Saprolegniosis in Carp (Gill Fungus)
Saprolegniosis is a common fungal infection in carp and other freshwater fish. It is caused by water molds from the genus Saprolegnia, naturally present in the water and sediment of ponds. This fungus normally does not attack a healthy fis...
What is it?
Saprolegniosis is a common fungal infection in carp and other freshwater fish. It is caused by water molds from the genus Saprolegnia, naturally present in the water and sediment of ponds.
This fungus normally does not attack a healthy fish. It takes advantage of a wound, a missing scale, stress, or weakened immunity to settle on the skin, fins, or gills. It is often called "cotton fungus" because it forms whitish or grayish filamentous tufts resembling wet cotton.
The disease affects both farmed pond carp and ornamental koi, especially during cold periods or after stressful handling.
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:
Filamentous white, gray, or brownish growths resembling cotton appear on the skin, fins, mouth, or gills of the fish.
The affected area may be red, irritated, or slightly swollen before the filaments appear. The fish rubs against the tank or pond surfaces, a sign of irritation.
When the gills are affected (gill fungus), the fish breathes faster, stays near the surface to seek oxygen, and may become lethargic.
In advanced cases, there is loss of appetite, the fish isolates itself, swimming becomes slow or unbalanced, and death may occur if the infection spreads widely.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Saprolegniosis is caused by water molds, mainly of the genus Saprolegnia, sometimes associated with related genera such as Achlya.
These fungi are opportunistic organisms: they are permanently present in freshwater, on decomposing organic matter, but only infect fish that are already weakened, injured, or stressed.
Unfertilized or dead carp eggs in spawning areas are also frequent targets, which can then favor spread to neighboring adult fish.
The main risk factor is a wound or skin lesion caused by injury, rough handling, an external parasite, or fighting between fish.
Stress plays a major role: transport, abrupt temperature changes, poor water quality, overstocking in the pond or tank, or lack of oxygen weaken the fish's natural defenses.
Cold water, particularly during seasonal transitions (autumn, winter, or after overwintering), favors the development of Saprolegnia. Water rich in decomposing organic matter also increases the risk.
Already sick fish, elderly individuals, or those carrying another infection (bacterial or parasitic) are more vulnerable.
Transmission
The fungus is naturally present in freshwater and spreads through contact with spores released from infected areas, promoted by fish living in close proximity and the presence of wounds.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
A veterinarian or aquaculture specialist diagnoses the condition by observing the characteristic cottony filaments with the naked eye, supplemented by a sample of the lesion examined under a microscope to confirm the presence of typical fungal filaments of Saprolegnia.
Water analysis of the pond or tank (temperature, oxygen, quality) is often performed in parallel, since the underlying cause of stress or injury must be identified to prevent relapse.
Treatment
Treatment is based on immediate improvement of water quality and stress reduction, which are essential for recovery.
Antifungal products suitable for freshwater fish can be used as bath treatments or added to the water, in the form of salts or specific compounds prescribed by a professional.
In some cases, local antiseptic treatment of the lesions may be performed by a professional. Never use treatments intended for other purposes without professional advice, as many fish are sensitive to certain substances.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
The best prevention is maintaining good water quality in the pond or tank: appropriate filtration, sufficient oxygenation, and reduction of decomposing organic matter.
Overstocking should be avoided, and carp should be handled carefully during grading, weighing, or transport to limit injuries and stress.
Regular monitoring of skin and fins helps detect any lesion early that could serve as an entry point for the fungus.
During cold periods, reducing handling and avoiding sudden water temperature fluctuations help preserve the fish's natural defenses.
Possible complications
Without treatment, the infection can spread and progressively destroy the skin, fins, or gills, leading to severe respiratory difficulties.
A secondary bacterial infection can occur on already damaged tissue, worsening the prognosis. In severely weakened fish or in breeding facilities, mortality can be significant if the infection affects the stock extensively.
When to see a veterinarian
Appearance of white or grayish tufts resembling cotton on the skin, fins, or mouth of the fish.
Abnormally rapid breathing or a fish remaining constantly near the water surface.
Loss of appetite, withdrawal, or abnormal swimming in one or more fish in the pond.
Unexplained or clustered deaths in a pond or tank.
Prognosis
The outlook is good when the disease is caught early and farming conditions are corrected quickly: surface lesions often heal well.
However, if the infection reaches the gills or spreads extensively before treatment, the outlook becomes more guarded, with a risk of death especially in fish already in poor condition.
Frequently asked questions
Can saprolegniosis be transmitted to humans?
No, this aquatic fungus does not infect humans. It is not a zoonotic disease.
Can a healthy fish catch this fungal infection?
It's rare. The fungus almost always takes advantage of a wound, stress, or poor water quality to establish itself.
Should you isolate the sick fish?
This can be useful for limiting competition-related stress and facilitating local treatment, but the root cause (water quality) should especially be addressed for the entire basin.
Does saprolegniosis also affect carp eggs?
Yes, unfertilized or dead eggs in spawning grounds are a frequent target for the fungus, which can then spread to nearby adult fish.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 22 August 2026
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