White spot disease in flatfish
White spot disease is an external parasitic infestation affecting the skin and gills of flatfish such as sole. It is caused by a ciliated protozoan that burrows into the skin, creating small visible cysts that appear as white spots on the f...
What is it?
White spot disease is an external parasitic infestation affecting the skin and gills of flatfish such as sole. It is caused by a ciliated protozoan that burrows into the skin, creating small visible cysts that appear as white spots on the fish's body.
This disease is common in aquaculture, especially when fish density is high or water quality is poor. It can progress rapidly and spread throughout a tank within days.
Without treatment, it can cause significant mortality, particularly in young sole or weakened fish.
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 distinctive sign is the appearance of multiple tiny white spots resembling grains of salt scattered across the skin and fins of the sole.
Affected fish often rub their bodies against the tank walls or bottom, as if scratching. This behaviour reflects the irritation caused by the parasite in the skin.
Disorderly swimming is also observed, with fish remaining motionless or becoming unusually agitated. Appetite decreases noticeably and the animal becomes lethargic.
If the gills are affected, respiratory distress may occur, with gill cover movements faster than normal.
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 ciliated protozoan parasite that embeds itself in the superficial layers of skin and gills. The parasite then multiplies in the tank water in the form of cysts, releasing numerous infective forms capable of rapidly contaminating other fish.
The parasite's life cycle is highly sensitive to water temperature: it develops faster in warmer water, which explains rapid outbreaks during warming periods.
High fish density in the tank promotes rapid spread of the parasite.
Poor water quality, inadequate filtration or insufficient water renewal weakens the natural defences of the sole's skin.
Stress from transport, handling or sudden changes in temperature or salinity makes fish more vulnerable.
Introduction of new fish without prior quarantine is a frequent source of parasite introduction into a disease-free facility.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian or aquaculture technician makes the diagnosis by observing white spots directly on the fish's skin, confirmed by microscopic examination of skin or gill scrapings to identify the characteristic parasite.
Water testing (temperature, quality, population density) also helps guide the diagnosis and identify triggering factors.
Treatment
Treatment involves antiparasitic medications appropriate for flatfish, applied to the tank water under professional supervision, taking into account the sole's particular sensitivity to certain substances.
Improvement of water quality and stress reduction accompany antiparasitic treatment to support recovery.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies primarily on maintaining good water quality: efficient filtration, regular water changes, and monitoring of temperature and salinity.
It is recommended to quarantine all new fish before introducing them to a healthy population, to detect any potential infestation before the group is exposed.
Keeping stocking density low and reducing stress factors (frequent handling, transport) also helps prevent the disease from developing.
Regular disinfection of equipment and tanks helps interrupt the parasite's life cycle.
Possible complications
Severe infestation can cause significant damage to the skin and gills, allowing secondary bacterial or fungal infections to take hold.
Without prompt care, mortality can be high, particularly in young soles or those already weakened by other stress factors.
When to see a veterinarian
Appearance of white spots on the skin or fins of multiple fish
Fish rubbing against tank walls or bottom
Reduced appetite or unusual lethargy in the population
Unexplained mortality in the tank
Signs of respiratory distress in the fish
Prognosis
The prognosis is good if the disease is detected early and treated promptly, with improved water quality. However, it becomes guarded in cases of heavy infestation or delayed treatment, particularly in juvenile soles.
Frequently asked questions
Can this disease be transmitted to humans?
No, this parasite is specific to fish and poses no risk to humans.
Do white spots disappear on their own?
Rarely without intervention. Without treatment and improved husbandry conditions, infestation tends to worsen rapidly.
How can we prevent the disease from spreading throughout the entire basin?
Quickly isolate affected fish, improve water quality, and consult a professional for appropriate treatment of the entire pond.
Are farmed soles in Tunisia particularly vulnerable?
Like any aquaculture operation, sole farms can be affected if water quality or stocking density favor parasite development, regardless of region.
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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