Gastrointestinal parasitism in sole (roundworms)
Gastrointestinal parasitism caused by roundworms (nematodes) affects sole, a flatfish raised in aquaculture or living in coastal waters. These parasites settle in the fish's digestive tract, where they feed at the expense of the host. They...
What is it?
Gastrointestinal parasitism caused by roundworms (nematodes) affects sole, a flatfish raised in aquaculture or living in coastal waters.
These parasites settle in the fish's digestive tract, where they feed at the expense of the host. They are common in flatfish and cause particular problems in intensive farming, where high stocking densities promote their multiplication.
In most cases, light infestations go unnoticed. Clinical signs and economic losses appear only when the parasite burden increases significantly.
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:
Parasitized sole often show few visible signs initially.
When infestation becomes severe, progressive weight loss is observed despite sometimes normal feeding, slowed growth compared to other fish in the group, and an overall weakened condition.
Some fish swim abnormally or remain at the bottom of the tank or pond. Irritation may cause the fish to rub against equipment or the substrate. In severe cases, inflammation of the intestinal wall can cause digestive disturbances that are difficult to detect with the naked eye in fish.
Visual clinical diagnosis is limited: only specialist analysis can confirm the presence and extent of parasitism.
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 condition is caused by nematodes (roundworms) that colonize the stomach or intestine of sole. Several genera of marine nematodes may be involved depending on geographic location and farming method.
The life cycle of these parasites often involves one or more intermediate hosts, such as small crustaceans or other marine invertebrates, which the sole ingests while feeding.
High stocking density of fish in ponds promotes parasite transmission and accumulation.
Feeding on uncontrolled natural prey (live small invertebrates) increases the risk of ingesting infective larvae.
A rearing environment with poor water quality, chronic stress, or weakened immunity makes fish more vulnerable to severe infestation.
Wild-caught sole from coastal areas can also harbour these parasites, though this does not necessarily pose a problem.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis relies on post-mortem examination or necropsy performed by a fish pathology specialist, who looks for worms in the digestive tract.
Additional techniques such as microscopic examination of intestinal scrapings or digestive contents help identify and count parasites.
In a farming context, batch monitoring with regular sampling helps assess the overall level of infestation.
Treatment
Treatment involves using antiparasitic drugs suitable for fish, prescribed and supervised by an aquaculture specialist.
Improvement of farming conditions—water quality, reduced stocking density, and feed management—accompanies medication to prevent relapse.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Water quality management and control of stocking density help reduce the risk of heavy infestation.
The use of formulated feeds instead of uncontrolled live prey reduces exposure to parasitic larvae.
Regular health monitoring of batches, with periodic parasite testing by a specialist, allows early detection of infestation before it becomes problematic.
Quarantining newly introduced fish also helps prevent parasite introduction into the farm.
Possible complications
Severe untreated infestation can lead to chronic intestinal inflammation, nutrient malabsorption, marked growth retardation, and in extreme cases, increased mortality within the batch.
Weakened fish also become more susceptible to secondary bacterial or fungal infections.
When to see a veterinarian
Progressive weight loss in one or more fish despite apparently normal feeding.
Slowed or uneven growth within the same batch.
Abnormal behaviour: lethargy, disorganised swimming, repeated rubbing against tank décor.
Unexplained deaths in a sole farm.
Prognosis
The prognosis is generally good for light to moderate infestation detected and managed promptly.
However, neglected severe infestation can substantially compromise growth and survival of affected fish, with significant economic impact on the farm.
Frequently asked questions
Is gastrointestinal parasitism in sole transmissible to humans?
No, these nematodes are specific to fish and do not pose a zoonosis risk under normal conditions of handling or consumption of cooked fish.
Can you see worms with the naked eye?
Sometimes, during a necropsy, certain worms of sufficient size are visible in the digestive tract. However, confirmation and accurate counting require examination by a specialist.
Can an infested sole heal on its own?
A light infestation may have no noticeable consequences, but a heavy parasitic load requires veterinary intervention to prevent complications.
How can this disease be prevented in livestock farming?
Controlling stocking density in ponds, maintaining water quality, and using formulated feeds rather than uncontrolled live prey are the main preventive measures.
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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