A. P. Desbois
4.3 INFECTIOUS DISEASES OF FISH
Concepts and Types of Vaccines 53
history, only in the past few decades it has become a well-established protective measure against bacterial infections (32).
The first commercially available bacterial vaccines were against enteric red mouth disease and vibriosis, introduced in the USA in the late 1970s. Fish immersion vaccines were found to be effec- tive against vibriosis in the USA. However, a new disease, furunculosis, appeared and injectable vaccines containing adjuvants were developed in the early 1990s.
The first viral vaccine was produced by Bioveta Company for fish in 1982. The vaccine was against a carp rhabdovirus, and was based on two inactivated strains of rhabdovirus emulsified in oil and administered by injection. Most available virus vaccines are based on inactivated virus or recombinant subunit proteins for aquaculture.
With the exception of the introduction of a recombinant virus vaccine in 1995, vaccination strat- egies in the fish farming industry have been more or less unchanged over the last 20 years. In the present scenario, trial and error has been the important strategy for the designing of fish vaccines.
Vaccination against the most serious bacterial diseases has been quite successful for the large-scale commercial farmed fish varieties with a few exceptions. The usage of modern sophisticated techniques is applied as a new trend for the development of new vaccines. The development of fish genomic stud- ies and their information can be incorporated for the invention of new vaccines. Usage of DNA vac- cines are safer than live attenuated vaccines but the legal issues related with the usage of DNA and genetically modified organism vaccines limited their availability. If only the method of administra- tion, efficacy, and cost of production were considered, live attenuated vaccine would be chosen as an optimal type of vaccine. But a consumer safety measure prevents the usage of live attenuated vaccine type because of the reversal of infectivity features (4). To date, there are many vaccines used to control infections such as killed vaccines, live attenuated vaccines, subunit vaccines, and DNA vaccines (33).
The field of fish vaccinology has shown an amazing development recently. The comprehensive scientific research and valuable practical experience are responsible for the production of first gen- eration fish vaccines, which will immensely contribute to social, environmental, and economical sustainability in aquaculture worldwide.
54 Fish Vaccines
4.3.1 BacterIal dIseases
Vaccines produced against many of the bacterial diseases that are causing major problems for the aquaculture industries are listed in Table 4.1 (6). The inactivated bacterin vaccine effectively con- trols vibriosis, but for other bacteria, it is difficult to control by vaccination process. A live attenu- ated E. ictaluri vaccine is efficacious through the process of immersion of fish (4).
4.3.2 VIral dIseases
Fishing industry suffers from serious setbacks due to viral diseases that can be rapidly spread throughout fish farms (38). Vaccines are available for the most common viral diseases with respect to salmonid farming. The commercially available viral fish vaccines are summarized in Table 4.2 (6). An inactivated viral vaccine against pancreas disease is available in Ireland and a vaccine against infectious salmon anemia (ISA) is available in Canada and the USA. Different recom- binant subunit vaccines based on the infectious hematopoietic necrosis virus (IHNV) and viral
TABLE 4.1
Vaccines for Fish Bacterial Diseases
Sl.
No.
Fish Type Bacterial
Diseases
Causative Organisms
Type of Vaccine
Delivery Method
1 Salmonids. Cod/halibut.
Sea bass/bream.
Amberjack/yellow tail, rainbow trout, turbot, and
eel
Vibriosis Listonella
anguillarum; Vibrio ordalii
Inactivated Vibrio spp.
Intraperitoneal;
Immersion
2 Atlantic salmon and rainbow trout
Coldwater vibriosis
Aliivibrio salmonicida Killed bacterins Immersion or injection
3 Salmonids Furunculosis Aeromonas
salmonicida
Inactivated Intraperitoneal;
Immersion 4 Atlantic salmon and
rainbow trout
Winter ulcer Moritella viscosa Inactivated Injection
5 Salmonids Yersiniosis Yersinia ruckeri Inactivated Immersion
or oral 6 Marine Fishes Pasteurellosis Pasteurella piscicida Inactivated Immersion
or injection
7 Marine Fishes Warm water
vibriosis
Vibrio alginolyticus, Vibrio
parahaemolyticus, and Vibrio vulnificus
Inactivated Immersion or injection
8 Catfish Edwardsiellosis Edwardsiella ictaluri Inactivated, live attenuated vaccine
Intraperitoneal Immersion 9 All freshwater finfish
species, bream, bass, turbot, salmon
Flavobacteriosis Flavobacterium columnaris
Inactivated, live attenuated vaccine
Immersion
10 Salmonids Bacterial kidney
disease
Renibacterium salmoninarum
Avirulent live;
culture inactivated
Intraperitoneal;
Immersion 11 Salmonids Piscirickettsiosis Piscirickettsia
salmonis
Inactivated, live attenuated vaccine
Intraperitoneal;
Immersion
Concepts and Types of Vaccines 55
hemorrhagic septicemia virus (VHSV) membrane glycoprotein are also available. DNA vaccines encoding the viral glycoproteins are remarkably efficacious. The lack of effective viral vaccines is one of the main problems facing fish vaccinology. In near future, new and improved virus vaccines will probably be developed for many viral diseases (4).
4.3.3 protozoan dIseases
Amoebic gill disease (AGD) is caused by Neoparamoeba perurans and represents a significant threat to Atlantic salmon marine farming in several countries worldwide (39). Ichthyophthirius multifiliis is a ciliated protozoan that causes “itch” or “white spot disease.” It is difficult to control it by conventional methods.
4.3.4 typesof VaccInes
Different types of vaccines are practiced in the fish farming industry. The variety of vaccines and their preparation mode along with the administration methods were depicted in Figure 4.1 (40).
In this section, inactivated vaccines, live attenuated vaccines, subunit vaccines, virus-like particle (VLPs) vaccines, DNA vaccines, and RNA vaccines were discussed.
4.3.4.1 Inactivated Vaccines
Inactivated or killed vaccines are produced from virulent microbes, and their infectivity and patho- genic features are mutated. Without compromising the antigenicity of the microbes, these changes can be induced through physical, chemical, or radiation processes. Early vaccine trials focused on TABLE 4.2
Vaccines for Fish Viral Diseases
Sl. No. Fish Type Viral Diseases Causative Organisms
Type of Vaccine Delivery Method
1 Salmonids Infectious
pancreatic necrosis
Infectious pancreatic necrosis virus
Subunit vaccines or inactivated
Intraperitoneal;
Injection
2 Atlantic salmon Infectious
salmon anemia
Infectious salmon anemia virus
Subunit vaccines or inactivated
Intraperitoneal 3 Salmonids, sea bass,
sea bream, turbot, Pacific cod
Infectious hematopoietic necrosis
Infectious hematopoietic necrosis virus
DNA-plasmid Intramuscular injection
4 Salmonids Salmon
pancreas disease
Salmon pancreas disease virus
Inactivated Intraperitoneal
5 Rainbow and brown
trout, turbot, Japanese flounder
Viral hemorrhagic septicemia
Viral hemorrhagic septicemia virus
Inactivated; live attenuated;
DNA
Immersion;
oral 6 Catfish and trout Spring viremia
of carp virus
Rhabdovirus carpio
Subunit vaccines or inactivated
Intraperitoneal;
oral
7 Carp Koi herpesvirus
disease
Koi herpesvirus Live attenuated Intraperitoneal;
Immersion 8 Several marine fish
species, e.g., sea bass, groupers,
barramundi, halibut
Viral nervous necrosis
Betanodavirus Inactivated Intraperitoneal
56 Fish Vaccines
inactivated vaccines in aquaculture. A. salmonicida bacterin, an A. salmonicida-Vibrio anguillarum- Vibrio ordalii-Vibrio salmonicida bacterin, an ISA virus vaccine, and Y. ruckeri bacterin are the four of the eight licensed vaccines for aquaculture in the USA and all of these are killed vaccines. Killed vaccines are less efficient against diseases caused by intracellular bacterial and viral infections (40).
Inactivated vaccines are administered through injection method to achieve high efficacy (41).
4.3.4.2 Live Attenuated Vaccines
Live attenuated vaccines are prepared by neutralizing the infectivity nature of the live microbes.
The attenuated microbes are modified avirulent type. The major advantage of live attenuated vac- cines is the ability to induce both humoral and cell-mediated immunity, which includes the induc- tion of interferon, largely important in fighting viral infections. In addition to this, a smaller amount of attenuated vaccines is required to induce an effective immune response. Moreover, they can be administered through natural routes and also provide longer lasting protection, so that the need for a booster dose is not required. The licensing of live attenuated vaccines is difficult because of the safety concerns related to the reversion to a virulent form. Because of this, only few live attenuated vaccines are commercially available. However, with the usage of recent molecular methods and the identification of new and safer vectors, the introduction of more live vaccines is possible in the coming years. AQUAVAC-ESC is one of the commercially available vaccines for the catfish indus- try. AQUAVAC-ESC is produced from a strain of E. ictaluri (RE-33) and is attenuated by multiple FIGURE 4.1 Vaccine types.
Concepts and Types of Vaccines 57
passages in ascending concentrations of rifampin. Very few live attenuated viral vaccines are avail- able currently. A koi herpesvirus mutant produced by serial cell culture transfer followed by irradia- tion to completely attenuate the virus and reduce the chances of reversion to a virulent form. A live attenuated vaccine against spring viremia of carp was also produced in China (42).
4.3.4.3 Subunit Vaccines
Subunit vaccines provide chances to target immune responses toward specific microbial determinants and they can be produced in a highly characterized state. They can be freeze-dried allowing for nonre- frigerated transport and storage (40). Subunit vaccines are produced using protein expression systems like bacteria, yeast, insect cells or tissues, protozoa, mammalian cell culture, and rarely even plants.
One of the advantages of subunit vaccines is that they are safer than live attenuated vaccines since they are unable to invade the host genome or replicate within the host. But one of its main problems is the generation of incorrect or misfolded processed antigens that fail to induce a protective immune response. In recent years, the usage of Tetrahymena thermophile as an expression system reduces the problem of misfolding to a large extent. The commercially available subunit vaccines include hemag- glutinin esterase/fusion proteins, infectious salmon anemia virus (ISAV), and the VP2 and VP3 capsid proteins against infectious pancreatic necrosis virus (IPNV). The VP28 envelope protein vaccine of white spot syndrome virus (WSSV) is also used to enhance survival of shrimp (42).
4.3.4.4 Virus-Like Particle Vaccines
The self-assembly of viral capsid proteins into virus-like particles is used as VLP vaccine because they mimic the natural structure of virus. Due to the lack of genomic materials, VLPs are unable to revert to their virulent form. Because of the presence of a particulate nature, VLPs can activate both innate and adaptive immune responses. Recently, various VLP vaccines for fish diseases have been developed. Nervous necrosis virus (NNV) VLP vaccines and two IHNV recombinant viruses displaying IPNV VP2 protein were generated against both IHNV and IPNV infection. Based on recent studies, VLPs have been shown to elicit strong immunogenicity and constitute a safe alterna- tive to inactivated or attenuated vaccines (40).
4.3.4.5 DNA Vaccines
DNA encoding an antigenic protein is transformed in an expression plasmid vector under the con- trol of a strong eukaryotic promoter applied in DNA vaccine production (42). DNA vaccines are produced by combining a specific gene that codes for an antigenic protein and an expression plas- mid as vector. When DNA vaccines are administered, they expressed the antigenic protein in the host and are expected to elicit an effective immune response. They can strongly activate cellular and humoral immunity. The minimum requirement for the development of DNA vaccine is the iden- tification of a protective antigen for the respective disease (40). The first DNA vaccine was tested in rainbow trout against IHNV in aquaculture (43). Another salmonid alphavirus subtype 3 DNA vaccine is also available in the European Union. DNA vaccines are considered safer than attenuated vaccines since they only express the antigenic protein and not the entire virulent organism (40).
4.3.4.6 RNA Vaccines
There are two types of RNA-based vaccines, namely, nonamplifying mRNA vaccine and self- amplifying mRNA vaccine. RNA-based vaccines are more advantageous because RNA is nonin- fectious and there is no potential risk of insertional mutagenesis. They are also potent stimulators of immunity. The alphavirus replicate functions in fish. Thus, the self-amplifying RNA vaccine produced by replacing the genes for the structural proteins of the alphavirus with a fish patho- genic antigen of interest could potentially protect against a number of important fish diseases. The untranslated regions of the salmonid alphavirus 3 (SAV3) genome might be used to construct an SAV3-based replicon. This replicon vaccine provides high protection against ISA. Hence, the SAV- based replicon represents a good vaccine candidate for aquaculture (40).
58 Fish Vaccines