Parasitism by Gyrodactylus and Polychaetus (gill monogeneans)
Gyrodactylus and Polychaetus parasitism includes infestations of the skin and gills of salmon by small flatworms called monogeneans. These parasites live attached to the fish using hooks, without passing through an intermediate host. They a...
What is it?
Gyrodactylus and Polychaetus parasitism includes infestations of the skin and gills of salmon by small flatworms called monogeneans. These parasites live attached to the fish using hooks, without passing through an intermediate host. They are found both in breeding and in the natural environment, in fresh water and in sea water depending on the species concerned. A light infestation often goes unnoticed, but a significant multiplication of parasites can seriously weaken the fish.
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:
Affected salmon rub against the walls of the pool or the gravel, as if to scratch themselves. They may swim abnormally, zigzag or staying close to the surface. The fins and skin sometimes show small damaged, paler or irritated areas. Significant damage to the gills results in more rapid breathing, with the fish “panting” near the water inlet or the surface. The most affected animals lose their appetite, become less active and gradually weaken. In fry and young salmon, the signs are often more marked than in adults.
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 due to the presence of monogeneans of the genus Gyrodactylus (parasites of the skin and fins, capable of reproducing very quickly) and of the genus Polychaetus (associated in particular with the gills). These parasites attach themselves directly to the fish using hooks and feed on mucus, skin cells or gill tissue. Unlike other parasitic worms, they do not need an intermediate host: transmission occurs from fish to fish by simple contact.
The high density of fish in ponds or cages favors the direct transmission of parasites. Stress linked to transport, handling or poor water quality weakens the salmon's natural defenses and facilitates the multiplication of monogeneans. Poorly oxygenated water, too hot or loaded with organic matter aggravates the sensitivity of the gills. The introduction of carrier fish without quarantine is a common cause of their appearance in a previously unaffected farm. Young salmon and animals already weakened by another disease are more vulnerable.
Transmission
Transmission occurs by direct contact between fish, without an intermediate host. It is favored by promiscuity in tanks or cages, as well as by contaminated breeding equipment passing from one batch to another.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian or aquaculture specialist examines the fish and observes its behavior in the water. Definitive diagnosis requires a sample of skin mucus or gill tissue, examined under a microscope to identify parasites and count them. This examination makes it possible to distinguish Gyrodactylus from Polychaetus and to evaluate the intensity of the infestation, which guides the treatment and prognosis.
Treatment
Treatment involves antiparasitic treatments adapted to the aquatic environment, applied in the form of baths or added to water according to the instructions of a professional. Supportive measures (improvement of water quality, reduction of stress, sometimes isolation of affected fish) accompany the treatment. The choice of product and duration of treatment should be determined by a veterinarian or fish health specialist, as some antiparasitics are toxic at the wrong dose or for certain species.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention is mainly based on good management of livestock: avoiding excessively high densities, maintaining quality water (correct oxygenation, suitable temperature, low organic load) and limiting sources of stress. Any introduction of new fish should go through a separate observation period before mixing with the rest of the flock. Regular monitoring of the skin, fins and gills allows an emerging problem to be identified early. Equipment used from one pool to another must be cleaned to avoid transporting parasites.
Possible complications
A massive, untreated infestation can lead to extensive skin lesions, a gateway for secondary bacterial or fungal infections, and severe respiratory compromise through damage to the gills. In fry or already weakened fish, this can lead to significant general weakening, or even death in the most serious cases.
When to see a veterinarian
The salmon repeatedly rubs against the decor or the walls of the pool.
Irritated, pale or damaged areas of skin appear on the body or fins.
The fish breathes unusually quickly or stays near the surface or inlet of water.
Loss of appetite or depression affects several fish from the same batch.
Unusual mortality is observed in the tank or cage.
Prognosis
The prognosis is generally good when the infestation is detected early and treated promptly, with improvements in water quality and breeding conditions. On the other hand, a massive infestation, especially in young salmon or in the presence of infectious complications, may have a more guarded prognosis.
Frequently asked questions
Can these parasites be transmitted to humans?
No. Gyrodactylus and Polychaetus are specific parasites of fish and do not pose a risk to humans.
How do these parasites arrive on a farm?
Most often by the introduction of fish that are already carriers, or by contaminated material transported from one pond to another without cleaning.
Does a salmon that rubs against the decor necessarily have monogeneans?
Not necessarily: other parasites or water irritation can cause the same behavior. Only a microscopic examination of the mucus or gills can confirm the cause.
Can you treat your pelvis yourself without professional advice?
This is not recommended. Antiparasitics used in aquatic environments must be chosen and dosed precisely, because some are toxic to fish or other species present in the pool.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
ESCCAP — European Scientific Counsel Companion Animal Parasites
Content reviewed by the TunisieVet editorial team · Updated 25 September 2026
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