These are caused by various species of aquatic fungi, including Saprolegnia and Achlya. In fact, many fungal infections attributed to Saprolegnai are actually caused by other types of fungi, and sometimes several fungal species may simultaneously colonize a lesion. Most fish pathogenic fungi generally invade only those tissues which have already been damaged through injury or disease. Fungi may also attack fish eggs. In addition to injuries, poor pond husbandry, adverse water chemistry, chills, and other stressors are predisposing factors in fungus outbreaks. Fish which are very old are more prone to fungus attack.
Fungus and fungal spores are quite common in aquatic environments and are particularly abundant where there is plenty of decaying organic matter. The infectious spores of fungus may transmit the disease from fish to fish. However, the layer of mucus that covers the skin of healthy, undamaged fish is normally a very effective barrier to the spores. If, for one reason or another, this mucus layer become damaged, following rough handling, fighting or spawning activity, for example, this will provide an opportunity for the fungus to gain a foothold. Only rarely do fungal infections invade deep within the tissues. One infrequently encountered systemic fungus is Ichthyophonus (which is extremely rare in freshwater fish), whose status as a true fungus has been questioned. Ichtyophonus invades the major organs and causes tissue nodules which are similar to those found in Fish Tuberculosis. Confirmation is based on the microscopical examination of diseased tissue for the presence of fungal spores.
Fungal infections can progress swiftly, eventually encasing the entire fish. Fungus also attacks fish that have died from other causes.
Fungus may also invade the lesions left by other diseases, such as white spot or ulcer diseases. A sudden change of temperature, unhygienic pond conditions and poor water quality may all predispose fish to fungus. In the case of fish eggs, fungus attacks the dead eggs and may spread to the adjacent healthy eggs, killing the whole batch.
There are approximately 50,000 species of fungus, and they are usually thought of as plants. However, they do not contain chlorophyll, the green photosynthetic pigment of plants, and cannot manufacture their own food. Some fungi are saprophytes, feeding on dead organic matter and thus helping bacteria to break down and release nutrients for recycling. Others are parasites of animals or plants, and some are an important source of antibiotics used in the treatment of bacterial disease of animals and man.
Fungi range in size from tiny single-celled yeasts, mildews and rusts (often seen on crops) to the familiar mushrooms and toadstools in damp fields. Fungi are widespread in damp terrestrial habitats and in aquatic habitats. The saprolegnia group of water molds will be familiar to most fish keepers as the cause of fish and egg fungus. These fungi are made up of thin threads, or hyphae, which weave together to form an obvious fungal mat, or mycelium. These threads often need to gain access through damaged skin, but once inside can then penetrate deep into tissue using special enzymes. Fungi can reproduce by asexual or sexual means. Either process usually results in the liberation of spores, certain types of which can be quite resistant to adverse conditions. It is important to note that the fish disease "mouth fungus", although it looks like a typical, fluffy fungal infection, is, in fact, the result of an infection with the Flavobacterium bacterium.
Treatment and Control
As soon as fish show any signs of fungus, treat them with a proprietary brand of fungus remedy. Treat lightly affected fish in the pond, but deal with heavily infected fish and all pond fish in an isolation tank.
To avoid egg fungus, remove all dead (opaque) fish eggs promptly and carefully from the batch using a fine pair of forceps or a pipette. Some proprietary brands of fungus remedy can be used to treat eggs, but be sure to check instructions for use before proceeding.
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