Gastrointestinal roundworm parasitism in tench
Gastrointestinal roundworm parasitism in tench is a common digestive disease in freshwater fish raised in ponds or tanks. These roundworms live in the fish's intestine and feed at its expense. They are present in many natural bodies of wate...
What is it?
Gastrointestinal roundworm parasitism in tench is a common digestive disease in freshwater fish raised in ponds or tanks. These roundworms live in the fish's intestine and feed at its expense. They are present in many natural bodies of water, but become problematic when their numbers increase significantly, particularly in farms where fish density is high. The disease mainly affects young or weakened tench, in which it can slow growth.
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:
Parasitised tench gradually lose weight despite sometimes maintaining their appetite, or conversely eat less.
Growth delay is often seen in young fish compared to other fish in the same batch.
The belly may appear hollow or slightly swollen depending on the stage of infestation.
Some fish become sluggish, swim abnormally, or rub against the tank or plant walls, a sign of irritation.
Abnormal stools that are stringy or soft may sometimes be visible trailing behind the fish.
In severe cases, deaths may occur, especially in fry or when combined with stress (poor water quality, overcrowding).
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 parasitic roundworms in the intestine, with several species able to infest cyprinids such as tench. The life cycle of these parasites often involves an intermediate host, such as certain aquatic invertebrates (crustaceans, insects, or their larvae), which the tench ingests while feeding on the pond bottom. Once swallowed, the larvae develop in the fish's intestine to adult stage, where they lay eggs that are shed with faeces, recontaminating the environment.
Overstocking fish in tanks or ponds increases transmission.
Poor water quality rich in organic matter, or a muddy bottom harbouring many intermediate hosts raises the risk.
Young tench and fish already weakened by other diseases or stress (transport, temperature change) are more susceptible.
Lack of water changes and a natural diet based on the pond bottom (mud, invertebrates) expose tench more to parasites.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is made by a veterinarian specializing in fish farming or an aquaculture specialist.
Microscopic examination of faeces or intestinal contents can reveal eggs or worms.
Necropsy of a dead or sacrificed fish may be necessary to identify the species of roundworm and assess the severity of the infestation.
Treatment
Treatment uses antiparasitic drugs suitable for fish, usually given through food or as a bath, under the supervision of a veterinarian or aquaculture specialist.
Improvement of rearing conditions (water quality, stocking density) should accompany medication to prevent recurrence.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Avoid overcrowding of fish in rearing ponds.
Maintain good water quality through regular cleaning of the pond bottom and aquatic vegetation.
Quarantine new fish before adding them to the existing population.
Monitor the general health and growth of tench regularly, particularly in young fish.
In commercial farms, periodic parasitological testing helps detect infestation before it becomes severe.
Possible complications
Severe, untreated infestation can cause lasting stunted growth, general weakness that makes the fish more susceptible to other infections, and in severe cases mortality, especially in young tench.
When to see a veterinarian
Unexplained weight loss or stunted growth in one or more tench in the pond.
Abnormal behaviour: lethargy, repeated rubbing against surfaces, unusual swimming patterns.
Abnormal faeces visible behind the fish.
Unexplained mortality in the population, particularly among young fish.
Prognosis
Prognosis is generally good if infestation is detected early and treated, along with improvements to rearing conditions. Without treatment, infestation can worsen and have lasting effects on the growth and survival of younger or weaker fish.
Frequently asked questions
Are roundworms from tench transmissible to humans?
No, the specific intestinal nematodes found in tench are not known to infest humans. The consumption of properly cooked fish presents no risk in this regard.
How to tell if your tench are infested without performing an autopsy?
Signs such as thinness, stunted growth, or rubbing against the decor can alert you, but only an examination by a specialist (stool analysis or intestinal content examination) can confirm the presence of roundworms.
Can the entire basin be treated in one go?
Some antiparasitic treatments can be administered to the entire batch through feed, but this must be decided and supervised by a veterinarian or aquaculture specialist to tailor the product and avoid adverse effects.
Can parasitism return after treatment?
Yes, if conditions favoring its presence persist (overcrowding, poor water quality, numerous intermediate hosts in the pond bottom), reinfestation is possible.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 22 August 2026
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