Twenty years ago, aquatic products played a secondary role in people’s food choices. However, now aquatic products have become one of the mainstream food categories. Looking back on the development of global aquaculture from 1997 to 2017, aquaculture has made a substantial contribution to food production throughout the world, especially in Asia. According to the current consumption, aquaculture production needs to increase from 82,087 kilotons in 2018 to 129,000 kilotons in 2050 to meet global needs ^[1][2][1,2]. By 2050, aquaculture will dominate the global seafood supply [3].

Vibrio is one of the important pathogenic microorganisms of humans and marine animals. It widely exists in marine and freshwater ecosystems. Because of its high abundance and biomass, Vibrio plays a crucial role in the aquatic environment. More than 80 species of Vibrio have been reported, some of which are pathogenic to animals, especially aquatic animals, some to humans, and some to both animals and humans [4]. The outbreak of vibriosis will not only seriously affect marine biomass but also lead to serious economic losses in Asian fish culture.

With the rapid development of Asian fish culture in recent decades, the cases of Vibrio infection through aquatic products at home and abroad, causing human disease or huge economic losses, are also increasing year by year. At the same time, the prevention and control measures for Vibrio are also developing. At present, the use of antibiotics is the most important treatment for vibriosis in Asian fish culture [5]. At the same time, the overuse of broad-spectrum antibiotics has resulted in an increase in the number of drug-resistant bacteria. The resistance genes of these bacteria can be transferred to other bacteria that have never been exposed to the antibiotic [6]. Therefore, it is necessary to develop some antibiotic-free methods. For example, using vaccines, probiotics, bacteriophages and other technologies.

Before considering the prevention and control of Vibrio, it is essential first to identify the exact pathogen. At present, the mainly used identification methods are still conventional physiological, biochemical analyses, 16S rDNA sequencing and drug sensitivity test. In addition to these widely used assays, some convenient, fast and highly sensitive detection methods have been developed in recent years, for example, the identification of biomarkers based on host genes [7], exosomic miRNAs [8] and so on.

Vaccination in Asian fish culture can prevent or mitigate the spread of disease and is effective against many related pathogens [9]. Vaccination is usually a secure and economic precaution. For this reason, illness prevention based on stimulating the immune system of aquatic animals has proved to be the basis of the development of modern Asian fish culture. Nevertheless, there are only a few Vibrios with vaccine control technology.

The control and prevention strategies of seven Vibrio species that are seriously harmful to Asian fish culture, including Vibrio harveyi, Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio mimicus, Vibrio anguillarum, Vibrio alginolyticus and Vibrio cholerae. For each Vibrio, rwesearchers describe their prevention and treatment methods (Figure 1), especially vaccine prevention methods, in order to provide views for better prevention and control of vibriosis in Asian fish culture in the future.

2. Control and Prevention Strategies of Vibrios

2.1. V. harveyi

V. harveyi is a luminous marine bacterium and is also a well-recognized and acute pathogen of marine fish [10]. The research on the control and prevention measures of V. harveyi started early, and now there are a variety of control technologies (Table 1).

2.1.1. Antibiotics

Antibiotic methods are generally used in the initial stage of prevention and treatment of vibriosis or emergency treatment. In the case of skin ulcer disease of young hybrid groupers, researchers confirm that the pathogen of this sickness is V. harveyi ML01 strain, which is sensitive to minocycline, doxycycline and ceftriaxone. In other words, these three antibiotics can be used for emergency treatment of V. harveyi infection [11]. In 2017, the drug sensitivity test of V. harveyi extracted from the diseased cultured hippocampus was carried out, and the results showed that V. harveyi is highly sensitive to doxycycline and tetracycline. This provides a reference for the prevention strategy of vibriosis in seahorse culture in eastern China [12]. Although antibiotics are widely used, the rapid increase in antibiotic resistance is really puzzling.

2.1.2. Bacteriophages

As people gradually realize the risk of using antibiotics in Asian fish culture, probiotics, bacteriophage, antimicrobials from natural sources and so on are gradually replacing antibiotics. In bacteriophage therapy, phages such as lytic Vibrio phage VhKM4 [13] can resist V. harveyi efficiently due to its strong lytic activity. Although several research studies have proven these methods effective, there have not been enough similar studies of each method to prove that they should be promoted in practical application. At the same time, whether these biological control methods have potential threats still needs further study in future research.

2.1.3. Vaccines

One of the research hot spots of Vibrio prevention is vaccine development. Most research on V. harveyi vaccine is targeted at fish. Many excellent achievements have been made in the research of V. harveyi vaccine. Vaccine exploration started from the traditional whole-cell inactivation method, followed by the study on the method of purifying subcellular components, making vaccine technology enter the era of modern vaccines represented by DNA vaccine [10]. Whole-cell vaccines can be categorized into two types, attenuated live vaccine and inactivated vaccine. The production cost of these vaccines is not high [14]. This traditional vaccine is the most widely used in the prevention of aquatic animal diseases.

• Inactivated vaccines

FKVh, a vaccine mainly composed of formalin-inactivated V. harveyi Vh1 strain, may be an effective vaccine, and the survival rate of hybrid tilapia increased from 20% to 87% after vaccination [15]. There is also a combined vaccine (VICV) against V. vulnificus, V. alginolyticus, V. harveyi and infectious spleen and kidney necrosis virus (ISKNV). Huang et al. have proved its immunization effectiveness by immunizing orange-spotted grouper Epinephelus coioides with the VICV vaccine and attacking the above four pathogens [16]. Compared with the monovalent vaccine, this kind of vaccine can more conveniently protect fish from a variety of pathogens. Although the production cost of inactivated vaccines is relatively lower compared with other kinds of vaccines, their performance still needs to be continuously improved. This can be achieved by combining adjuvants, liposome embedding and other methods. Research has proposed a greatly effective vaccine that can prevent V. harveyi. The vaccine consists of inactivated V. harveyi cells and ISA763 AVG adjuvant. The experiment observed that the RPS of grouper inoculated with this vaccine was 100% in the sixth week and 91.7% in the twelfth week after being attacked by V. harveyi [17]. The formalin-inactivated cell of V. harveyi adjuvanted with Montanide TMISA 763 AVG induced efficient immune protection in turbot [18]. Similarly, the application of liposomes-entrapped V. harveyi WCV or V. harveyi WC can actively strengthen the immune system and provide protection for V. harveyi infection in Epinephelus bruneus [19]. The expression standard of various immune substances in the grouper‘s spleen is significantly up-regulated after inoculation in the laboratory, using a vaccine made of inactivated V. harveyi ZJ0603 combined with β-glucan [20].

• Attenuated live vaccines

An attenuated live vaccine has been developed to highly protect Japanese flounder (Paralichthys olivaceus) infected with V. harveyi in the experiment. The vaccine is made of live Escherichia coli, which can express and secrete Vhp1 with impaired cytotoxicity [21]. Moreover, a study shows that V. harveyi WC13DH51 strain can be made into a live attenuated vaccine and has a significant protective effect on groupers [22]. Furthermore, an attenuated live vaccine was developed by constructing recombinant Et15VhD. The infection experiment shows that this vaccine can effectively prevent the infection of V. harveyi and E. tarda [23]. Similarly, the attenuated mutant strain T4DM of V. harveyi can also be used as a live attenuated vaccine. On the medium containing rifampicin with increased concentration, T4DM was obtained by selecting T4D mutants repeatedly with a relatively narrow antibiotic resistance profile and no detectable plasmid. T4DM is also a cross-protection vaccine, which can effectively protect Japanese flounder from the infection of V. alginolyticusvia and V. harveyi, especially through immersion (10⁸ CFU/mL) and intraperitoneal injection (10⁸ CFU/mL) [24].

• Subunit vaccines

A subunit vaccine made of purified recombinant Vhp1 can effectively render Japanese flounder V. harveyi-resistant [21]. There is also a V. harveyi subunit vaccine encoding TssJ antigen that was found to emerge a moderate protective role against V. harveyi in fish. The full-length sequence of TssJ was obtained from the V. harveyi strain QT520 and was predicted as a new candidate antigen, whose relative percentage survival was 52.39% [25]. Moreover, based on VirB11, a recombinant protein vaccine was developed and became a candidate vaccine to prevent V. harveyi infection [26]. In addition, recombinant cell vaccines expressing the DnaJ and OmpK have strong cross-protection against V. alginolyticus, V. parahaemolyticus and V. harveyi [27].

• Anti-idiotypic vaccines

A great deal of studies have shown that antibodies may have a regulatory effect on the immune system. Consequently, they have the conditions for making vaccines. The vaccine developed according to this principle is called an anti-idiotypic vaccine. As an anti-Id vaccine, anti-Id IgG is a vaccine that can provide protection by imitating the antigen epitope of V. harveyi. It may have a good application prospect in Asian fish culture against V. harveyi [28].

• DNA vaccines

A series of experimental results suggest that DNA vaccines represented by pDV are positive vaccines against V. harveyi [29]. DNA vaccine can also be obtained by cloning the ompU gene into pEGFP-N1 plasmid. After the infection test of the turbot, the RPS was 51.4% [30]. Moreover, a V. harveyi DNA vaccine encoding TssJ antigen could produce a moderate protective role against V. harveyi in fish, and the relative percentage survival was 69.11% [25]. However, the exact route of protection in fish for these vaccines is still unclear at present [14].

• mRNA vaccines

In 1990, the successful use of in vitro transcription (IVT) mRNA in animals was first reported, and related research has developed extremely rapidly since then. The production cost of the mRNA vaccine is low. The application safety is high, and the development turnaround time is short and pretty efficient. Therefore, the mRNA vaccine may have a better prospect compared with the traditional vaccine [31]. mRNA vaccines applied to aquatic animals are rare. ResWearchers found a study on the mRNA vaccine against V. harveyi infection in fish. TIn this study, the researchers first used computational techniques to find potential T-and B-cell epitopes in V. harveyi hemolysin proteins and then sutured these epitopes into multi epitope mRNA vaccines. However, more experiments are needed to further prove the effectiveness of this vaccine [32].

2.2. V. vulnificus

V. vulnificus is a gram-negative bacterium that can cause wound infection and septicemia. Unlike other Vibrios, it is able to ferment lactose. According to genetic, biochemical and serological tests and host infection, V. vulnificus is currently classified into three biotypes. Biotype 1 strains are the source of most human infections, and biotype 2 strains mainly infect eels. The recently discovered biotype 3 has the biochemical characteristics of biotype 2 and 1 and can result in human wound infection [33]. In this era of environmental protection and sustainable development, the biological control strategy of Vibrio is gradually emerging, but there are few examples in fish farming.

Vaccines

 

• Inactivated vaccines

Since V. vulnificus is one of the most harmful Vibrios to Asian fish culture, its vaccine control technology has been continuously developed (Table 2). The early vaccines against V. vulnificus are generally inactivated vaccines. For example, a formalin-inactivated V. vulnificus vaccine was effective at exciting a humoral antibody response in sex-reversed hybrid tilapia [34]. The combined vaccine (VICV) also can be effective against V. harveyi [16].

• Subunit vaccines

There are also a few subunit vaccines against V. vulnificus under development. Some scientists have found that the expressed OmpU of V. vulnificus was capable of resisting the infection of V. vulnificus in Japanese eels and evidently raised the immune ability of eel. Therefore, the OmpU is proposed as a potential subunit vaccine against V. vulnificus [35].

• Multivalent vaccines

Multivalent vaccines may be more practical in aquaculture due to their multiple protective effects. There is a bivalent protein that can be used against V. vulnificus and Edwardsiella anguillarum in Japanese eel as a vaccine. This fresh recombinant Omp vaccine with OmpA and OmpU shows its strong immunogenicity by significantly increasing the RPS rate of eels when infected with E. anguillarum and V. vulnificus [36]. In addition to these bivalent vaccines, a trivalent outer membrane protein, OmpII-U-A, containing part sequences of OmpU from V. vulnificus, OmpA from E. anguillarum, and OmpII from A. hydrophila, can also be made into a vaccine. According to the study of He et al. [37], the OmpII-U-A is able to prevent eel from being infected by V. vulnificus and A. hydrophila. This is the first time the expression and immunogenicity of a trivalent Omp are being reported, and the outcomes of this research will supply valuable guidelines for the exploration of multiplex vaccines in fish. V. vulnificus has many bivalent and trivalent vaccines that can protect aquatic animals from A. hydrophila, E. anguillarum and other pathogens. These vaccines can provide ideas for the advancement of aquatic animal multiplex vaccines.

2.3. V. parahaemolyticus

V. parahaemolyticus is a Gram-negative, slightly halophilic bacterium that inhabits brackish aquatic environments such as coastal and estuarine waters. Apart from being pathogenic to aquatic organisms, V. parahaemolyticus is also known as a global food-borne pathogen and one of the most common causes of gastroenteritis in East Asia due to the local dietary habit of eating raw fish and shellfish ^[38][39][38,39]. V. parahaemolyticus is antibiotic-resistant, so it cannot be treated with antibiotics which is currently the most commonly used measure in Asian fish culture [40]. Consequently, there is a pressing need to exploit fresh, effective alternatives for antibiotics against V. parahaemolyticus (Table 3), while the vaccine is the most promising approach due to its economy, efficacy and safety in public awareness ^[40][41][42][40,41,42]. [44]. For this reason, using modern methods for understanding and developing new V. parahaemolyticus immunogenic proteins and antibodies are necessary ^[45][46][45,46].

2.4. V. cholerae

V. cholerae is a Gram-negative motile bacterium that can cause fatal pandemic diseases. There are millions of cholera cases worldwide every year, and the mortality rate is extremely high [47]. Consuming contaminated seafood by mistake is one of the reasons why people are infected with V. cholerae. As an important food-borne pathogen, V. cholerae is widely distributed in fish, which brings serious safety hazards to human and aquatic animal health [48].

2.4.1. Antibiotics

V. cholerae is one of the important pathogens related to fish vibriosis. In the bluegill sunfish that died in the farms of Guangdong around 2018, the pathogen identified was non-O1/non-O139 V. cholerae. The antibiotic sensitivity displayed that the isolated strain was sensitive to azithromycin, chloramphenicol, neomycin, norfloxacin, doxycycline, etc. The possible method to prevent infection of bluegill sunfish is to give neomycin or doxycycline for seven days [49].

2.4.2. Vaccines

After consulting a large number of data, rwesearchers found that the current prevention and control of V. cholerae in Asian fish culture is still based on antibiotics, and no V. cholerae vaccine for aquatic animals has been found yet. Nevertheless, in recent decades, the misuse of antibiotics has resulted in the emergence and spread of drug-resistant bacteria in the environment, which is likely to pose a threat to public health [50]. Therefore, people are also constantly exploring new methods for the prevention and control of V. cholerae (Table 4).

Vaccines

• Inactivated vaccines

A polyvalent Vibrio vaccine had already been commercially used in Indonesian fish farming recently for tiger grouper (Mycteroperca tigris) and had shown effective protection against V. parahaemolyticus and two other Vibrio pathogens [43].

• Recombinant vaccines

A study on cross-protection also found a recombinant cell vaccine had successfully induced an immune response to V. parahaemolyticus in juvenile sea bass by expressing the OmpK of Vibrio [27]. At the same time, another study has pointed out the limitation of recombinant OmpK in preparing diagnostic antibodies

2.4.3. Edible Antibodies

In 2015, a dominant non-O1 V. cholerae L1 strain was isolated from diseased carp in a breeding farm in Jiangsu, China. The researchers used egg yolk powder (IgY) against non-O1 vibrio cholerae to prove its effective effect on diseased carp [51]. This is one of the new methods to control V. cholerae.

2.4.4. Bacteriophages

In addition to the above emerging strategies, a phage prevention and control method has also been proposed [52], which has become a highly potential prevention and control measure in the future. Nevertheless, researchwers have not found any information about the bacteriophage therapy of V. cholerae. This may be due to the difficulty in developing efficient phage administration mechanisms, different types of aquaculture systems, and the lack of a specific regulatory frame [53]. As a kind of pathogenic bacteria that is very harmful to Asian fish culture, the lack of V. cholerae vaccine prevention technology is indeed a big gap in the prevention of aquatic animal diseases.