Aquaculture activities in the country have suffered a lot from the occurrence and spread of diseases. The issue of aquatic animal diseases is one of the main challenges of aquaculture in Iran and the world which annually causes heavy damage to the farmers. In recent years, aquaculture products have been seriously threatened due to the lack of control over many common diseases caused by various pathogens. One of the most important measures in controlling and preventing damage caused by diseases is vaccination along with health management and optimal management of aquaculture, which purposeful and program - oriented processing should be on the national agenda. Vaccines stimulate the fish immune system to produce antibodies and protect the fish from diseases. Vaccination strategy is determined based on the following items: Type of disease, type of vaccine, method of vaccination, time of vaccination and need for repeat vaccination. An important point for the development and commercialization of a vaccine is that the methods used for the target fish species should be appropriate to the ecological and epidemiological conditions of the disease such as the seasonal prevalence of disease, the size of the fish and the geographical area in which the disease occurs. Another point to consider is that the fish immune system should be fully developed both visually and functionally at the time of vaccination. Side effects (environmental effects and drug residues) and risks of drug therapy have limited the use of drugs for therapeutic purposes, and this limitation is increasing day by day. Therefore, increasing fish resistance to disease through natural or acquired immunity is an effective and important factor for the health of farmed fish. Vaccination is generally a safe and economically justifiable preventive measure. Therefore, in modern aquaculture, disease prevention based on stimulating the immune system of fish is the basis for reducing the economic losses caused by diseases.
Vaccine efficacy is expressed as the mortality rate of the vaccinated group relative to the unvaccinated group after experimental challenge in the laboratory with the pathogen or in the farm and natural environment after exposure to the pathogen.
Factors affecting the effectiveness of vaccination
1. Type of vaccine
1-1- Inactivated vaccine (Killed vaccine): Most bacterial vaccines used in aquaculture are inactivated vaccines called bacterins and because all bacterial cells are used together with extracellular products, they work well.
1-2- Live attenuated vaccines: One of the advantages of these vaccines is the stimulation of cellular and blood immunity and one of its disadvantages is the persistence of the pathogen in fish and the environment, the return of the severity of the disease and complete uncertainty about their safety.
1-3- Subunit vaccines: These vaccines contain a protein from an infectious agent that stimulates the immune system. One of these vaccines is the IPN vaccine. These vaccines are safe but do not provide adequate cellular immunity.
1-4- DNA vaccines: These vaccines encode a gene, so it is not possible to return to the acute form of the disease caused by the pathogen which is an important factor in relation to environmental safety for aquatic organisms.
1-5- Autogenous vaccines (indigenous - regional): These types of vaccines are produced on a specific farm based on the strain. Many researchers consider the use of autogenous vaccines to be a better way to prevent disease in aquaculture. Public demand for such vaccines which are produced under the control and supervision of regional aquaculture authorities is now increasing.
2. Monovalent and polyvalent vaccines: The most ideal formulation of the vaccine is polyvalent that at the same time, it provides protection against most diseases to which a particular species of fish is exposed. In addition, polyvalent vaccines should cover all major serotypes of each pathogen in a particular geographic area. Of course, it is important to note that sometimes antigenic competition occurs especially when the vaccine is given by injection.
3. Method and strategy of using the vaccine
3-1- Oral method: The simplest method for vaccinating fish in large numbers is the oral method. Bacterins can be taken orally and supplied to the immune system. The efficiency of this method can be increased by using the carrier – mediated delivery method. From an economic point of view, the oral method is the most ideal method for vaccinating fish, but it requires a booster dose. The main drawback of oral vaccination is its high error rate.
3-2- Immersion vaccination (dip and bath): In this method, the fish are placed in a solution containing the desired antigen for a certain period of time to be vaccinated.
3-3- Injection method: In the injection method, various injection methods such as intraperitoneal, intramuscular, subcutaneous, etc. can be used. Among these methods, intraperitoneal injection method is more common. Although the use of injection method on a large scale is associated with administrative problems such as catching and injecting individual fish (and of course stress, spending more time and money), but due to the direct and reliable delivery of sufficient amounts of antigen to each fish, it is always superior to other methods in terms of efficiency.
The point to consider in the vaccination of aquatic organisms is that, unfortunately, all commercial vaccines on the market are made against common strains in specific geographical areas and countries and do not work well against strains in other regions. Therefore, it is necessary to identify the dominant pathogens in each region and make vaccines from native strains.