Strengths and weaknesses of different challenge methods

The development of effective fish vaccines is greatly dependent on reproducible standardised challenge methods. Different test protocols involving bath exposure, intra-peritoneal and intra-muscular injection and co-habitation are all well described in the literature. Ideally, to represent the many facets of infectious diseases in fish, a test challenge method should closely mimic natural exposure to the pathogen and ensure that immune mechanisms located in the body surfaces play their role. Bath and co-habitation challenges best fulfil this requirement. On the other hand, these methods are more difficult to control and standardise than injection methods. The pathogenicity of some strains of bacteria may also be insufficient to induce a satisfactory outbreak of disease in the target species when using bath or co-habitation challenge. A common measure of efficacy of a vaccine is by Relative Percent Survival (RPS), which expresses the proportional relationship between mortalities in a vaccinated group compared to unvaccinated controls. It has been demonstrated that RPS may vary significantly between challenge methods in groups of Atlantic salmon vaccinated with the same batch of vaccine. Also, RPS may vary for the same batch of vaccine when used in identical test systems at different times. Great care should therefore be taken when competitive products evaluated in different tests are compared. As several vaccines have been marketed and shown to have very good efficacy under field conditions, such products could be introduced as reference standards when testing new candidates for marketing authorisation.