Enteric Redmouth Disease (ERM)

Enteric Redmouth Disease (ERM)

Enteric Redmouth Disease (ERM)*

Enteric Redmouth Disease or (ERM) is a bacterial infection found in both freshwater and marine fish. Redmouth disease was first observed in in the 1950s in Idaho rainbow trout hatcheries. It is caused by the pathogen Yersinia ruckeri which is primarily found in cold water fishes like rainbow trout and other salmonids.

Being a cold water disease, it is relatively rare in tropical fish keeping circles where water temperatures are elevated, but it is occasionally found in Koi and goldfish ponds and aquariums. Mortality rates are generally high when not promptly treated.


Enteric redmouth disease is characterized by hemorrhaging of the fish’s mouth, fins, and eyes.  It is seen as a reddening of the mouth, gill tips, throat, fins, and eventually (if not treated) total erosion of the jaw and palate of the fish.  As the disease progresses, hemorrhaging will also occur in the internal organs which causes the abdomen of the fish to be filled with a yellowish fluid.  The fish will exhibit a pot bellied appearance, show abnormal behavior and become anorexic.


As with most preventable tropical fish diseases, Enteric Redmouth Disease is caused by poor water quality and overcrowding.

Although Redmouth Disease is mainly transmitted by direct contact with the infected fish, asymptomatic fish will often carry the bacteria in their lower intestines until a stressful situation occurs.   The infected fish then sheds the bacteria into the surrounding water through it’s feces where it can survive for several months in the sediment and other biofilm on surfaces of the aquarium or pond.


Several antibiotics are available for the treatment of Redmouth Disease in fish.   API Furan-2, API Melafix, and Fish Aid Amoxicillin are among some of the fish medications that can be used to treat Redmouth Disease.


The best way to prevent Enteric Redmouth Disease in ponds and cold water aquariums is by maintaining pristine water quality in the system.   Maintaining a low stocking level in ponds and aquariums is absolutely essential in the prevention of Redmouth Disease as well as most other bacterial infections.  Poor water quality is usually the culprit with most tropical fish diseases.

*Photo credit Alaa Eldin Eissa

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