Isolation of Aeromonas salmonicida subspecies salmonicida from Lake Whitefish (Coregonus clupeaformis) inhabiting Lakes Michigan and Huron

Herein we describe the first report of Aeromonas salmonicida subspecies salmonicida infections in lake whitefish (Coregonus clupeaformis) collected from four sites in lakes Michigan and Huron, Michigan, USA. The bacterium was isolated from the kidneys of four out of 1286 lake whitefish that were tested over a three-year period. The four isolates were phenotypically similar to one another and exhibited the morphological, colonial, and biochemical traits typical of A. salmonicida subspecies salmonicida. Amplification of 16S rRNA genes specific to A. salmonicida subspecies salmonicida via polymerase chain reaction and subsequent gel electrophoresis analyses confirmed the identity of the four lake whitefish isolates. Clinical signs associated with infection included extensive external hemorrhaging, exophthalmia, splenomegaly, splenic and renal congestion, fibrinous adhesions of the spleen and liver, and hemorrhagic enteritis. Histopathological examination of infected fish revealed multi-focal hemorrhage and infiltration of lymphocytes and histiocytes in subdermal adipose tissues and musculature. A low infection incidence of A. salmonicida salmonicida in Great Lakes lake whitefish does not preclude the fact that overt signs of disease were observed in infected individuals and that lake whitefish may act as a reservoir for this bacterium that is highly pathogenic to numerous fish species.