Bacterial Skin Infection in Flatfish
Bacterial skin infection in flatfish is a disease affecting the skin of this bottom-dwelling or farmed fish. It causes lesions, redness, or small wounds on the body surface. The disease is common in weakened fish or those living in poor wat...
What is it?
Bacterial skin infection in flatfish is a disease affecting the skin of this bottom-dwelling or farmed fish. It causes lesions, redness, or small wounds on the body surface. The disease is common in weakened fish or those living in poor water conditions. It can affect both wild and farmed fish in aquaculture or pond settings.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Red or discolored patches appear on the skin, sometimes with small open sores or crusts. The fish may rub against the bottom or tank walls, as though scratching at an itch. The fins may appear frayed or damaged. The affected fish often swims abnormally, remains still, or stays near the surface. It may stop eating and appear weak.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Several bacteria naturally present in seawater can cause infection when the fish's skin is compromised. These bacteria take advantage of a minor wound, stress, or lowered natural defenses to establish themselves and multiply in the skin.
Poor water quality, excessive warmth, low oxygen, or pollution favors the disease. Overcrowding in farm ponds, injuries from handling or contact with rough surfaces, and stress from transport also increase risk. Fish already weakened by another disease or parasites are more vulnerable.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on observation of the skin lesions and examination of the fish's general condition. A specialist in fish pathology may take a sample from the lesion to identify the bacteria responsible and adjust treatment accordingly.
Treatment
Treatment uses antibiotics suitable for fish, typically administered in the water or through food, under specialist supervision. Improvement of water quality should accompany antibiotic treatment as standard practice.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Maintaining good water quality is essential: appropriate temperature, adequate oxygenation, and absence of pollutants. Overcrowding in tanks should be avoided, and fish should be handled carefully to prevent skin damage. Quarantining new fish before introducing them into an existing population helps reduce the risk of introducing bacteria.
Possible complications
Without treatment, lesions can spread and deepen, leading to more severe ulcers. The infection can also weaken the fish significantly and make it more vulnerable to other diseases, or even lead to death in severe cases.
When to see a veterinarian
Red patches or open wounds appearing on the fish's skin. Fish rubbing repeatedly against decorations or tank walls. Frayed or damaged-looking fins. Loss of appetite or unusual behavior in the fish. Unexpected deaths in the tank or aquarium.
Prognosis
The prognosis is favorable when the disease is caught early and water quality is corrected. Without treatment, the outcome can be poor, especially in fish that are already weakened.
Frequently asked questions
Can this disease be transmitted to humans?
No, it is not a transmissible disease to humans. It remains specific to fish.
Can you treat a fish on your own, without specialist advice?
It is strongly advised against using medications without specialist advice, as improper treatment can worsen the fish's condition or disrupt an entire breeding pond.
How to prevent this disease in livestock farming?
Prevention relies mainly on good water quality, reasonable fish stocking density, and gentle handling to avoid injuries.
Does this infection affect only diseased flatfish?
No, even apparently healthy fish can develop this infection if their skin is injured or if the water quality is poor.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 22 August 2026
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