Finfish Health: Key Diseases, Pathogens & Treatments

Aeromonas spp.

A group of Gram-negative, aerobic bacteria frequently isolated from freshwater fish; major cause of hemorrhagic septicemia, dropsy, fin rot, skin ulcers and furunculosis.

Pseudomonas spp.

Gram-negative bacteria commonly affecting freshwater fish, often associated with hemorrhagic septicemia, fin and tail rot, and skin ulcer disease.

Vibrio spp.

Marine Gram-negative bacteria that cause septicemia and ulcerative conditions in salt-water fish.

Hemorrhagic Septicemia (fish)

Systemic bacterial infection marked by external reddening, body‐wall hemorrhage and visceral bleeding; morbidity rises with stress, poor water quality and hypoxia.

Dropsy

Fish disease caused mainly by Aeromonas hydrophila; characterized by fluid accumulation in the body cavity, loose scales and a swollen abdomen that exudes water when pressed.

Fin and Tail Rot

Progressive disintegration of fins and tail with whitish thread-like edges; usually due to Aeromonas or Pseudomonas infections.

Skin Ulcer Disease

Bacterial condition (Aeromonas/Pseudomonas) beginning as reddish pimples that develop into hemorrhagic ulcers across the skin.

Furunculosis

Deep ulcerative disease of fish caused by Aeromonas salmonicida, producing open red sores (furuncles).

Columnaris Disease

Bacterial infection by Flexibacter (Flavobacterium) columnaris causing grey patches on head and back and ‘saddle-back’ appearance.

Flexibacter / Cytophaga columnaris

Slender, filamentous bacteria responsible for columnaris disease in freshwater fish.

Cotton Wool Disease (Saprolegniasis)

Fungal infection by Saprolegnia or Achyla producing cottony, grey-white growths on skin and eggs.

Branchiomycosis (Gill Rot)

Fungal disease caused by Branchiomyces demigrans; hyphae block gill blood vessels leading to respiratory distress and death.

Epizootic Ulcerative Syndrome (EUS)

Seasonal (Nov–Feb) ulcerative condition of warm-water fish involving invasive Aphanomyces fungus and granulomatous skin lesions.

Aphanomyces spp.

Water moulds that invade tissue during EUS, producing extensive necrotic ulcers in freshwater and estuarine fish.

White Spot Disease (Ich)

Parasitic infection by Ichthyophthirius multifiliis; seen as tiny white nodules on skin, fins and gills.

Argulosis

Infestation by the crustacean parasite Argulus (fish louse) causing irritation, hemorrhagic patches and erratic swimming.

Anchor Worm (Lernaeasis)

Parasitism by Lernaea species; worms embed in skin causing ulcers, scale loss and severe stress.

Whirling Disease

Myxosporean infection (Myxosoma cerebralis) leading to deformities of vertebrae and characteristic tail-chasing ‘whirling’ behavior.

Slime Disease

Protozoan skin infection (Ichthyobodo, Chilodonella or Trichodina) producing grey-blue mucus film and pale gills in aquarium fish.

Broken Back Disease

Skeletal deformity in fish caused by vitamin C deficiency, resulting in a curved or ‘broken’ spine.

No Blood Disease

Anemia and emaciation linked to folic acid deficiency in fish; leads to gradual weakness and death.

Environmental Diseases

Non-infectious health problems from poor water quality—low dissolved oxygen, elevated ammonia/nitrite, or toxins—major cause of losses in aquaculture.

Potassium Permanganate (KMnO₄)

Broad-spectrum oxidizing agent widely used as a bath or pond treatment (1–5 mg L⁻¹) against bacterial, fungal and parasitic infections.

Quarantine Measures

Isolation of new or sick fish before introduction to a stock, reducing risk of spreading infectious agents.

Hypoxia (Low Dissolved Oxygen)

Environmental stressor that predisposes fish to bacterial outbreaks, increases mortality and must be managed in culture systems.