Bacterial Infections (Aeromonas) in Fish

By PetMD Editorial on Aug. 11, 2008

Aeromonas Infection in Fish

Many types of bacteria can infect multiple organs of a fish. One such common infection is caused by the Aeromonas salmonicida bacteria. It is generally due to poor sanitation or nutrition, and is recognized by the red ulcers which cover the fish.

Koi and goldfish are the pet fishes most susceptible to Aeromonas infections, as are most warm water and freshwater fishes. In severe cases, it can prove fatal for fishes.

Symptoms and Types

The Aeromonas bacterial infection affects multiple systems in the fish's body, resulting in such symptoms as:

  • Enlarged eyes (exophthalmos)

  • Accumulation of fluids in the abdomen (ascitis)

  • Renal dropsy (kidney damage)

  • Ragged fins

Most infected fish have reddening of the body, with hemorrhagic spots in the gills, tail, fins, body wall and internal organs of the animal, while others display skin and gill ulcers.


Although the Aeromonas salmonicida bacteria causes the infection, injuries, seasonal changes, sharp changes in water temperature, and poor sanitation or nutrition can all put the fish in state where it is more susceptible to the bacteria.


Depending on the type of Aeromonas bacteria the fish has, the veterinarian will prescribe medication to eliminate the infection -- usually antibiotics. This medication can either be injected into the fish or added to the fish's water.

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