Parasites of Freshwater Fish

What is a parasite?

A parasite is an organism that lives in or on another larger organism of a different species (the host), upon which it depends for food. Although the parasite benefits from the association, the host is harmed. Depending on the species, the host/parasite relationship may be temporary or permanent. Bacteria and viruses are classified as parasites in some branches of biology.

Can parasites kill fish?

Parasites generally don't kill their hosts (it's a dumb parasite that kills its free lunch), but some can severely stress fish populations to the point of becoming biological and economical concerns. Parasites have a stake in the survival of their host. Sometimes, when parasites are numerous or the fish is stressed from another cause, the fish will die. Parasites can weaken a fish by:

  • destroying tissue
  • removing blood and cellular fluids
  • diverting part of its nutrient supply
  • allowing secondary infections to develop

Do all fish have parasites?

Yes, in fact, parasitism is natural and normal. It occurs throughout the plant kingdom and in every major group of the animal kingdom. Usually, fish parasites aren’t noticeable, but sometimes anglers will catch a fish with obvious signs of infection or parasitism.

Can I eat a diseased or parasitized fish?

It is advisable to thoroughly cook or hot smoke all fish to 140° F for at least five minutes, or freeze them at 0° F for 48 hours. If you are planning to eat an infected or infested fish, see the recommendations in the Parasites Table.

Although some parasites make fish look and taste unappetizing, very few fish parasites can be transferred to humans. Even when a fish exhibits obvious signs of disease or parasites, most likely the fish is still edible when cooked, hot smoked, or frozen. People have been infected with tapeworms (Diphyllobothrium latum) after consuming marinated, uncooked walleye and northern pike. In one incident, anglers marinated freshly caught fish overnight in lemon juice and ate them the next day. They were following a recipe for seviche, a South American dish. Generally, Minnesotans don't prepare seviche, but they do pickle raw fish. Pickling alone may not destroy a larval tapeworm.

Can anglers help prevent infections in fish?

Yes! Fish secrete a mucous coating over the entire length of their body that wards off fungal, viral, and bacterial infections. If this mucous coating is damaged, the fish becomes more susceptible to infection. Anglers can help prevent infections by taking extra care when returning fish to the water. To protect the mucous coating, remove the hook while the fish is still in the water or wet your hands before handling the fish. Release the fish gently and as quickly as possible after the hook is removed.

How do I identify freshwater fish parasites?

External Parasites (Ectoparasite)

Parasite Type Symptom Description Edible Recommendation
Lymphocystis | Click to Enlarge
virus Cluster of white or cream colored warts. May be pinkish to red and resemble a raspberry Infects walleye (occasionally perch and other species). Enters skin abrasions and attacks connective tissue. Cells grow to enormous size and cause lumpy growths. Yes Remove infected skin; clean fish and prepare as usual
Dermal Sarcoma | Click to Enlarge
virus   Infects walleye; similar to lymphocystis, except cells are normal size. Yes Remove infected skin; clean fish and prepare as usual
Lymphosarcoma | Click to Enlarge
virus Cauliflower-like reddish tumor; small to several inches; may spread to internal organs Infects muskies and northern pike. More prominent in cooler water (fall, winter). Usually fatal in muskellunge. No; lack of scientific understanding Dispose of fish or entrails in household waste or bury. Never discard into a waterbody
BACTERIA (such as Aeromonas spp.) bacteria Fish popeyed; scales puffed with fluid (dropsy). Bloody wounds, inflammation around mouth Infects stressed fish. Severely infected fish probably won't bite on hook, but may be found dead or distressed. May have open bloody wounds. Maybe If wound is superficial, cut out infected tissues. Do not eat if fish has puffy body and swollen eyes
COLUMNARIS DISEASE (Flexibacter columnaris) bacteria Grey-white to yellowish slime on skin, gills or fins; also frayed fins Infects catfish, trout, and possibly other species. Yes Clean fish and prepare as usual
FUNGAL INFECTION (Saprolegnia spp.) fungus White hair-like growths on body. Appears cottony or furry; collapses when fish removed from water Infects fish injured during spawning, by improper handling, or other causes. Can cover and kill fish. Yes; may taste peculiar Skin fish; cut out infected tissue and adjacent flesh. Prepare as usual
ICH (Ichthyophthirius multifilis) protozoan Tiny white spots or clusters on skin or gills Most common protozoan encountered by anglers; visible with magnifying glass. Burrows under skin and may cause lesions. Yes Clean fish and prepare as usual
ANCHORWORM (Lernaea spp.) copepod Red pustule on or near base of fins; thread-like body may protrude from wound 1/8 to 1 inch; buries anchor-shaped head into fish. Body protrudes; inflamed pustule may form. Parasite may drop off, leaving inflamed area. Yes Cut out inflamed area; clean fish and prepare as usual
FISH LOUSE (Argulus spp.) copepod Bloody area under scales. Louse-like organism, resembling miniature turtle under scales Rarely seen; up to ˝ inch; often leaves fish after removed from water. Pierces skin, destroying fish's mucous coat. Secondary infection from bacteria or fungus can result. Yes Clean fish and prepare as usual
Ergasilus spp. copepod White, cream, or yellow cysts or sacs on gills or in mouth When numerous, can kill young fish. Presence indicated by V-shaped white egg sacs on inner edges of gills. Yes Clean fish and prepare as usual
Achtheres and Salmincola spp. copepod   Infects mainly trout and salmon. White; short plump bodies with arm-like appendages that cling to fish. Yellow egg sacs. Attach in the mouth or on gills. Yes Clean fish and prepare as usual
YELLOW GRUB (Clinostomum spp.) fluke   Cream-colored cysts contain larval flukes (about ˝-inch long); numerous at times. Will emerge if cyst broken. Become adults in fish-eating birds. Also parasite of flesh. Yes Clean fish and prepare as usual
BLACK SPOT (Neascus spp., Uvulifer spp.)
Neascus | Click to Enlarge
fluke Small round black spots under skin or in flesh Easy to recognize; caused by larval flukes burrowing under skin. Become adults in fish-eating birds. Yes Clean fish and prepare as usual
EYE FLUKE (Diplostomulum spp.) fluke Eye trouble: bulging (popeye); opaque or shrunken; fish apparently blind Microscopic larval flukes live in fluid of eye, often causing popeye. Bulging subsides but blindness may result. Yes Clean fish and prepare as usual
Camallanus spp. roundworm Red thread-like worms extending from anus Found throughout intestine. Occasionally species living in lower large intestine extends from anus. Yes Clean fish and prepare as usual
Philometra spp. roundworm White to pink thread-like swelling on head or fins Typically infests carp, buffalo, and suckers; lives just under skin. Yes Clean fish and prepare as usual
LEECHES leech Flattened, segmented worms attached to body, gills, fins, or mouth; sucker on both ends. Can be greenish to black; undulating Conspicuous blood-feeder; produce small circular wound that persist though leech may drop off. Up to 1˝-inches long. Yes Clean fish and prepare as usual
LAMPREY Three species of parasitic lamprey in Minnesota: chestnut and silver from inland lakes and rivers; sea lamprey from Lake Superior. lamprey Attached Eel-like fish, 6- to 25-inches long; OR circular wound or scar ˝- to 1-inch diameter Sucker mouth with horny teeth and rasping tongue used to cut through scales and skin; feed on blood and body fluids. Yes If fish has an open sore, remove the inflamed area; clean fish and prepare as usual
CROOKED SPINE (Myxosoma cerebralis causes deformities in young trout) protozoan; other Crooked spine Other causes include genetic and nutritional defects, bacterial infections, chemical exposure, electric shock. Difficult to pinpont cause. Yes Clean fish and prepare as usual
Glochidia (larval stage of many freshwater mussels) mussel Tiny freshwater mussels attached to gill filaments or fins. May also appear as lumps on gill filaments or fins Soon after attaching, become encysted; appear as enlargements. Cysts open, mussels fall to substrate, develop into nonparasitic adults. Yes Clean fish and prepare as usual


Internal Parasites (Endoparasite)

Parasite Type Symptom Description Edible Recommendation
YELLOW GRUB (Clinostomum spp.) fluke White, cream, or yellow cysts embedded in muscle, especially at base of tail or fins. Heavy infections may be found in body cavity, head, throat, and gills. Flukes very active when squeezed from cysts Cream-colored cysts contain larval flukes (about ˝-inch long); numerous at times. Will emerge if cyst broken. Become adults in fish-eating birds. Also parasite of flesh. Yes Clean fish and prepare as usual
WHITE GRUB (Posthodiplostomum spp.) fluke   Smaller and whiter than yellow grub. Numerous at times; can cause small round transparent cysts on internal organs, typically the liver. Yes Remove cysts from flesh; clean fish and prepare as usual
BLACK SPOT (Neascus spp., Uvulifer spp.)
Neascus | Click to Enlarge
fluke Black spots under the skin or in flesh Easy to recognize; caused by larval flukes burrowing under skin. Become adults in fish-eating birds. Yes Clean fish and prepare as usual
BROADFISH TAPEWORM (Diphyllobothrium spp.). tapeworm White, glistening, worm about ˝ to 1˝- inches long found in muscle. May be coiled and encysted Infects northern pike, walleye, and yellow perch, and other species. Adults develop in intestines of humans and other mammals. Symptoms include nausea, vomiting, weakness, dizziness, diarrhea. Yes, but… Destroyed by normal cooking or hot smoking to 140° F for minimum of 5 min, or by freezing for 48 hours at 0° F. Cold smoking, marinating, and pickling without cooking or freezing may not kill the tapeworm
Heterosporis | Click to Enlarge
microsporidian Areas of flesh appear white, opaque, and freezer burned or cooked Infects yellow perch, but also found in northern pike, walleye, burbot, sunfish, and rock bass. Not widespread. Forms spores in muscle cells. Yes, but… decreases quality and changes texture of fillet Cut out infected areas and prepare as usual. Dispose of fish or entrails in household waste or bury. Never discard into a waterbody
Myofibrogranuloma | Click to Enlarge
virus Areas of flesh appear yellowish brown with granular deposits and be semi-translucent or opaque Infects walleye in Minnesota, Wisconsin, and the Dakotas; genetic and environmental stressors may play a role. No; lack of scientific understanding Dispose of fish or entrails in household waste or bury. Never discard into a waterbody.
BASS TAPEWORM (Proteocephalas spp.) tapeworm Whitish segmented flat worms in intestines of fish Infects large and smallmouth bass but also in trout, perch, pike, and carp; larvae may invade fish reproductive organs. Yes Clean fish and prepare as usual
RIBBON TAPEWORM (Ligula spp.) tapeworm Large, white, unsegmented flat worm tapered at ends; found free in body cavity Infects minnows, carp, suckers, and other species. Thick fleshy body uncommonly large and may create abdominal bulge. Yes Clean fish and prepare as usual
LARVAL TAPEWORM tapeworm White, threadlike worms lying on or moving through the internal organs Not found in cysts. Numerous worms may infect the ovaries of bass. Yes Clean fish and prepare as usual. Roe can be cleaned by removing worms with tweezers before preparing
SPINY-HEADED WORM (Acanthocephala spp.) spiny-headed worm White or orange worm in the body cavity, attached to the inside of the intestine. Adults typically live inside intestine and are not seen. Yes Clean fish and prepare as usual
LARVAL SPINY-HEADED WORM OR LARVAL TAPEWORM spiny-headed worm; tapeworm Irregular white cysts in or on the internal organs. These cysts are larger, whiter and not as round as those described for White Grub Yes Clean fish and prepare as usual
Contracaecum spp. and Eustrongylides spp. roundworm Coiled worm (like a watch spring) encysted on internal organs Found on internal organs or wall of body cavity; immobile. Become adults in fish-eating birds. Yes Clean fish and prepare as usual
LARVAL ROUNDWORM roundworm Tiny gold-brown cysts on the internal organs Often numerous; cysts give sandy appearance to entrails. Kidney roundworm, found in bullheads and northern pike, may infect humans. Yes, but… Clean fish and prepare as usual. Thorough cooking kills this parasite
INTESTINAL WORMS flukes, tapeworms, roundworms, spiny-headed worms White, undulating worms emerging from ruptured intestines Normally not seen unless intestine is accidentally cut. Yes Clean fish and prepare as usual


What causes tumors in fish?

Tumors (fibrous or fatty tissue in well-defined lumps) are uncommon in fish. Tumors may be caused by contaminants, so check fish consumption advisories for the waterbody of origin before eating a fish with a tumor. Continuous tissue injury, infections, and damage to genes by viruses or chemicals can cause cells to alter or replicate quickly and produce tumors.

What are the most common parasites?

Because they are the easy to see, "grubs" (flukes, Trematods) are the most commonly reported parasite living on fish. The most common flukes in Minnesota fish are:

Yellow grub
Spends most of its life in the mouths of herons. Leaves the heron, and invades snails and eventually a particular species of snail. Next, it burrows into the muscle of a fish.
White grub
Typically found in the liver, heart, and other internal organs of sunfish. It's visible as numerous white "specks."
Black grub
Ironically, they're actually white. Dark pigment from the fish encysts the flukes, making them look black. Adults live in the intestines of kingfishers. Fluke eggs drop into the water with the birds' feces. They hatch and enter aquatic snails. After maturing somewhat, they leave the snails and infect fish.

What's the latest about Viral Hemorrhagic Septicemia virus (VHS)?

VHS has infected fish in all the Great Lakes, except Lake Superior, and some inland lakes in New York, Michigan, and Wisconsin. It has not been reported in Minnesota as of early 2008. Fish afflicted with VHS may appear limp or swim abnormally, and show bleeding from their eyes, skin, gills, skeletal muscles, and and fin bases. They also may have abnormal eyes (popeye or sunken eyes). The virus can cause significant fish die-offs. The USDA Animal and Plant Health Inspection Service has listed 28 species of fish located in the Great Lakes Basin that have been infected with VHS. Fish mortality from VHS is greatest at water temperatures between 38 and 54°F and rare above 60°F.

For more information, see:

What should I do if I see dead and dying fish?

Minnesota anglers should report suspicious fish kills to the Minnesota DNR's State Duty Office, (651-649-5451 or 1-800-422-0798). Viral Hemorrhagic Septicemia (VHS) and Spring Viremia of Carp (SVC) have not been reported in Minnesota but fisheries managers are watching for these viruses.

Where can I learn more about fish parasites?


Includes a large group of one-celled microscopic organisms. While bacteria cannot be seen with the unaided eye, the signs of bacterial infection are usually visible.
Member of a group of small crustaceans. It is very abundant in aquatic systems and commonly used as food by fish. Although most copepods are free-swimming, those mentioned here are fish parasites.
A nonliving sheet-like structure enclosing a parasite. The sheet-like structure or membrane may be produced by the host, the parasite, or both.
Another name for TREMATODE or GRUB. These parasites cause flattened worm-like cysts in and on fish.
Minute thread-like plants that lack chlorophyll. Common in fresh water, most fungi grow on decaying organic matter. Fungi usually attack fish only when the skin has been injured through abrasion or by other parasites.
An immature stage between egg and adult. Many parasites may go through several life stages and hosts before becoming adults.
A single-celled organism of the lowest division in the animal kingdom. Although a few may be seen with a magnifying glass, a microscope is required to see most species. Not all protozoa are parasites.
A class of organisms also known as NEMATODE. They have round elongated bodies tapering at both ends and lack segmentation and suckers. They are among the most common of fish parasites. There are free-living (nonparasitic) as well as parasitic roundworms.
A common name for a group of parasitic worms called Acanthocephalans. Usually under one-and-a-half inches long, they live in the digestive tract of various animals.
Also known as CESTODE; adults are white, flattened, segmented worms that inhabit the intestine. Some tapeworms occur only as larvae in fish and develop into adults in other animals such as predatory fish, birds, or mammals. All tapeworms are parasitic.

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By Jeff Gunderson

This page last modified on October 15, 2014     © 1996 – 2015 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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