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Saprolegnia Fungal Disease

A genus of fungus belonging to the Class Oomycetes. In the aquarium hobby, Saprolegnia is frequently used as a general name to describe any type of cotton-wool like fungal growth on the body surface of fish.

This is the most common fungal pathogen of freshwater ornamental fish. These fungi are usually saprophytes, which means that they normally feed and grow upon dead animal and plant material. If spores attach to a fish, normally substances in the mucous layer of the fish prevent their germination. However, if spores of this fungus find their way into a suitable place like a wound, they will germinate and send out branching stems called hyphae. These hyphae penetrate the skin and often spread into the deeper tissues. Eventually the fungus produces a set of hyphae to bear the fruiting bodies known as zoosporangia. It is from these that spores are released into the surrounding water. These raised hyphae give the classic “cotton-wool” appearance of a Saprolegnia fungal infection.

The fungus gets its sustenance from the fish, and in doing so causes extensive damage. Eventually the fish dies because the fungal infection damages the skin to the extent that the osmotic balance of the fish is disrupted, which means that the fish cannot maintain its body fluid and salt balance. On occasion the gills are targeted, often as a sequel to bacterial gill disease, where pathological changes to the gills favor lodging and establishment of fungal spores. Other fungal species do occur as opportunist pathogens and include the Calytralegnia, Achlya, Aphanomyces, Dictyuchus, Leptolegnia, Pythiopsis, and Thraustotheca.

What to Look for

All freshwater species are potentially susceptible to a Saprolegnia fungal infection. The cotton wool-like patches are very characteristic and are often associated with areas of damage such as ulceration, and in extreme cases these fungal masses can completely envelop parts of the fish. In pond fish, fungal infections that have been present for some time may be green or brown due to secondary colonization by algae. Note that finding dead fish covered in fungal mats does not necessarily mean that the fish died from a fungal infection-spores are ever present, and Saprolegnia will rapidly establish itself on a cadaver. One particular manifestation of Saprolegnia is “staff’s disease,” where the fungal growths are present only in the nostrils. This is seen in common carp in Poland during the winter months in one to two-year-old carp.

For treatment, proprietary medications containing malachite green are strongly recommended. Remove any visible hyphae by swabbing the affected area with a 10% providone-iodine solution once daily. Maintaining the fish in a salt solution will not only help to control the fungal infection, but it will also help with the osmotic imbalance resulting from the infection. Even salt solutions as low as 10 parts per thousand (mg/100mls) will inhibit Saprolegnia infections. Ideally, aim for 1 to 3g/l as a permanent solution until the problem has been resolved.

The reduction of stress appears to be the single greatest factor to help fish resist saprolegniasis.

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