Skip to Local Navigation

Parasites of Freshwater Fish

What is a parasite?
Can parasites kill fish?
Do all fish have parasites?
Can I eat a diseased or parasitized fish?
Can anglers help prevent infections in fish?
How do I identify freshwater fish parasites? (External | Internal)
What causes tumors in fish?
What are the most common parasites?
What's the latest about Viral Hemorrhagic Septicemia virus (VHS)?
What should I do if I see dead and dying fish?
Where can I learn more about fish parasites?
Related Pages

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)

Symptom	Parasite	Type	Description	Edible	Recommendation
Cluster of white or cream colored warts. May be pinkish to red and resemble a raspberry	LYMPHOCYSTIS	virus	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	virus	Infects walleye; similar to lymphocystis, except cells are normal size.	Yes	Remove infected skin; clean fish and prepare as usual
Cauliflower-like reddish tumor; small to several inches; may spread to internal organs	LYMPHOSARCOMA	virus	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
Fish popeyed; scales puffed with fluid (dropsy). Bloody wounds, inflammation around mouth	BACTERIA (such as Aeromonas spp.)	bacteria	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
Grey-white to yellowish slime on skin, gills or fins; also frayed fins	COLUMNARIS DISEASE (Flexibacter columnaris)	bacteria	Infects catfish, trout, and possibly other species.	Yes	Clean fish and prepare as usual
White hair-like growths on body. Appears cottony or furry; collapses when fish removed from water	FUNGAL INFECTION (Saprolegnia spp.)	fungus	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
Tiny white spots or clusters on skin or gills	ICH (Ichthyophthirius multifilis)	protozoan	Most common protozoan encountered by anglers; visible with magnifying glass. Burrows under skin and may cause lesions.	Yes	Clean fish and prepare as usual
Red pustule on or near base of fins; thread-like body may protrude from wound	ANCHORWORM (Lernaea spp.)	copepod	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
Bloody area under scales. Louse-like organism, resembling miniature turtle under scales	FISH LOUSE (Argulus spp.)	copepod	Rarely seen; up to ½ 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
White, cream, or yellow cysts or sacs on gills or in mouth	Ergasilus spp.	copepod	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 ½-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
Small round black spots under skin or in flesh	BLACK SPOT (Neascus spp., Uvulifer spp.)	fluke	Easy to recognize; caused by larval flukes burrowing under skin. Become adults in fish-eating birds.	Yes	Clean fish and prepare as usual
Eye trouble: bulging (popeye); opaque or shrunken; fish apparently blind	EYE FLUKE (Diplostomulum spp.)	fluke	Microscopic larval flukes live in fluid of eye, often causing popeye. Bulging subsides but blindness may result.	Yes	Clean fish and prepare as usual
Red thread-like worms extending from anus	Camallanus spp.	roundworm	Found throughout intestine. Occasionally species living in lower large intestine extends from anus.	Yes	Clean fish and prepare as usual
White to pink thread-like swelling on head or fins	Philometra spp.	roundworm	Typically infests carp, buffalo, and suckers; lives just under skin.	Yes	Clean fish and prepare as usual
Flattened, segmented worms attached to body, gills, fins, or mouth; sucker on both ends. Can be greenish to black; undulating	LEECHES	leech	Conspicuous blood-feeder; produce small circular wound that persist though leech may drop off. Up to 1½-inches long.	Yes	Clean fish and prepare as usual
Attached Eel-like fish, 6- to 25-inches long; OR circular wound or scar ½- to 1-inch diameter	LAMPREY Three species of parasitic lamprey in Minnesota: chestnut and silver from inland lakes and rivers; sea lamprey from Lake Superior.	lamprey	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	CROOKED SPINE (Myxosoma cerebralis causes deformities in young trout)	protozoan; other	Other causes include genetic and nutritional defects, bacterial infections, chemical exposure, electric shock. Difficult to pinpont cause.	Yes	Clean fish and prepare as usual
Tiny freshwater mussels attached to gill filaments or fins. May also appear as lumps on gill filaments or fins	Glochidia (larval stage of many freshwater mussels)	mussel	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)

Symptom	Parasite	Type	Description	Edible	Recommendation
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	YELLOW GRUB (Clinostomum spp.)	fluke	Cream-colored cysts contain larval flukes (about ½-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 spots under the skin or in flesh	BLACK SPOT (Neascus spp., Uvulifer spp.)	fluke	Easy to recognize; caused by larval flukes burrowing under skin. Become adults in fish-eating birds.	Yes	Clean fish and prepare as usual
White, glistening, worm about ½ to 1½- inches long found in muscle. May be coiled and encysted	BROADFISH TAPEWORM (Diphyllobothrium spp.).	tapeworm	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
Areas of flesh appear white, opaque, and freezer burned or cooked	HETEROSPORIS	microsporidian	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
Areas of flesh appear yellowish brown with granular deposits and be semi-translucent or opaque	MYOFIBROGRANULOMA	virus	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.
Whitish segmented flat worms in intestines of fish	BASS TAPEWORM (Proteocephalas spp.)	tapeworm	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
Large, white, unsegmented flat worm tapered at ends; found free in body cavity	RIBBON TAPEWORM (Ligula spp.)	tapeworm	Infects minnows, carp, suckers, and other species. Thick fleshy body uncommonly large and may create abdominal bulge.	Yes	Clean fish and prepare as usual
White, threadlike worms lying on or moving through the internal organs	LARVAL TAPEWORM	tapeworm	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
White or orange worm in the body cavity, attached to the inside of the intestine.	SPINY-HEADED WORM (Acanthocephala spp.)	spiny-headed worm	Adults typically live inside intestine and are not seen.	Yes	Clean fish and prepare as usual
Irregular white cysts in or on the internal organs.	LARVAL SPINY-HEADED WORM OR LARVAL TAPEWORM	spiny-headed worm; tapeworm	These cysts are larger, whiter and not as round as those described for White Grub	Yes	Clean fish and prepare as usual
Coiled worm (like a watch spring) encysted on internal organs	Contracaecum spp. and Eustrongylides spp.	roundworm	Found on internal organs or wall of body cavity; immobile. Become adults in fish-eating birds.	Yes	Clean fish and prepare as usual
Tiny gold-brown cysts on the internal organs	LARVAL ROUNDWORM	roundworm	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
White, undulating worms emerging from ruptured intestines	INTESTINAL WORMS	flukes, tapeworms, roundworms, spiny-headed worms	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?

Definitions

BACTERIA: 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.
COPEPOD: 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.
CYST: A nonliving sheet-like structure enclosing a parasite. The sheet-like structure or membrane may be produced by the host, the parasite, or both.
FLUKE: Another name for TREMATODE or GRUB. These parasites cause flattened worm-like cysts in and on fish.
FUNGUS: 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.
LARVA: An immature stage between egg and adult. Many parasites may go through several life stages and hosts before becoming adults.
PROTOZOAN: 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.
ROUNDWORM: 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.
SPINY-HEADED WORM: 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.
TAPEWORM: 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.

By Jeff Gunderson, 2008

Fish:

Contact:

Jeff Gunderson: Fisheries Specialist