Ichthyobodo (black gill disease)
Ichthyobodosis is a parasitic disease that affects the skin and gills of salmon. It is caused by a tiny single-celled parasite called Ichthyobodo (formerly called Costia). This parasite lives on the surface of the fish and feeds on the skin...
What is it?
Ichthyobodosis is a parasitic disease that affects the skin and gills of salmon. It is caused by a tiny single-celled parasite called Ichthyobodo (formerly called Costia). This parasite lives on the surface of the fish and feeds on the skin cells and gills.\n\nIt is sometimes called "black gill disease" because the affected gills can take on a darker shade, due to irritation and reactions of the fish's body. This disease mainly affects young salmon, particularly in breeding (fish farms, aquaculture farms), when water conditions are not optimal.
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 become lethargic and swim less actively. They may stay near the surface or near oxygen sources because their gills have difficulty maintaining normal breathing.\n\nRaped breathing is often observed, with fish "panting" with the opercula (the flaps that protect the gills) moving faster than normal.\n\nInfected fish will sometimes rub against the walls of the tank or objects present, as if to relieve an itch. The skin may appear dull, with excess mucus visible in spots.\n\nIn later stages, the gills may appear darker or blotchy, hence the name of the disease. Appetite decreases, leading to slowed growth and sometimes weight loss in older 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
The disease is caused by a microscopic flagellate parasite of the genus Ichthyobodo. This parasite adheres to the surface of the skin and gills of salmon thanks to a specific attachment device.\n\nIt multiplies rapidly by division, especially when water conditions favor its proliferation. Once attached, it damages the surface cells, which causes inflammation and excessive production of mucus by the fish to defend itself.
Young salmon (fry and juveniles) are more vulnerable than adults. Poor quality water, with excess organic matter or stagnant water, encourages the proliferation of the parasite.\n\nUnsuitable water temperature, stress linked to transport, handling or a high density of fish in the same space also increase the risk. A weakened immune system, for example after another illness, also makes salmon more susceptible.
Transmission
The parasite is transmitted directly from one fish to another in the water, especially when the salmon are in high density, such as in farms. It can also survive for some time in open water before finding a new host, making it easier to spread in a pond or enclosure.
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 fish farming specialist first examines the fish and their environment. The definitive diagnosis is made by taking a small amount of mucus or a fragment of gill, then observed under a microscope to identify the parasite thanks to its shape and its characteristic movements.\n\nThis examination must be carried out quickly after the sample, because the parasite degrades quickly outside of water.
Treatment
The treatment is based on parasiticidal baths adapted to fish, prescribed and dosed by a veterinarian or an aquaculture specialist. Salt-based products or other antiparasitic compounds can be used depending on the case.\n\nImproving breeding conditions (water quality, reduction in density, stress management) is an integral part of the treatment, in addition to drug treatment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention is based above all on good management of water quality: effective filtration, sufficient renewal, and monitoring of parameters (oxygen, temperature, organic matter).\n\nIt is recommended to avoid overdensities of fish in breeding ponds or cages, and to limit sources of stress (rough handling, prolonged transport).\n\nQuarantining new fish introduced into a farm helps limit the introduction of the parasite. Regular cleaning of installations and equipment also reduces the risk of contamination.
Possible complications
Without treatment, damage to the gills can worsen and seriously compromise the fish's breathing. Skin and gill lesions also open the door to secondary, bacterial or fungal infections.\n\nIn farms, a significant infestation can lead to significant economic losses, particularly among young, more fragile salmon.
When to see a veterinarian
Salmon that swim close to the surface or breathe abnormally quickly\nA loss of appetite or an unexplained slowdown in growth in a batch of fish\nFish that frequently rub against the walls or objects of the tank\nUnusual or increasing mortality in the farm\nGills that appear darker or damaged upon observation
Prognosis
The prognosis is generally good if the disease is detected early and breeding conditions are corrected quickly. On the other hand, a delay in treatment, especially in young salmon, can lead to serious respiratory complications and greater losses.
Frequently asked questions
Is ichthyobodosis dangerous for humans?
No, this parasite is specific to fish and poses no known risk to humans or other land animals.
How do I know if my salmon are affected?
Warning signs include rapid breathing, lethargic behavior, loss of appetite and rubbing against surroundings. Only a microscopic examination by a specialist confirms the diagnosis.
Can we prevent this disease in breeding?
Yes, good water quality management, reasonable fish density and quarantine of new arrivals greatly reduce the risk of infestation.
Can the treatment be done without veterinary advice?
No, it is essential to consult a veterinarian or an aquaculture specialist for an accurate diagnosis and appropriate treatment, because the wrong dosage of antiparasitic product can harm the fish.
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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