Conger eel anisakiosis

What is it?

Anisakiosis is an infestation by larvae of parasitic worms called Anisakis. These worms belong to the nematode family, microscopic roundworms measuring a few centimeters. \n\nThe conger eel, like many marine fish, can harbor these larvae in its flesh and viscera without necessarily showing visible signs. It is a link in the natural cycle of this parasite at sea. \n\nThis disease is also of public health concern because humans can be infested by eating raw or undercooked fish containing these live larvae.

Most infested conger eels show no visible outward signs. The infestation is often discovered only when the fish is gutted or cut up. \n\nIn cases where the parasitic load is significant, we may observe weight loss of the animal, a general weakening, or small whitish nodules coiled in a spiral visible on the surface of the viscera, in the abdominal cavity or in the flesh itself. \n\nThese larvae can also cause local irritation or inflammation of the fish's digestive tissues when they are very numerous.

Commonly observed signs

These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.

Causes and risk factors

The disease is caused by larvae of nematodes of the genus Anisakis, mainly Anisakis simplex. \n\nThe cycle of this parasite involves several successive hosts. The eggs are released by marine mammals (dolphins, seals, whales) which are the definitive hosts where the adult worm reproduces. The larvae hatch at sea and are ingested by small crustaceans. \n\nThe conger eel becomes infested by eating prey (small fish, squid, crustaceans) already carrying these larvae. The larvae then migrate into its flesh and internal organs without becoming adults, because the conger eel is only an intermediate or paratenic (transport) host in this cycle.

Certain factors increase the likelihood that a conger eel will carry Anisakis larvae. \n\nA habitat and diet rich in small fish and cephalopods that are themselves parasitized increase the risk. Fishing zones where marine mammals circulate also promote the maintenance of the parasitic cycle. \n\nOlder, larger fish have statistically had more opportunities to become infested over the course of their lives because larvae accumulate over time without being removed.

Transmission

The cycle passes through marine mammals (definitive hosts), crustaceans then fish and cephalopods (intermediate hosts). The conger eel becomes infested by consuming prey already carrying larvae. Humans become infected by consuming raw or undercooked fish containing these larvae.

Diagnosis and care

Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.

Treatment

There is no antiparasitic treatment applicable to conger eels living in a natural or farmed environment to eliminate these larvae. \n\nThe management of this disease is essentially based on food safety measures applied after fishing, namely the removal of visibly parasitized parts, freezing or appropriate cooking of fish intended for consumption.

Prevention

Possible complications

In conger eels, a massive infestation can cause weight loss and weakening, sometimes with damage to the digestive organs. \n\nThe main risk nevertheless remains for the human consumer who eats raw, poorly cooked or insufficiently frozen fish: the live larvae can then cause acute digestive disorders or allergic reactions in humans.

When to see a veterinarian

Transmission to humans

Anisakiosis is a recognized foodborne zoonosis. Humans become infected by consuming raw, marinated, cold-smoked or undercooked fish containing live larvae. \n\nIn humans, it can cause abdominal pain, nausea, vomiting, and sometimes allergic reactions which can lead to severe reactions in sensitized people. \n\nSufficient cooking or proper freezing of fish before raw consumption eliminates this risk.

Prognosis

For the conger eel itself, the prognosis is generally good: the infestation is most often well tolerated and does not significantly affect its survival in the natural environment. \n\nFor food safety, the prognosis depends entirely on compliance with fish preparation measures: fish properly cooked or frozen according to recommendations no longer presents a risk.

Frequently asked questions

References

WOAH — Organisation mondiale de la santé animale (ex-OIE)

FAO — Organisation des Nations unies pour l'alimentation et l'agriculture

CDC — Centers for Disease Control and Prevention

Content reviewed by the TunisieVet editorial team · Updated 25 September 2026