorporating multiwalled carbon nanotubes, chitosan, bismuth, and lead sulfide nanoparticles for detecting pathogenic Aeromonas. This biosensor has a high detection limit of 1.0 × 10⁻¹⁴ M and can identify Aeromonas concentrations as low as 10² CFU/mL in spiked tap water.

Artificial intelligence (AI), image processing, and computer vision technology

Integrating AI into disease diagnostic methods can enhance the efficiency and accuracy of pathogen detection in several ways, such as through image analysis, analysis of fish behavior, genetic data analysis, real-time monitoring, and monitoring of water quality (Mandal and Ghosh 2023). Machine learning algorithms can detect anomalies indicating health issues in farmed fish populations, predict disease outbreaks, and optimize feeding schedules (Ditria et al. 2022). Additionally, AI can help determine the most effective treatments and dosages for sick fish, minimizing the use of antibiotics and other medications. AI-driven platforms can assist farmers in making informed decisions using real-time data and predictive models (Jaies et al. 2024).

Automated fish disease detection can also be established through the integration of image-processing technology with computer vision (Torres and Arroyo 2018; Li et al. 2022a, b, c). Developing AR 3D images of fish diseases, utilizing standardized and shared datasets, along with deep learning and data fusion techniques, can enhance the accuracy and efficiency of diagnosing fish diseases (Li et al. 2022a, b, c). This diagnostic approach offers a real-time, non-invasive, and cost-effective method for disease diagnosis (Ohnuma et al. 2017; Li et al. 2022a, b, c).

The complexity of data interpretation, host–pathogen interactions, and high costs are among the most significant challenges faced by these innovative diagnostic approaches (Bahassi and Stambrook 2014; Jaies et al. 2024.

Conclusion

Disease outbreaks pose a substantial obstacle to the sustainability of aquaculture, resulting in massive financial losses. Traditionally, the prevention and control of fish diseases have predominantly leaned on the application of conventional chemotherapeutics and antibiotics. Nevertheless, the prolonged and uncontrolled utilization of these compounds contributes to the rise of microbial resistance, harms fish health, degrades aquaculture environments, leads to the presence of drug residues in aquatic products, induces financial losses, and impedes the trade of fishery products.

These challenges serve as strong motivation to explore innovative and effective therapeutic approaches that can overcome these impediments. Advancements in biotechnology have greatly expanded the development of effective therapies to combat various pathogens in aquaculture. A variety of bio-based and immunoprophylaxis therapies have emerged, including phytotherapeutics, nanotherapeutics, probiotics, prebiotics, synbiotics, phage therapy, vaccination, quorum quenching, antimicrobial peptides, biosurfactants, and bacteriocins. These promising therapies demonstrate significant potential for application in aquaculture. Additionally, germline stem cells and germ cell transplantation offer new prospects for the genetic breeding of disease-resistant fish strains. Regularly screening and monitoring the health status of cultured species are essential for the early detection of diseases. Therefore, developing effective disease diagnostic technologies enables accurate pathogen detection, provides fish farmers with precise information about the health of their farmed fish, and facilitates the application of efficient therapies. Further research is essential to refine the utilization of these alternative therapeutic approaches. There is a need for a deeper understanding of their successes and failures, cost implications, efficacy, risks, practical applications (especially for small fish producers), adverse effects on the aquatic environment, and how these alternatives contribute to improving health and enhancing host immunity. The objective is to devise the safest, economically viable, and most effective methods to prevent aquatic animal diseases. A multidisciplinary approach involving scientists, researchers, industry stakeholders, and regulatory bodies is essential to navigate these challenges for the responsible and sustainable application of these disease-alternative therapeutics in aquaculture.

Recommendations

The primary critical measure in averting disease outbreaks in fish farms revolves around adhering to good farm management and implementing robust biosecurity practices before the event of any disease outbreaks. The second crucial aspect involves the swift and precise diagnosis of invading pathogens. The third focal point is the implementation of cost-effective and safe therapeutic interventions. The fourth critical issue is conducting extensive environmental risk assessment studies before applying any therapy in aquaculture facilities. Application of therapeutic combinations can strongly enhance the efficacy of these therapies. Immunoprophylaxis should be carried out before the occurrence of a disease outbreak to reduce the losses of fish production. While the aforementioned therapeutics show considerable promise in protecting fish from various harmful pathogens, their application carries inherent risks. Therefore, a deeper understanding through research is imperative to address these limitations. A multidisciplinary approach, engaging scientists, industry stakeholders, and regulatory bodies, is essential to navigate these challenges and establish guidelines for responsible and sustainable application of these therapies in aquaculture.

Non-therapeutic preventive measurements for controlling fish diseases

(1) Application of biosecurity and good farm management practices in aquaculture. (2) Development and breading of disease-resistant fish strains. (3) Feeding farmed fish on well-balanced diets. (4) Conducting antibiotic sensitivity testing before administering any antibiotic treatment in fish farms. (5) Strengthen oversight of antibiotic and chemotherapeutic use in fish farms by regulating their sales and ensuring supervision by adequately trained aquatic health professionals. (6) Developing a national strategy on aquatic animal health. (7) Promote ongoing education and training for aquaculturists and fish farmers. (8) Mechanical (filtration) and ozone treatment of water in recirculating aquaculture systems. (9) Employing biological methods to control parasites by using cleaner fish. (10) Enforcing legislation and policies that regulate the use of chemicals in aquaculture.

Phytotherapeutics

(1) Developing standardized phytotherapeutic products with well-defined active ingredients and improving extraction methods. (2) Developing phytotherapeutics with multiple antimicrobial properties through synergistic combinations of different phytotherapeutic compounds or their combination with other bio-enhancers to enhance overall efficacy. (3) Improving the bioavailability and absorption of phytochemicals via adopting encapsulation techniques or nanoformulations. (4) Adjusting dosages and avoiding aquatic environmental conditions that can influence the efficacy and absorption of phytotherapeutics.

Nanotherapeutics

(1) Ensure the safety and biocompatibility of nanotherapeutics for aquatic organisms by conducting comprehensive toxicity studies. (2) Optimize the size of nanoparticles to enhance their uptake and absorption by farmed fish. (3) Conduct thorough environmental impact assessments to understand the potential consequences of nanotherapeutics use on aquatic ecosystem. (4) Improve effective and safe delivery systems for nanotherapeutics, ensuring that nanoparticles reach the target organisms in a controlled and sustained manner without causing harm to the surrounding environment. (5) Adopting green synthesis methods to produce nanoparticles to enhance their therapeutic efficacy. (6) Providing economically viable nanotherapeutics for fish farmers. (7) Enforce legislations controlling the application of nanotherapeutics in aquaculture.

Probiotics, prebiotics, and synbiotics

(1) Selecting probiotic strains that are well-adapted to the target fish species and have proven beneficial effects on growth, disease resistance, and overall health. (2) Conduct microbial analyses to ensure the dominance of beneficial probiotic strains. (3) Select probiotic strains that do not adversely interact with other treatments. (3) Develop new probiotic strain combinations to overcome mutations in microbial strains. (4) Use probiotics with optimal dosage based on the species, size, and environmental condition. (5) Maintain a stable beneficial microbial community through repeated application of probiotics.

Phage therapy

(1) Selecting an appropriate phage that has antagonistic effects against the targeted pathogen. (2) Using phage mixtures, phage cocktails, and new combinations of phages to ensure effective therapy against mixed infections and to avoid bacterial resistance. (3) Phages should be tested for survival against a range of environmental parameters to ensure long-term survival. (4) Phages should be purely lytic to prevent recombination and mutation processes in bacterial strains. (5) Phage therapy should be cost-effective and affordable to farmers. (6) Enforcing legislations regulating the application of bacteriophages in aquaculture.

Vaccines

(1) Confirm the safety requirements of fish vaccines, especially modified live vaccines that need to pass certain safety guidelines. (2) Increasing the availability of commercially authorized vaccines against a wide range of fish pathogens. (3) Developing safe, cost-effective polyvalent vaccines that confer protection against a wide range of pathogens. (4) Fish vaccines should be environmentally friendly, cost-effective, and affordable for small fish farmers. (5) Vaccine technology should be suitable for large-scale production. (6) Developing a new generation of vaccines using recent genetic engineering technology and gene editing to create safer and highly effective vaccines. (7) Improve vaccine delivery methods for fish.

Quorum quenching

(1) Achieving high specificity in targeting the quorum sensing (QS) systems of pathogens without adversely affecting beneficial microorganisms or the host. (2) Using naturally-sourced QSIs and developing newer QSIs agents for resistant bacteria.

Antimicrobial peptides, biosurfactants, and bacteriocins

(1) Improve the purifying and screening process for novel agents. (2) Reduce the cost of applying these therapies. (3) Improve the stability of these therapeutic agents and mitigate potential toxicity, thereby enhancing their overall safety and effectiveness.

Stem cell-based therapy

(1) Adhering to ethical guidelines for the use of stem cells. (2) Optimize the technical aspects of culturing techniques and delivery methods. (3) Conduct continuous research and development to understand the potential of this therapy and identify sources of stem cells. (4) Assess the safety and efficacy of stem cell therapy in aquaculture species. (5) Evaluate the cost-effectiveness of stem cell therapy in aquaculture.

Diagnostic therapy

(1) Selecting appropriate diagnostic tests. (2) Developing reliable antibodies, primers, and probes specific to the pathogens of interest. (3) Standardizing and validating diagnostics to confirm the sensitivity, specificity, and reproducibility of diagnostic tests. (4) Integrating molecular and immunological diagnostics with artificial intelligence based-technologies to significantly enhance the accuracy of pathogen detection. (5) Regular screening and monitoring of the health status of cultured species to detect early signs of disease.