Fish Diseases and Management

7.3.6.Types of vaccines

Formalin killed – formalin acts on amino and amide groups in proteins and on non-hydrogen bonded amino groups in the purine and pyramidine bases of nucleic acids to form cross links which preserve the structural integrity of immunogenic and antigenic determinants. Formalin 0.3% v/v has been used to kill Aeromonas hydrophila, Edwardsiella ictaluri, Pasteurella piscida, Pseudomonas, Vibrio anguillarum, V. ordalli. Other chemicals like chloroform and phenol, physical disruption by sonication, lysis with sodium hydroxide at pH 9.5 or the detergent sodium sulphate has been used to inactivate bacteria for experimental vaccines. Heat killing denatures proteins, oxidises lipids and destroys crucial antigenic sites present on the pathogens.

Attenuated vaccines have the advantage of mimicking a natural infection by wild type virus. This type of vaccination also has the advantage of delivering protective antigens to the appropriate site and in the proper context to generate correct immune response. Attenuated vaccines are however, not normally used in fish due to the scare of reversion of the pathogen to a more virulent one. This might happen when the organism is exposed to unfavourable culture conditions. Apart from reversion, there is a possibility that the pathogen may be avirulent in the target species but may cause infection in unrelated species and cause widespread infection.

Recently, new trends in vaccination have emerged incorporating latest techniques in biotechnology. These include production of subunit vaccines, synthetic peptides, DNA vaccines and mutant viruses. Currently several commercial fish vaccines are available in the market. These include those against enteric red month, vibriosis, furunculosis, infectious pancreatic necrosis virus, viral hemorrhagic septicaemia etc. More and more vaccines are being tried against other infectious diseases as well.