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Also known as: Winter Fungus


This term describes infection with fungi of the Saprolegnia complex. Some of the species are primary pathogens whereas others only cause disease only when there is preexisting illness, mechanical injury or environmental stress.

Saprolegnia filaments are long with rounded ends, containing the zoospores.

Saprolegnia can also infect fish eggs, establishing itself first on dead ones and then extending to neighbouring healthy ones.

A significant component of the pathogenesis is the action of the distinctive recurved attachment hairs which may function like burrs.

Factors which may be involved in the development of saprolegniasis include traumatic damage to the skin, concurrent infections and low water temperatures.

Clinical Signs

Saprolegnia lesions are focal grey-white patches on the skin which have a cotton wool-like appearance under water when the hyphal elements extend out. The early lesions are circular and extend out until they merge.

The patches can then become dark grey or brown as the mycelium traps mud or debris.

The head region is most commonly involved, but any part of the skin or gills, even internal organs, can be affected.

Respiratory distress may be evident if the gills are involved, and death can follow rapidly.


The clinical signs are quite characteristic.

Skin scrapes examined under the microscope may help identify the fungal hyphae which belong to Saprolegnia.

Histopathology reveals: the fungus invading the stratum spongiosum of the dermis and then the epidermis, causing erosions as it spreads. Numerous hyphae will be seen, and underneath, dermal necrosis and oedema. There is also a slight inflammatory response.

Treatment and Control

A variety of chemical treatments can be used to control the infection, such as malachite green, copper sulphate, potassium permanganate, salt and formalin.

Prevention involves maintaining fish under good husbandry conditions, by correcting nutrition, ensuring good water quality and avoiding overcrowding.

Even so, fish may still succumb from the disease.

Saprolegniasis Learning Resources
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Test your knowledge using flashcard type questions
Ornamental Fish Q&A 14


Roberts, R. (2001) Fish Pathology Elsevier Health Sciences

Durborow, R. (2003) Saprolegniasis (Winter Fungus) and Branchiomycosis of Commercially Cultured Channel Catfish Southern Regional Aquaculture Center Publication

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