Gastrointestinal parasitism of conger eel (roundworms)
Gastrointestinal parasitism of conger eel is an infestation of the digestive system by roundworms, called nematodes. These parasites live in the stomach or intestine of the fish. The conger eel (Conger conger) is an elongated marine fish, o...
What is it?
Gastrointestinal parasitism of conger eel is an infestation of the digestive system by roundworms, called nematodes. These parasites live in the stomach or intestine of the fish. The conger eel (Conger conger) is an elongated marine fish, often found in Tunisian waters, which can harbor these parasites throughout its life without necessarily showing visible signs. This parasitosis is common in wild marine fish and concerns both individuals living in a natural environment and those kept in ponds or public aquariums.
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:
Most infested conger eels show no external signs, especially if the number of worms is low.\n\nWhen the infestation becomes significant, we may observe progressive weight loss, a drop in appetite or a refusal of food. The fish may appear less lively, swim abnormally, or remain prostrate near the bottom.\n\nIn some cases, worms may be visible directly in the stool, in vomit, or when opening the digestive tract during post-mortem examination or when preparing the fish.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
These are nematodes belonging to several families which naturally parasitize the digestive tract of marine fish. In the conger eel, we find in particular larvae or adults of worms belonging to the Anisakidae group, as well as other gastrointestinal nematodes specific to marine fish.\n\nThe cycle of these parasites generally passes through several hosts: small crustaceans or marine invertebrates host the first larval stages, then fish such as the conger eel become infested by eating these prey. The worms then develop in the digestive tract of the conger eel.
Wild conger eels living in coastal areas rich in intermediate prey are more exposed. A varied natural diet increases the risk of contact with intermediate hosts carrying larvae.\n\nIn captivity, the use of uncontrolled whole fish as food can encourage the introduction of parasites. Chronic stress or poor water quality can worsen the consequences of an already present infestation.
Transmission
The conger eel becomes infested by feeding on prey (small fish, crustaceans, marine invertebrates) carrying nematode larvae. There is no direct transmission between fish.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is often based on direct observation of worms in stools, vomit or during examination of the digestive tract. A parasitological examination of the stools (search for parasite eggs) can be carried out by a veterinarian specializing in aquatic animals.\n\nIn certain cases, an autopsy or examination of the fish after its death can confirm the presence and extent of the infestation.
Treatment
Treatment is based on the use of antiparasitics belonging to the family of dewormers active against nematodes, prescribed and adapted by a veterinarian specializing in aquatic animals.\n\nSupportive measures, such as improving the quality of water and diet, often accompany the treatment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
In captivity or breeding, it is recommended to freeze whole fish intended for food long enough before distributing them, which helps kill nematode larvae.\n\nRegular health monitoring of fish newly introduced into a pond or aquarium, as well as good water quality, help to limit infestations and their consequences.
Possible complications
A large, untreated infestation can lead to severe weight loss, poor general health and increased susceptibility to other diseases.\n\nIn rare cases, a heavy parasitic load can cause obstruction or inflammation of the digestive tract.
When to see a veterinarian
The conger eel is visibly losing weight without any apparent cause.\nThe fish has refused to feed for several days.\nWorms are visible in the stools or when handling the fish.\nSwimming behavior becomes abnormal or the fish remains prostrate.
Prognosis
The prognosis is generally favorable when the infestation is detected early and treated by a professional. A light infestation may never cause a visible problem. On the other hand, a massive and neglected infestation can permanently weaken the animal.
Frequently asked questions
Is the parasitized conger eel dangerous for humans?
Consumption of raw or undercooked fish containing larvae of nematodes such as Anisakidae may pose a risk to humans. Sufficient cooking or freezing beforehand eliminates this risk.
How do I know if my conger eel is parasitized?
Often there are no visible signs. Weight loss, decreased appetite or the presence of worms in the stools should lead you to consult a veterinarian specializing in aquatic animals.
Can we prevent this parasitosis in captivity?
Yes, by sufficiently freezing the prey intended for food and by controlling the quality of the water and the origin of the fish introduced into the pond.
Does this disease only affect conger eel?
No, many marine fish can harbor similar gastrointestinal nematodes; The conger eel is not a particularly risky species compared to other marine fish.
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 25 September 2026
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