White spot disease in flounder
White spot disease is a parasitic infestation affecting the skin and gills of flounder. It is caused by a tiny single-celled ciliated organism that burrows beneath the fish's skin. It is easily recognized by the small white spots that appea...
What is it?
White spot disease is a parasitic infestation affecting the skin and gills of flounder. It is caused by a tiny single-celled ciliated organism that burrows beneath the fish's skin. It is easily recognized by the small white spots that appear on the body, somewhat like grains of semolina. The disease primarily affects farmed flounder in tanks or captivity, where high fish density and poor water quality favour its spread. In the wild, it usually remains subtle and causes little harm.
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:
The most visible sign is the appearance of many small white spots, rounded in shape, scattered across the skin, fins, and sometimes the gills.
Affected fish rub against the tank walls or bottom, as if trying to scratch.
Disordered swimming, jerky movements, or conversely a fish resting listlessly on the bottom are often observed.
When the gills are affected, breathing becomes faster or laboured.
In advanced cases, the fish loses appetite, weakens, and the fins may fray or become damaged.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by a microscopic ciliated parasite that penetrates the superficial layer of the fish's skin. It develops there, feeding on cells and tissue fluid, then forms a cyst that releases numerous free-swimming infective forms into the water. These new forms seek out another fish to colonize, explaining the rapid spread of the disease within the same tank.
High fish density in the same volume of water greatly increases transmission risk.
Poor water quality, lacking oxygen or overloaded with organic matter, weakens the skin's natural defences.
Sudden temperature swings or stress from transport, handling, or environmental change make flounder more vulnerable.
Young fish and individuals already weakened by another disease are more susceptible.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
A veterinarian or aquaculture specialist diagnoses the condition by observing the characteristic white spots on the fish's skin, supplemented if necessary by microscopic examination of a mucus scraping or fin fragment. This examination allows visualization of the parasite and confirms that this disease is present rather than another skin condition.
Treatment
Management focuses on immediate improvement of water conditions and, as needed, use of antiparasitic treatments suitable for fish, applied to the tank water under professional supervision. The choice of medication must take into account the particular sensitivity of flounder and any other species present in the same tank.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies above all on maintaining good water quality: efficient filtration, regular water changes, and careful monitoring of temperature and oxygen levels.
Avoiding overcrowding in rearing tanks or aquariums is strongly recommended.
Any new flounder introduced to a tank should be observed separately for a period before mixing with other fish, to detect any signs of disease.
Reducing sources of stress—such as frequent handling or transport—helps maintain the natural resistance of fish.
Possible complications
Significant infestation of the gills can seriously hamper breathing and lead to death.
Open skin lesions provide entry for secondary infections, particularly bacterial or fungal.
In a rearing facility, an uncontrolled outbreak can quickly spread to a large number of fish and cause substantial economic loss.
When to see a veterinarian
Appearance of white spots on the skin or fins of one or more flounder
Abnormal behavior: repeated rubbing, erratic swimming, or lethargy
Rapid or difficult breathing
Persistent loss of appetite
Unusual mortality in the tank
Prognosis
The outlook is favorable if the disease is detected early and rearing conditions are promptly improved. However, advanced infestation affecting the gills, or combined with overcrowding, may result in significant losses—particularly in young fish or already weakened individuals.
Frequently asked questions
Can this disease be transmitted to humans?
No, this parasite is specific to fish and poses no risk to humans.
Can you save a fish that's already in very bad condition?
It depends on the extent of gill damage. Rapid treatment combined with water quality improvement provides the best chances of recovery.
Does the disease only affect flounder?
No, similar parasites causing white spots exist in many fish species, both marine and freshwater, but each parasite species is generally quite specific to its host.
How can you prevent the disease from coming back?
By maintaining good water quality, avoiding overstocking, and temporarily isolating any new fish introduced to the pond.
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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