Gastrointestinal parasitism in turbot (roundworms)
Gastrointestinal parasitism in turbot caused by roundworms (nematodes) is a parasitic disease affecting the digestive tract of this flatfish, whether farmed in aquaculture or living in the wild. Microscopic to visible cylindrical worms set...
What is it?
Gastrointestinal parasitism in turbot caused by roundworms (nematodes) is a parasitic disease affecting the digestive tract of this flatfish, whether farmed in aquaculture or living in the wild.
Microscopic to visible cylindrical worms settle in the turbot's stomach or intestines. They feed on the animal's tissues or digestive contents, disrupting its digestion.
This disease mainly affects turbot raised in tanks or cages, where overcrowding promotes parasite transmission between individuals.
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:
Infested turbot eat less or refuse food.
They progressively lose weight despite sometimes maintaining appetite, because nutrients are diverted by the parasites.
Slowed growth is observed, which is problematic in farming where growth rate affects profitability.
Fish may become lethargic, swim abnormally, or remain at the bottom of the tank.
In severe infestations, the belly may appear distended and overall condition notably deteriorates, with possible mortality if additional stress factors are present.
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 nematodes (roundworms) that colonize the turbot's digestive tract. Several species of marine nematodes can parasitize flatfish.
The life cycle of these parasites often involves an intermediate host, such as a small crustacean or marine invertebrate, which the turbot ingests while feeding. The released larvae then develop in the fish's digestive tract.
High stocking density in farming promotes parasite transmission.
Poor water quality, chronic stress, or inadequate diet weaken the turbot's natural defenses and increase its susceptibility.
The use of live food or uncontrolled natural prey (containing infected intermediate hosts) represents a significant risk factor.
Young turbot and animals already weakened by other diseases are generally more vulnerable.
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 pathology specialist first examines the clinical signs and the farm's history.
Confirmatory diagnosis requires necropsy of representative fish from the group, with direct microscopic examination of the digestive tract contents to identify and count the worms present.
Additional tests may be performed to rule out other causes of poor growth or weight loss (bacteria, water quality, nutritional deficiencies).
Treatment
Treatment relies on specific antiparasitic drugs suited to aquatic environments, most often administered through feed (medicated feed) under veterinary supervision.
Supportive measures such as improving water quality and reducing stress on the group promote recovery.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies on good sanitary management of the farm: monitoring fish stocking density, maintaining water quality, and providing balanced feed from a reliable source.
Avoiding feeding turbot with wild-caught prey or uncertified live food reduces the risk of parasite introduction.
Regular health monitoring of the stock, with periodic examinations by a veterinarian specializing in aquaculture, allows for early detection.
Possible complications
Untreated infestation can lead to lasting stunted growth, which is costly for the farm.
It can also weaken the fish's immune system, making it more vulnerable to secondary bacterial or fungal infections.
In severe cases, intestinal damage may occur, with increased mortality risk if the group experiences additional stress (transport, water temperature changes).
When to see a veterinarian
Loss of appetite or persistent refusal to feed in several fish of the group.
Abnormally slow growth compared to expected standards.
Visible weight loss despite apparently normal feeding.
Abnormal behavior: lethargy, isolation, unusual swimming patterns.
Unexplained deaths associated with weight loss in the group.
Prognosis
The prognosis is generally favorable if infestation is detected early and treated promptly, with growth resuming after treatment.
In cases of delayed diagnosis or heavy infestation combined with other stressors, economic and health losses may be more significant.
Frequently asked questions
Is parasitism of turbot by roundworms dangerous for humans?
Some nematodes in marine fish can theoretically pose a risk if the fish is consumed raw or undercooked, but adequate cooking or proper freezing eliminates this risk. Following good fish preparation practices protects consumers.
How do you know if a turbot farm is affected?
The signs are often subtle at first: slower than normal growth, irregular appetite. A veterinary examination with necropsy of a few fish allows confirmation of the presence of worms in the digestive tract.
Can this disease be completely prevented in livestock farming?
We cannot guarantee complete prevention, but good management of water quality, stocking density, and feed sourcing significantly reduces the risk of infestation.
Is antiparasitic treatment safe for fish?
Treatments must be selected and dosed by a veterinarian or aquaculture specialist, as improper use can stress the fish or disrupt the balance of the tank.
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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