Saprolegniosis in Roach (Gill Fungus)
Saprolegniosis is an infection caused by aquatic fungi of the genus Saprolegnia. These fungi naturally occur in most freshwater environments. In roach, they take advantage of skin or gill damage already present to establish themselves and f...
What is it?
Saprolegniosis is an infection caused by aquatic fungi of the genus Saprolegnia. These fungi naturally occur in most freshwater environments. In roach, they take advantage of skin or gill damage already present to establish themselves and form whitish or grayish filaments resembling cotton. The disease affects both wild fish in rivers and ponds as well as those kept in tanks or aquaculture facilities.
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:
Whitish to grayish filamentous tufts resembling cotton appear on the skin, fins, or gills of the roach.
When the gills are affected, the fish breathes with difficulty: it remains near the surface or in well-oxygenated areas, and its gill movements (opening and closing) may appear faster or irregular.
The fish may become lethargic, rub itself against tank décor or the bottom, swim abnormally, and lose appetite. In advanced cases, ulcerated skin areas may appear beneath the fungal filaments.
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 oomycete fungi of the genus Saprolegnia, particularly Saprolegnia parasitica. These organisms normally live in water and feed on decomposing organic matter. They become pathogenic when the fish has a skin lesion, wound, injury from handling, or general immunosuppression.
Cold water, poor water quality (pollution, high organic matter levels), and chronic stress favor disease development.
Skin wounds from parasites, fighting between fish, handling, or netting are common entry points for infection.
The breeding season is a high-risk period, as fish may sustain small lesions during spawning.
An immune system weakened by another disease, overcrowding, or sudden temperature fluctuations also increases susceptibility.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis begins with observing the characteristic cotton-like filaments on the skin or gills. A veterinarian specializing in fish disease can confirm the infection by microscopic examination of a skin or gill sample, which reveals the typical fungal filaments of Saprolegnia.
Treatment
Treatment uses antifungal medications designed for fish, applied as baths according to professional guidance. Improving water quality and reducing stress go hand in hand with treatment.
Salt (sodium chloride) is sometimes used as a bath treatment under supervision for its antifungal properties and to reduce osmotic stress on the fish.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Maintaining good water quality (appropriate filtration, low organic matter levels) reduces the risk.
Avoiding stress and rough handling of fish prevents injuries that could allow fungi to enter the body.
Watching for and quickly treating other illnesses (parasitic or bacterial) that could weaken the fish is essential.
Preventing overcrowding and sudden temperature changes also helps prevent the condition.
Possible complications
If the infection spreads extensively across the gills, it can compromise the fish's breathing. A widespread infection or deep skin damage can cause severe weakening and, if left untreated, death.
When to see a veterinarian
White or greyish cotton-like filaments appearing on the fish's body.
Visible breathing difficulties (fish staying at the surface, abnormal gill movements).
Lethargy or persistent loss of appetite.
Multiple fish in the same pond or tank showing similar signs.
Prognosis
The outlook is good if the infection is caught early and limited to the skin or fins, especially if water quality is corrected. It becomes more guarded if the gills are extensively affected or if the infection is widespread, particularly in an already weakened fish.
Frequently asked questions
Is saprolegniosis contagious between fish?
The fungus occurs naturally in water and is not transmitted from one fish to another like a typical infectious disease. However, poor water quality can promote its appearance in several fish in the same tank at the same time.
Can saprolegniosis be prevented in pond roach?
Yes, by maintaining good water quality, avoiding stress and injuries during handling, and monitoring the overall health of the fish.
Is salt alone sufficient to treat saprolegniosis?
Salt may help in mild cases, but professional advice is recommended to choose the most suitable treatment, particularly in cases of gill damage or widespread infection.
Does this disease affect only roach?
No, Saprolegnia can infect many freshwater fish species, as well as their eggs, as long as the skin or gills are compromised.
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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