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Indiana Department of Natural Resources

Fish & Wildlife > Wildlife Resources > Wildlife Diseases > Fish Parasites & Diseases Fish Parasites & Diseases

Fish, like any animal, are exposed and susceptible to a wide range of diseases and parasites. In fact, it is unusual to find a fish completely free of disease organisms. It is normal to see a few dead fish from time to time. These fish usually die as the result of natural causes. Severe losses of fish due to disease organisms are rare in a natural environment. Most severe losses occur when fish are stressed due to unfavorable conditions, such as poor water quality.

Disease problems are most noticeable in the spring when fish are in a weakened state after winter and during the stressful spawning period. Also, as the water warms in spring, living conditions are better for growth of disease organisms.

There are no practical, and few safe methods, for treating disease problems in ponds. Disease outbreaks in ponds simply must run their course. Prevention or repression of disease is best accomplished by stocking only healthy fish from reputable sources and by maintaining a favorable pond environment.

With the exception of the broad fish tapeworm, fish parasites are not harmful to humans. Proper cooking destroys parasites, including tapeworms. Some of the more common parasites and diseases are described below.

Black-spot disease

These parasitic flatworms appear as tiny black spots on the skin, fins and flesh of fish. No method of control is available for the elimination of this problem. This organism does little harm to the fish. The main problem associated with black-spot is the unsightly appearance it may cause. Skinning infected fish will remove most black spots.

The life cycle of the parasite is quite complex. A fish-eating bird (typically a great blue heron or kingfisher) eats an infected fish. The black spot or worms are released and grow to sexual maturity in the bird's intestine. The adult worms pass eggs with the bird's droppings. When the eggs reach water, they hatch into free-swimming organisms which then penetrate snails for further development. Finally, after leaving the snails they burrow into the skin of fish and form a cyst. The fish surrounds the cyst with black pigment that gives the disease its name. If an infected fish is consumed by a bird, the cycle repeats itself.

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Yellow grub

The yellow grub (or white grub) is also a larval flatworm with a life cycle similar to parasites causing black-spot disease. The parasite appears as yellow or white spots in the flesh, often 1/4 inch long. While unsightly, it is harmless to man and in many cases can be removed during the cleaning process.

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Fungus

The most common fungus is saprolegnia and appears as gray-white threads resembling cotton balls growing on fish or fish eggs. Fungus usually occurs as a secondary infection caused by handling, parasites or bacterial attack. There is no practical way to control fungus in pond situations. Fungus rarely causes a fish to die, but can often be found on weakened or stressed fish before they die.

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Ichthyophthirius "Ich"

Ich is a large, ciliated, single celled animal (protozoan) that can be positively identified under a microscope by its horseshoe shaped nucleus (center). It is common on warm-water fish, and occasionally found on cold-water fish.

In the early stages of Ich, infected fish usually rub against the pond bottom in an effort to rid themselves of the parasite. This protozoan can be very harmful to fish. Losses due to "Ich" often occur.

The elimination of "Ich" in a pond situation is virtually impossible. Problems resulting from the parasite can be minimized by maintaining good water quality in the pond and by making sure only healthy fish are stocked

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Lymphocystis

Lymphocystis is a viral disease that causes a yellow or white wart like growth on the fish's body. Lymphocystis subsides naturally only to return, much like a cold-sore in humans. Generally, little harm is done to the fish but the infection is unsightly.

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Bacterial Diseases

Bacterial diseases are common in all fish and occur most often when environmental conditions, such as water quality, are not favorable. Inadequate oxygen levels in the pond can stress fish and make them susceptible to bacteria infections. These infections are often associated with spring die-offs in fish ponds. As the water warms during the spring, fish weakened by the winter months are often invaded by harmful bacterial that can cause death. This weakened condition can also be enhanced by frenzied spawning activity that further stresses the fish.

Bacterial losses are one of the most commonly noted causes of fish loss during May and June. No control is available for the treatment of bacterial problems in ponds. Fortunately, bacterial problems rarely reach epidemic proportions in ponds.

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