Aeromonas Hydrophila and Motile Aeromonad Septicemias of Fish

Aeromonas hydrophila and other motile aeromonads are among the most common bacteria in freshwater habitats throughout the world, and these bacteria frequently cause disease among cultured and feral fishes. From descriptions of fish diseases in the early scientific literature, Otte (1963) speculated that septicemic infections in fish caused by motile aeromonads were common throughout Europe during the Middle Ages. Although the bacterial etiology of these early reports was inconclusive, the pathology was similar to that observed with red leg disease in frogs, in which A. hydrophila was identified as the causal organism. Because many bacteria isolated from fish with hemorrhagic septicemias in fish were often misidentified, it is now recognized that certain isolations of bacteria ascribed to the genera Pseudomonas, Proteus, Bacillus, Aerobacter, and Achromobacter actually belonged to the genus Aeromonas. The exact etiology of disease involving aeromonads is complicated by the diverse genetic, biochemical, and antigenic heterogeneity that exists among members of this group. Consequently, motile aeromonads are often referred to as a complex of disease organisms that are associated with bacterial hemorrhagic septicemias and other ulcerative conditions in fishes.