Molecular approaches to fish vaccines

For more than 50 years, researchers have tested a variety of killed. attenu ated, and subunit preparations for control of fish diseases. The earliest f ish vaccines used killed preparations containing whole bacteria, viruses, o r parasites and today, several bacterins have become commercially successfu l with more expected as improved delivery systems and adjuvants are realize d. Live, attenuated vaccines have been developed by serial passage of a pat hogen in culture or by using naturally occurring mutants and cross-reacting strains. These generally offer excellent protection and are cost-effective , but concerns about residual virulence or their effects on other aquatic s pecies make them difficult candidates for licensing. In recent years, the t ools of molecular biology have been applied to construction of a variety of recombinant, engineered, or subunit vaccines for fish. Among the approache s to be discussed are: attenuated strains of viruses and bacteria created b y deletion of specific genes associated with virulence, in vitro synthesis of protective antigens from genes cloned into E. coli or baculovirus expres sion systems, chemical synthesis of peptides that represent protective epit opes, and direct immunization with DNA coding for protective antigens. Prep arations representing each of these approaches have been tested in laborato ry or field trials with various results and such vaccines promise to be saf e and relatively inexpensive if they are able to provide protection when de livered by immersion. A significant advantage of genetically engineered vac cines is the ability to construct multivalent preparations that can protect fish against several pathogens or different strains of the same pathogen. While many of these novel vaccine strategies have been effective at stimula ting specific immunity in the laboratory, more work is needed to develop be tter delivery systems and to overcome potential regulatory concerns.