Bacterial skin infection in sole
Bacterial skin infection in sole is a disease that affects the skin of this flat fish raised in aquaculture or found in the wild. It appears as lesions, redness, and sometimes ulcers on the body. This condition is common in farms where wate...
What is it?
Bacterial skin infection in sole is a disease that affects the skin of this flat fish raised in aquaculture or found in the wild. It appears as lesions, redness, and sometimes ulcers on the body. This condition is common in farms where water quality or fish density favors the spread of opportunistic bacteria. It may affect a single fish or spread quickly through a tank.
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:
Early signs are often subtle: the fish swims less actively or remains still at the bottom of the tank.\nReddened areas appear on the skin, potentially developing into open wounds or ulcers.\nFins may become frayed or appear ragged at the edges.\nThe affected fish sometimes rubs against the tank walls or substrate, a sign of irritation.\nLoss of appetite and general lethargy often accompany advanced cases.\nIn severe forms, wounds that fail to heal may expose underlying tissue.
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 opportunistic bacteria naturally present in water can be responsible, notably species of the genus Vibrio or Pseudomonas, among others. These bacteria take advantage of an initial skin injury (trauma, parasite, stress) to establish themselves and multiply, especially when the fish's natural defenses are weakened.
Poor water quality (low oxygen levels, high ammonia, unsuitable temperature) weakens the fish's protective skin barrier.\nExcessive stocking density promotes stress and transmission between fish.\nInjuries from handling, transport, or external parasites create entry points for bacteria.\nA weakened immune system from chronic stress or inadequate diet increases susceptibility.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
A veterinarian specializing in aquaculture or fish pathology examines visible lesions and the fish's overall condition. Skin samples may be analyzed in the laboratory to identify the responsible bacterium and guide the most appropriate treatment.
Treatment
Treatment typically relies on antibiotics suited to the species and identified bacterium, prescribed only by a veterinarian. Improved water quality and stress reduction always accompany management of the condition. Local disinfection care may be recommended depending on the severity of lesions.
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 the most important measure: regular monitoring of the tank's physical and chemical parameters.\nAvoid overstocking fish in farming facilities.\nHandle sole with care to minimize skin injuries.\nQuarantine any new fish before introducing it into an existing tank.\nProvide balanced nutrition to support natural defenses.
Possible complications
Without treatment, the infection can spread deep into muscle tissue and internal organs. Septicemia (generalized infection) is possible in severe cases and may lead to the fish's death. Spread to other fish in the same tank is also a significant risk in farming operations.
When to see a veterinarian
Appearance of redness, wounds, or ulcers on the fish's skin.
Abnormal swimming, prolonged immobility, or isolation of the fish.
Persistent loss of appetite.
Unusual or grouped mortality in the same tank.
Prognosis
The outlook is favorable if the disease is detected early and water quality is corrected quickly. However, advanced or generalized infections carry a more cautious outlook, particularly in high-density farming operations.
Frequently asked questions
Can this disease be transmitted to humans?
No, it is not a disease transmissible to humans. However, standard hygiene precautions are still recommended when handling sick fish.
Can you treat a diseased sole without veterinary advice?
It is strongly advised against using antibiotics without a precise diagnosis. Improper treatment can worsen the situation and promote the emergence of antibiotic-resistant bacteria.
How do you know if the entire basin population is affected?
Daily monitoring of fish behavior and appearance allows for quick identification of additional cases. Veterinary advice is recommended as soon as a fish shows any suspicious signs.
Can seawater be responsible for this infection?
Water itself naturally contains certain bacteria. It is mainly an imbalance in its quality that promotes their proliferation and the development of disease.
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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