Gastrointestinal parasitism of the torpedo ray (roundworms)
Gastrointestinal parasitism by roundworms (nematodes) is a digestive condition that affects the torpedo ray, a cartilaginous fish living in the Mediterranean Sea and near the Atlantic. These worms settle in the stomach or intestine of the a...
What is it?
Gastrointestinal parasitism by roundworms (nematodes) is a digestive condition that affects the torpedo ray, a cartilaginous fish living in the Mediterranean Sea and near the Atlantic.
These worms settle in the stomach or intestine of the animal and feed at its expense.
In cartilaginous fish such as torpedoes, this parasitosis is mainly encountered in captivity, in public aquariums or quarantine tanks, but it also exists in the wild.
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 parasitized animal eats less or refuses the food offered.
It gradually loses weight, which is sometimes difficult to see in a ray because the body is flattened.
A drop in activity and a longer resting time at the bottom of the pool than usual are often noted.
The abdomen may appear slightly swollen or tender to palpation in a clinically handled animal.
Abnormal stools, soft or containing threadlike elements resembling small worms, may be observed in the box or during an examination of the feces.
In severe infestations, marked weight loss and a general state of weakness set in.
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) which parasitize the digestive tract of the torpedo ray.
Several genera of nematodes are described in elasmobranchs (rays and sharks), some being quite specific to marine cartilaginous fish.
Infestation most often occurs by ingestion of prey (small fish, crustaceans) which harbor larval forms of the parasite, called intermediate hosts.
Once ingested, the larvae develop in the torpedo's digestive tract and become adult worms capable of laying eggs, which are then passed into the water with the feces.
Animals living in captivity, especially recently captured or transferred from one pool to another, are more exposed due to stress and promiscuity.
A diet based on uncontrolled whole fish from local fisheries increases the risk of ingesting parasitic larvae.
A poorly maintained pond or aquarium, with poor water quality, encourages the accumulation of parasite eggs and reinfestation of animals.
Young torpedoes and animals otherwise weakened (transport, injury, other illness) seem more susceptible to severe infestation.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian specializing in aquatic animals carries out a complete clinical examination of the animal, taking into account its behavior in the water and its body condition.
A stool examination under a microscope (looking for parasite eggs) is the primary diagnostic tool.
In some cases, an endoscopy or post-mortem examination (in an animal already deceased in the collection) allows the worms to be directly visualized.
The veterinarian also evaluates the quality of the water and the breeding conditions, because they strongly influence the progression of the disease.
Treatment
Treatment is based on the use of specific antiparasitics adapted to cartilaginous fish, the choice and administration of which must be supervised by a veterinarian experienced in fish medicine.
Rays and sharks are in fact particularly sensitive to certain classic antiparasitic molecules used in bony fish, which requires great caution.
Nutritional support and improved breeding conditions (water quality, stress reduction) often accompany drug treatment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention mainly involves good food management: offering prey that is frozen rather than fresh, or has undergone a sufficient freezing period to kill the parasitic larvae.
Rigorous maintenance of ponds and aquariums, with a good filtration system and regular cleaning of the bottom, limits the accumulation of parasite eggs.
Quarantining new animals before introducing them into a common pool makes it possible to detect an infestation before it spreads.
Periodic parasitological examination of stools, particularly in professional aquarium collections, helps detect the problem early.
Possible complications
A massive and untreated infestation can lead to severe weight loss, partial obstruction of the digestive tract and general weakening of the animal.
This makes it more vulnerable to other diseases, especially infectious ones, and can lead to death in severe or neglected cases.
When to see a veterinarian
The animal has refused to eat for several days.
Visible weight loss or weight loss is noted.
Worms or suspicious items are observed in the stool or the tray.
The animal remains abnormally still or appears weakened.
Several animals in the same pelvis show similar signs.
Prognosis
The prognosis is generally favorable if the infestation is detected early and treated by a competent aquatic veterinarian.
In the absence of treatment, a significant infestation can develop into severe weakening or even death of the animal, especially in captivity where additional stress plays an aggravating role.
Frequently asked questions
Can a parasitized torpedo ray contaminate other fish in the pond?
Yes, roundworm eggs passed in feces can contaminate water and, if suitable intermediate hosts are present, promote the spread of the parasite to other animals in the pond.
Is this parasite dangerous for humans?
The gastrointestinal nematodes described in the torpedo ray are not known to be transmissible to humans under normal conditions of handling or controlled consumption.
Can you treat your torpedo ray yourself with a commercial pest control product?
No, this is strongly discouraged. Cartilaginous fish are very sensitive to certain antiparasitic drugs and self-medication can be toxic, even fatal, for the animal.
How do I know if my torpedo is parasitized without examining it?
A change in behavior, a drop in appetite or progressive weight loss should alert you, but only an examination of the stool by a veterinarian can confirm the presence of worms.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 25 September 2026
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