parasitic disease Eel

Eel gastrointestinal parasitism (roundworms)

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Definition Signs Causes Transmission Diagnosis Treatment Prevention Complications When to see a vet Prognosis FAQ Sources Affected species

What is it?

Eel gastrointestinal parasitism includes several infestations by roundworms (nematodes) which live in the digestive tract or swim bladder of the fish. \nThe best-known parasite in the European eel is Anguillicola crassus, a nematode native to Asia which has spread widely in Europe and also affects Tunisian and Mediterranean populations. \nThis worm mainly settles in the swim bladder, the organ that allows the fish to float, but larval forms also pass through the intestine. \nOther less specific intestinal nematodes can also parasitize the eel's digestive tract. \nThis disease affects wild eels as well as those raised in aquaculture or kept in ponds.

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:

Weight loss Fatigue Abnormal swimming Worms or larvae visible on the skin Stunted growth Dull coat

Infested eels often show few visible signs at first, making the disease difficult to spot without examination. \nAt a more advanced stage, we can observe progressive weight loss despite a preserved appetite, less efficient or unbalanced swimming, the eel having difficulty controlling its buoyancy due to damage to the swim bladder. \nFish may seem slower, have difficulty rising or stay close to the bottom. \nWhen opening the fish (necropsy), the veterinarian or fish farmer may observe whitish to reddish worms in the swim bladder or intestine, as well as thickening of the swim bladder wall. \nIn breeding farms, a drop in general growth of the flock may be the only visible sign for a long time.

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), mainly Anguillicola crassus for swim bladder damage, and various other less specific intestinal nematodes. \nThese parasites have a life cycle that passes through intermediate hosts, most often small aquatic crustaceans (copepods), sometimes relayed by fish or other prey which serve as transport hosts. \nThe eel becomes contaminated by eating these intermediate hosts carrying infective larvae. \nThe larvae then migrate through the intestinal wall to reach the swim bladder, where they become adults and lay eggs, restarting the cycle.

Eels living in waters where intermediate hosts (copepods) are abundant are more exposed. \nHigh fish densities, typical of certain farms, favor the transmission and accumulation of parasites. \nA natural diet rich in small aquatic prey increases the risk of ingesting infective larvae. \nEels that are older or have lived longer in a contaminated natural environment often accumulate a greater parasitic load. \nStress, poor water quality or an unbalanced diet can worsen the consequences of the infestation without being the direct cause.

Transmission

Transmission occurs by ingestion of intermediate hosts (copepods) or prey carrying infective larvae; there is no direct transmission between eels.

Diagnosis and care

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

The diagnosis is based on the clinical examination of the fish, but especially on the autopsy or endoscopy of the swim bladder which allows the adult worms to be directly visualized. \nImaging techniques such as radiography can sometimes help visualize swim bladder abnormalities in live fish. \nExamination of stool or intestinal contents may reveal nematode eggs or larvae. \nIn farms, regular monitoring with autopsy of a sample of fish makes it possible to estimate the level of infestation in the batch without having to sacrifice all the animals.

Treatment

Treatment is based on the use of families of antiparasitics specific to nematodes, prescribed and adapted by an aquaculture veterinarian according to the situation. \nIn farms, overall management of the flock is often necessary in addition to individual treatment, including improving water quality and reducing fish density. \nNo treatment guarantees complete eradication of the parasite in a natural environment where intermediate hosts remain present.

Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.

Prevention

There is no vaccine against this parasitosis. \nPrevention is mainly based on farm management: limiting fish density, controlling water quality and avoiding the introduction of uncontrolled wild eels into healthy farms. \nRegular veterinary monitoring with examination of a few fish from the batch makes it possible to detect an infestation before it becomes severe. \nIn closed ponds, limiting the presence of copepods or intermediate hosts can reduce the risk of transmission. \nQuarantine and health control of eels before introduction into a new pond are useful measures in aquaculture.

Possible complications

A severe infestation of the swim bladder can lead to lasting buoyancy disorders, hampering the movements and survival of the fish in the natural environment. \nIn eels, this can also compromise their ability to migrate long distances, an important aspect of their natural life cycle. \nChronic infestation can weaken the overall condition of the fish and make it more susceptible to other secondary diseases or infections.

When to see a veterinarian

Progressive weight loss of the eel despite a normal appetite\nAbnormal, unbalanced swimming or difficulty maintaining its position in the water\nGeneral slowing down or unusual lethargy of the fish\nReduction in growth observed on several eels from the same batch in breeding\nDiscovery of worms when handling or observing a dead fish

Prognosis

The prognosis depends on the level of infestation and the speed of treatment. \nA light to moderate infestation can be managed without major consequences on the survival of the fish, especially with appropriate veterinary monitoring. \nIn the event of severe infestation of the swim bladder, the prognosis is more guarded, with a risk of lasting problems with swimming and buoyancy. \nIn breeding, early management of the flock generally makes it possible to limit economic and health losses.

Frequently asked questions

Is this disease dangerous for humans who eat eel?

These nematodes are specific to fish and are not considered transmissible to humans through consumption, especially if the fish is properly cooked. However, complete cooking is always recommended as a general precaution.

How does an eel get these worms?

It becomes contaminated by eating small aquatic crustaceans (copepods) or other prey that carry the infesting larvae of the parasite in their body.

Can we see worms with the naked eye?

Yes, adult worms are visible to the naked eye when the fish opens, usually in the swim bladder or intestine, as whitish to reddish filaments.

Can an infested fish completely recover?

A mild infestation treated early generally leaves few after-effects, but a severe infestation of the swim bladder can leave lasting swimming problems even after treatment.