Aeromonas salmonicida infection in wrasse (Labridae), used as cleaner fish, on an Atlantic salmon, Salmo salar L., farm

Abstract. Typical Aeromonas salmonicida with similar biochemical characteristics to A. salmonicida from Atlantic salmon, was isolated from wrasse, Ctenolabrus rupestris (L.) and Centrolabrus exoletus (L.), stocked as cleaner fish with these salmon. Although no external clinical signs were apparent, localized bacterial microcolonies were observed in muscle, gills, intestine, kidney and myocardial tissue. Mortalities attributed to A. salmonicida comprised 55% (n = 32) of total mortalities. No carriers of A. salmonicida were found in wild wrasse following stress testing. Although salmon post-smolts died when challenged with 1 × 10⁵ ml^-1 of a virulent strain, there were no mortalities in challenged wrasse. An oral route of infection is suggested rather than water-borne transfer as wrasse browsed on salmon mortalities. Wrasse were treated for A. salmonicida infection by injection with antibiotic and were also vaccinated, and in the latter case, elevation of antibody levels was noted.