The bacteria that make our fish sick are always present in aquaria and are opportunistic pathogens.that typically take advantage of fish compromised by poor water quality, stress and/or parasites.

The major groups of bacteria that affect fish are gram negative (the most common), gram positive, and acid-fast bacteria.

Gram-negative bacteria

These can be systemic or result in ulcers are the most common bacterial pathogens of fish. These include Aeromonas, Vibrio, Edwardsiella, Pseudomonas, Flavobacterium, etc..

Symptoms of systemic bacterial infections include: lethargy, lack of appetite or not eating, abnormal swimming patterns or spinning, hemorrhagic lesions on the skin, ulcers, swelling of the belly, fluid in the belly, changes in buoyancy, pop-eye or fluid around the eyeball, darkening of the skin, damage to the gills, and ultimately death. If the gill are damaged, symptoms reflect oxygen seeking behavior which can include increased respiratory rate, piping (gasping at the water surface), etc.

Septicemia caused by Aeromonas is generally secondary to stress or poor water quality. Symptoms may include hemorrhages of the skin and ulcers, bleeding of the internal organs, fluid retention to include  dropsy or ascites, and pop-eye or excess fluid around the eyes.

Ulcerative dermatitis may appear as red, raised and or missing scales and ulcers and may progress to septicemia. Loss of skin integrity may result in fluid overload with dropsy and.or pop eye.

Gram-negative bacteria that typically cause external infections include Flavobacterium columnarae, Flexibacter maritimus, yellow-pigmented bacteria, Cytophaga spp, etc..

Symptoms include cottonly patches on the skin and/or fin rot. It may also affect the gills.

Treatment: for gram-negative bacterial infections

• Make sure water quality is good and deal with external parasites if present.
• For internal infections, Injections or medicated food are preferred. Culture and sensitivity should be obtained if possible. Antibiotics include: Amikacin, Aztreonam, Ceftazidime (Fortaz), Enrofloxacin (Baytril), Florfenicol, Kanamycin, Oxolinic acid, Oxytetracycline, Nitrofurazone, Sulfadimethoxine/ormethoprim (Romet), and Trimethoprim sulfa.
• For external infections antibiotics baths may be used or potassium permanganate baths. Injections or medicated food may be necessary in severe cases.

Gram-positive bacteria

Gram-positive bacteria thar grow rapidly and cause systemic infections: Streptococcus and related species.

Symptoms are similar to the systemic gram-negative infections and include skin discoloration, pop eye, fluid in the belly, ulcers, bleeding, granulomas of the kidney spleen or liver. abnormal swimming behavior such as spiraling or spinning are very common in fish with streptococcal infections and indicate the central nervous system is affected.

Treatment: for gram-positive bacterial infections:

• Make sure water quality is good and deal with external parasites if present.
• Antibiotics include: erythromycin (1.5g/lb of food, feed 10–14 days), amoxicillin, ampicillin, and florfenicol.

Acid-fast bacteria

Acid-fast bacteria such as Mycobacterium grow slowly and cause systemic infections that result in granulomas  that affect internal organs. Mycobacteriosis is usually chronic, progresses slowly, and causes low-to-moderate mortalities. The most common species are M marinum and M fortuitum. Symptoms are typically nonspecific and may include ulcers, lack of appetite, weight loss lethargy, pop-eye, swollen abdomen, fin or tail rot, and skeletal abnormalities. There is no known treatment.

Summarized from:

Bacterial and Parasitic Diseases of Pet Fish

By Helen Roberts, Brian Palmeiro, and Scott Weber

Veterinary Clinics: Exotic Animal Practice, Vol 12 (2009), pp 609–638

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