Vaccine‐induced protective immunity against <i>Aeromonas salmonicida</i> tested in experimental carp erythrodermatitis

Abstract

Abstract The development and applicability of a dose‐controlled experimental infection with atypical Aeromonas salmonicida in carp is described. The proliferation and clinical manifestations of experimentally induced carp erythrodermatitis mimicked a natural infection. An in‐vivo assay was used to evaluate the lethal properties of cell‐free culture supernatants and a simple serum‐free growth medium was devised for maintaining the virulence of the challenge strain. Depending on the inoculation dose, a sublethal (chronic) to a lethal (acute) infection could be induced, and a dose‐response relation was observed between A. salmonicida inoculum size and carp mortality. The dose‐controlled experimental infection was used as a challenge test for laboratory evaluation of the efficacy of potential vaccine candidates. The vaccine candidates tested, a cell envelope preparation, purified lipopolysaccharide and purified A‐layer (ACE) protein showed no protection or only a feeble one at the best, while formalinized whole cells showed a consistent but only moderate protection. In contrast, when concentrated, detoxified culture supernatant was used, the carp were protected against a subsequent lethal challenge. These observations indicate that immunity against A. salmonicida extracellular products is of prime importance.