Atlantic salmon, Salmo salar L. are broadly susceptible to isolates representing the North American genogroups of infectious hematopoietic necrosis virus.

Beginning in 1992, three epidemic waves of infectious hematopoietic necrosis, often with high mortality, occurred in farmed Atlantic salmon Salmo salar L. on the west coast of North America. We compared the virulence of eleven strains of infectious hematopoietic necrosis virus (IHNV), representing the U, M and L genogroups, in experimental challenges of juvenile Atlantic salmon in freshwater. All strains caused mortality and there was wide variation within genogroups: cumulative mortality for five U-group strains ranged from 20 to 100%, four M-group strains ranged 30-63% and two L-group strains varied from 41 to 81%. Thus, unlike Pacific salmonids, there was no apparent correlation of virulence in a particular host species with virus genogroup. The mortality patterns indicated two different phenotypes in terms of kinetics of disease progression and final per cent mortality, with nine strains having moderate virulence and two strains (from the U and L genogroups) having high virulence. These phenotypes were investigated by histopathology and immunohistochemistry to describe the variation in the course of IHNV disease in Atlantic salmon. The results from this study demonstrate that IHNV may become a major threat to farmed Atlantic salmon in other regions of the world where the virus has been, or may be, introduced.

Route of vaccine administration: effects on the specific humoral response in rainbow trout Oncorhynchus mykiss.

The specific humoral response of teleost fish to extracellular bacteria was examined using a rainbow trout-Vibrio anguillarum model. Treatment groups were immunized by oral, immersion, and injection routes. All 3 delivery methods conferred full protection in controlled laboratory challenges (p < 0.01). Prior to boosting, serum antibody titers did not correlate with protection in the orally and immersion-vaccinated groups, but, contrary to previous studies, titers measured 10 and 17 d after boosting correlated positively with protection in all 3 vaccinated groups. The route of administration strongly affected the magnitude of the antibody response as measured by enzyme-linked immunosorbent assay (ELISA) and Western blots; however, the antigenic epitopes recognized were not substantially altered by delivery method as evidenced in immunoblot patterns. Given that the primary and booster vaccination protocols were identical, the data suggest that all 3 vaccinated groups may have had a specific humoral response following initial immunization but that specific serum antibody levels before boosting were too low to be detected by ELISA in fish vaccinated by oral and immersion routes. An anamnestic response was evident in all 3 groups. The data support the possibility that teleosts, like higher vertebrates, have a protective immune response to extracellular bacteria that is predominantly humoral. Route of delivery may primarily affect the efficiency with which the immunogenic constituents of the vaccine are presented to the relevant recognition and effector components of the immune system.

Polyvalent vaccines in fish: the interactive effects of multiple antigens.

Fermentation and cell-grown products commonly used as fish vaccines present a relatively crude mixture of antigenic and other immunologically active components to the immune system. The specific antigenic make-up and the potential protective immunogenicity of this mixture is dependent upon the strain of the pathogen, production parameters, inactivation methods, down-stream processing, product formulation, storage conditions and the delivery method under which the final product is applied. In the past ten years, commercial vaccine products for fish have more often consisted of mixtures of multiple products, including two, three, four and five vaccines. Such mixtures present an even more extensive and complex array of antigens to the immune system. Considering the fact that not all antigens stimulate a protective immune response, that antigens vary in their immuno-dominance relative to each other and that the immune system of fish has a defined and limited capacity to respond to individual antigenic substances, it becomes increasingly difficult to formulate these complex mixtures into safe and effective commercial products. The clonal capacity of fish to respond to multiple antigens may depend on a variety of factors including host species and age, water temperature, route of administration and temporal considerations. There is growing evidence that antigenic as well as non-antigenic components of vaccines can interact synergistically or antagonistically and that they can stimulate, cross-react with, inhibit or even suppress the immune response to specific antigens. This paper reviews current knowledge about the interactive effects of antigenic components of fish vaccines and gives specific examples of how these interactions might affect the formulation and performance of multivalent products. This paper also considers the potential basis for these interactive effects and discusses technical approaches that take advantage of this knowledge for the development of new multivalent products.