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Fish diseases

Fishing Wisconsin

Even though we are reporting on fish disease in Wisconsin, we encourage you to continue to enjoy your fishing experiences. The information we have gathered for you is to help prevent the spread of the diseases and as information so you know what disease you may encounter and what to look for in the fish you catch.

Anglers: You can continue to enjoy catching and eating your catch as long as you fully cook them before eating.

Name of Disease Pathogen (organism that can cause the disease) Fish Affected Description
Columnaris Bacteria- Flavobacterium columnare Columnaris is common in bluegills, crappies, yellow perch, and bullheads

An infection will usually first manifest in fish by causing frayed and ragged fins. This is followed by the appearance of ulcerations on the skin, and subsequent skin loss, identifiable as white or cloudy, fungus-like patches – particularly on the gill filaments. Mucus often also accumulates on the gills, head and back. Gills will change color (light or dark brown). Fish will breathe rapidly as a sign of gill damage. Anorexia and lethargy are also common.

Viral Hemorrhagic Septicemia Virus- Viral Hemorrhagic Septicemia virus Viral Hemorrhagic Septicemia virus can affect a variety of fish species. There are currently 25 susceptible species.

Some signs of VHS are: Hemorrhaging (bleeding, bulging eyes), unusual behavior, anemia, bloated abdomens and rapid onset of death

Lymphosarcoma Unknown - likely a virus Lymphosarcoma causes cancerous sores on the skin and in the muscle of northern pike and musky

Initial infection with Lymphosarcoma may appear as purple and pink blisters on the skin and then develop into a bulging tumor up to four inches in diameter. The sores or tumors are found most frequently on the sides of fish but also their head and fins, and may not develop for six to 18 months

Black Spot Parasite- A few different species of parasites can cause this disease (Uvulifer species, Neascus species). That black color is from the fish’s immune cells, not the parasite. Blackspot in bluegills and other panfish species

This parasite has a life cycle that uses birds, snails and fish as the host animal. Parasitic larvae burrow into the skin or muscle of the fish and stop their development. When a bird eats an infected fish, the larvae develop into an adult parasite in the bird and the cycle begins again. The black spot parasite uses the kingfisher or gull as the host bird.

Yellow Grub Parasite- Usually Clinostomum species Clinostomum (grubs in fish) is usually found in bluegills and other panfish

This parasite has a life cycle that uses birds, snails and fish as the host animal. The cercariae burrow into the muscle of the fish where they develop into third-stage larvae called metacercaria. When a heron eats the fish, the metacercariae develop into adult parasites in the bird

Heterosporis Parasite- Heterosporis sutherlanda Heterosporis is often referred to as the yellow perch parasite

The muscle (fillet) of infected fish appears white and opaque, almost as if the fish was already cooked or had freezer burn. The life cycle of the parasite has not been completely worked out, but the parasite's spores develop in muscle cells and cause the muscle tissue to degenerate. So far, fisheries biologists have not documented a decline in yellow perch abundance in lakes where Heterosporis is present.

Whirling Disease Parasite- Myxobolus cerebralis The whirling disease only affects salmonid fish, which include trout and salmon

A disease caused by the Myxobolus cerebralis parasite (Life cycle fact sheet) damaging the cartilage and nerve tissue of trout and salmon. Fish with whirling disease may have blackened tails, skeletal deformities, skull deformities, and/or they may swim in circles. Younger fish are more likely to develop disease. Not all fish infected with the parasite will develop whirling disease. Only salmonids, such as trout and salmon species, are susceptible to whirling disease.

If you catch a diseased fish or observe a fish kill take the following steps.

For more information on other fish parasites, please see the DNR's webpage on invasive species.