Koi Ulcer Disease (Aeromonas): Bacterial Ulcers in Pond Fish

Ulcer disease in koi and goldfish is a bacterial infection — usually Aeromonas — that turns bloody skin spots into deep open ulcers, often after stress, handling injury or cold-season immune suppression. Learn the signs, the triggers, and the water-quality, salt and veterinary steps that treat it.

The pathogen

Ulcer disease in koi and goldfish is a bacterial infection, most often by Aeromonas salmonicida (a non-motile agent that the Merck Veterinary Manual calls a very important disease of koi and goldfish) or by motile Aeromonas hydrophila, which causes motile aeromonad septicemia. Pseudomonas can act as a secondary invader.

Signs

Early signs are bloody spots and ulcers on the body, ragged fins, bulging eyes (exophthalmia) and sometimes 'dropsy' — a pinecone appearance from abdominal fluid. Chronic cases develop focal swelling, hemorrhage and tissue necrosis in the muscle that progress to deep crater-like ulcers discharging through the skin, with internal-organ damage in severe infections.

Triggers

Aeromonas takes hold when the fish is stressed or its immune system is suppressed. Documented predisposing factors include poor water quality, organic loading, handling and transport, marked temperature changes and hypoxia. Netting and handling injuries give the bacteria an entry route through broken skin, and cold-season immune suppression makes outbreaks more likely.

Treatment

Fix the underlying stressor first: correct water quality, reduce organic load, ensure oxygen and avoid temperature swings and rough handling.
Isolate affected fish and apply topical antimicrobial wound care to superficial ulcers (for example twice daily), which works well in pet fish.
Use salt as supportive therapy to reduce osmoregulatory stress across damaged skin — UF/IFAS lists a 0.1 to 0.3% prolonged level to cut osmotic stress, a 1% bath for 30 minutes to a few hours, or a 3% short dip for 30 seconds up to 10 minutes (remove the fish if it loses equilibrium).
For severe or systemic infections, have a veterinarian run culture and sensitivity (blood culture is an effective non-lethal method) and prescribe the appropriate antibiotic — avoid blind antibiotic use, which drives resistance.

Sources: www.merckvetmanual.com ; ask.ifas.ufl.edu (UF/IFAS VM007); pmc.ncbi.nlm.nih.gov