What’s Growing on My Fish—and Is It Safe to Eat?

Common Fish Diseases, What They Look Like, and When Fish Are Safe to Eat

Fish sometimes get diseases. This northern pike is heavily infected with neascus, which is commonly called black spot disease. | Joe Shead

While these fish diseases may look alarming, most common parasites and infections are harmless to humans if the fish are properly cleaned and cooked. Identifying what you’re looking at and knowing how to handle it can help anglers make safe, informed decisions.

What Causes Fish Diseases?

Poor water quality, overcrowding, increased water temperatures or spawning stress can make healthy fish susceptible to disease. Congregating during the spawn can accelerate the transmission of diseases. Although infections can occur year-round, they are more common when water is warm.

Below are descriptions, images and safety recommendations for some of the most common fish diseases and parasites anglers encounter.

Neascus / Black Spot Disease (Common and Harmless)

This bluegill shows neascus infection on the fins and the head. When infected with neascus, a fish looks like it was sprinkled with black pepper. When thoroughly cooked, it is safe to eat. | Joe Shead

Fish infected with neascus, or black spot disease, look like they were sprinkled with black pepper. The disease is caused when fish are parasitized by the larval form of a trematode. The larvae encyst in the skin, fins and sometimes flesh of fish. Fish release melanin at the infection site, resulting in raised black spots. When birds eat infected fish, the parasite’s eggs are released by feces into the water and the cycle continues. It’s usually obvious when a fish is infected with neascus. Although unsightly, the fish are safe to eat if cooked properly.

Yellow Grub (Unsightly, But Safe When Cooked)

Yellow grub often infects perch, although it can occur in other species. It often appears externally under the gills or near the base of the tail. | Joe Shead

Perch anglers are probably familiar with yellow grub, although this parasite can affect other species. These yellow or white grubs are sometimes visible on the throat or the base of the tail of infected fish. Fish-eating birds deposit the eggs of a trematode (fluke) into the water through their feces. The first larval stage of the parasite, known as miracidia, infect snails. The parasite eventually develops into another larval stage called cercariae, which leaves the snail and attaches to a fish. The grubs are about the size of a pimple. Although unsightly, the parasite does not pose a threat to humans. Pick them out with your fillet knife, or just thoroughly cook the fillet.

Walleye Dermal Sarcoma (Cut It Out, Cook the Rest)

Walleyes sometimes get infected with Walleye Dermal Sarcoma or Lymphocystis. These diseases look like clusters of warts on fish. When the warts are cut off and the fillet is thoroughly cooked, the fish are safe to eat. | Joe Shead

Walleye Dermal Sarcoma is a benign tumor caused by a virus that affects walleyes and sauger. The tumor develops in the fish’s skin, causing a white, pink or purple cluster-like growth. Transmission is via direct contact between fish, and thus, is often transmitted while spawning, when infected cells rupture, releasing the virus. The virus cannot infect humans, but it is best to cut away the tumor from the fillet and thoroughly cook the meat.

Lymphocystis (Looks Bad, Low Risk)

Lymphocystis is similar in appearance and develops in a similar manner as Walleye Dermal Sarcoma. Although it is commonly found in walleyes, it infects other species like crappies as well. The disease looks like a cluster of white or pink warts. Lymphocystis is caused by a virus that infects fish by entering skin abrasions and then attacking cells. These cells grow to a millimeter or more in diameter. Eventually these growths rupture, releasing more viruses into the water.

Lymphocystis tumors are composed of grossly enlarged cells. However, dermal sarcomas consist of irregularly shaped, but normal-sized cells. Both diseases can infect the same fish at the same time. In fact, both diseases have been seen in the same tumor. However, they are caused by different viruses.

Tapeworms (The One to Take Seriously)

Tapeworms may grow in several fish species. Unlike many other fish ailments, tapeworms can infect humans if the fish are not frozen long enough or cooked thoroughly enough to kill them. | Joe Shead

Fish can harbor tapeworms as well. And unlike the previously mentioned fish diseases, tapeworms can infect humans.

Fish can be infected with tapeworms when mammalian hosts excrete tapeworms into the water through their feces. Tapeworm eggs hatch and the larvae are taken up by crustaceans as the first intermediate host. These larvae then infect fish as the second intermediate host.

You may encounter tapeworms several inches long in fish fillets. Northern pike, walleyes, perch, trout, whitefish and other species may harbor tapeworms.

If not frozen or cooked properly, tapeworms can grow inside human intestines. Cold pickling of infected fish will not kill the tapeworm. Freeze the fillets at 0 degrees Fahrenheit for at least 48 hours to kill the parasite before pickling. Undercooked tapeworms can grow in human intestines. People may not show symptoms of infection, but may exhibit abdominal pain, diarrhea, weakness or weight loss. Medication can kill the tapeworm.

Quick Safety Guide for Eating Diseased Fish

• Most visible fish diseases are harmless to humans
• Always remove affected tissue
• Cook fish thoroughly—do not consume rare fish
• Cold pickling alone does not kill all parasites
• When in doubt, don’t eat the fish

Knowledge Beats Guesswork at the Dinner Table

These are just a handful of the most common fish ailments anglers may see, but there are many more. While they may look quite alarming, most of these fish infections do not pose a risk to humans when fish are handled and prepared properly. Always cook fish thoroughly, freeze fish intended for pickled preparation, and do your research. And remember, if you’re concerned, don't eat the fish.