You Don't Want to Eat That Raw
Everything you ever wanted to know about fish parasites—and a few things you might rather not.
By Tim Brastrup and John McCue
It was one of those picture-perfect days on Lake of the Woods—clear blue sky, sparkling waters, crisp spruce, and ancient rocks lining the pristine shore in the distance. I was out fishing with my dad.
"It doesn't get any better than this," Dad said.
"Yeah?" I countered. I could think of at least one thing in the vicinity that was less than picture-perfect. To prove it, I tossed out my line and hauled in a healthy-sized walleye. Later at camp I sliced open its stomach—to reveal a 12-inch-long tapeworm.
My dad was astounded. How could such an undesirable creature be part of such an idyllic scene?
Which was exactly my point. Parasites are part of life for pretty much all fish, everywhere. Consequently, they are part of life for anglers too. No matter how unspoiled the setting, the fish you catch are probably going to come complete with uninvited guests. The good news is that most fish parasites are benign to humans, and all can be killed by properly cooking your catch.
What Are Parasites?
Parasites are living things that live off other living things. The fleas on a fox, the worms in your cat, and the gut-busting microorganisms you boil or filter out of your water when camping are all parasites. They are arguably among the most successful animals on Earth.
To anybody who is fascinated by the diversity of life, it doesn't get any better than parasites. Many have elaborate life cycles during which they change form and move from one animal to another. They tolerate a wide range of environmental conditions. They can produce tremendous numbers of young. And they have a variety of amazing adaptations that help them find, hang onto, and draw sustenance from other creatures.
Here are a few of the most common and most visible fish parasites.
Black Spot Disease, Uvulifer ambloplitis
Have you ever caught a fish that looked like someone had sprinkled it with coarse-ground pepper? You were likely looking at black spot disease, caused by the larval stage of certain flukes that live as adults in fish-eating birds. Any fish can become infected. The most frequently infected are sunfish, yellow perch, northern pike, bass, and other fish that live in shallow water and vegetated areas where birds and snails are found.
Life Cycle: 1 The adult fluke lives in the digestive tract of fish-eating birds, typically kingfishers or herons. It produces eggs, which pass from the bird in its droppings. The eggs hatch in water, liberating larvae called miracidia that swim around until they encounter a snail.
2 After digging their way into the snail, the miracidia form mother sporocysts. Mother sporocysts produce sporocysts that invade the snail's liver. In about six weeks, each sporocyst produces another form called a cercaria.
3 Cercariae emerge from the snail into the water. They seek out a fish and penetrate its flesh, forming cysts just under the skin and in the flesh.
4 If a bird eats infected fish, the parasites leave the cysts and mature in the digestive tract of the bird, starting the cycle again.
Effects on Fish: Black spot is rarely fatal to fish. A heavy infection may make a fry's eyes bulge. If heavily infected fish live in stressful conditions (poor nutrition, injury, crowding, etc.), they could die.
Effects on People: Black spot disease is not harmful to humans. However, you may find the spots unappetizing. Your best bet is simply to remove the skin and cysts in the flesh before cooking the fish.
Broad Fish Tapeworm, Diphyllobothrium latum
Adult broad fish tapeworms are found in the small intestine of fish-eating mammals such as bears, cats, and otters. They are huge worms, often more than several yards long. Several species of fish, primarily northern pike, carry these tapeworm larvae. If you eat improperly cooked fish infected with these larvae, they are likely to grow in your intestines too.
Life Cycle: 1 The eggs pass in the droppings of the host mammal to water, where they hatch into free-swimming forms called coracidia.
2 Coracidia are eaten by microscopic crustaceans called copepods. Inside the copepods, they develop into larvae called procercoids.
3 When a fish eats an infected copepod, the procercoids move into the fish tissues. They develop into wormlike forms called plerocercoids.
4 When a mammal eats an infected fish, the plerocercoid attaches to the wall of its intestine and grows into a mature tapeworm.
Effects on Fish: A broad fish tapeworm does not weaken a fish or stunt its growth. If a healthy northern pike, for example, eats an infected fish, the plerocercoid will migrate into the pike's flesh and encyst again. The plerocercoid will not develop into an adult tapeworm until a mammal eats the fish.
Effects on People: Warning—if you eat an infected fish, tapeworms are likely to grow in your digestive tract. They are not only gross, but they also can cause a nutrient deficiency, especially a vitamin deficiency. People have died after getting this type of tapeworm from sources such as inadequately cooked or cold-processed pickled northern pike.
To avoid this tapeworm, only eat thoroughly cooked fish. Cold-process pickling does not kill tapeworms or cysts! If you pickle fish, be sure to use heat in the processing.
Yellow Grub, Clinostomum complanatum
Adult yellow grubs are flukes and live in a heron's mouth. The larval stage, called a metacercaria, is the form referred to as a yellow grub. It appears as a yellowish cyst in the flesh or just under the skin of fish, most commonly yellow perch but also black bass and sunfish. Yellow grub doesn?t infect humans.
Life Cycle: 1 The eggs move from an infected heron's mouth into the water while the heron is catching fish. Once in the water, the eggs hatch almost immediately into miracidia.
2 When the miracidia come into contact with a ram's horn snail, they burrow into it and migrate to the snail's digestive gland or liver. The miracidia develop into sporocysts, which produce larvae called mother rediae. Each mother redia produces three to 15 daughter rediae.
3 Daughter rediae form cercariae, which escape from the snail into the surrounding water. When the cercariae encounter a fish, they burrow into it and form a cyst, often near the tail or front fins. They develop into metacercariae —yellow grubs—within about 20 weeks.
4 When a heron eats infected fish, the metacercariae migrate to its mouth and matures into flukes. There the flukes produce eggs, beginning the cycle again.
Effects on Fish: Yellow grubs usually have no significant effect on fish.
Effects on People: Like other parasites of birds, this fluke will not infect people. If you want, you can remove the cyst with a knife. When the fish is cooked thoroughly, the cysts will die and not be noticeable.
Bass Tapeworm, Proteocephalus ambloplitis
Adult bass tapeworms infect largemouth and smallmouth bass. They can harm bass fisheries by reducing reproduction. The tapeworm is introduced to new populations through migration or stocking. The transfer of harmful parasites is one reason it is illegal to transport live fish and to stock fish without a permit.
Life Cycle: 1 The tapeworm matures in the bass. Segments at the tail end detach from the worm and pass out of the intestine in the fish's feces. When they reach water, they swell, rupture, and release large numbers of eggs.
2 Copepods eat the eggs. Inside the copepods, the eggs release larvae called oncospheres. The oncospheres then develop into procercoids.
3 When young bass eat infected copepods, the procercoids migrate through the intestinal wall of the bass and develop into plerocercoids in the liver or ovaries, where they can cause infertility.
4 When an adult bass eats a young bass with plerocercoids, the plerocercoids migrate to the intestine and transform into an adult tapeworm.
Effects on Fish: The plerocercoid can destroy a fish's reproductive organs, making it sterile. Where bass and trout are found in the same lake, trout can become infected with bass tapeworms, making them less robust and causing slower growth.
Effects on People: Bass tapeworm has no known effects on humans. But if you find one in its plerocercoid form while filleting a bass, you almost certainly won?t be able to tell the difference between this tapeworm and those that harm humans. So be sure to thoroughly cook the fish.
Whitefish Tapeworm, Triaenophorus crassus
The adult whitefish tapeworm lives in the intestine of northern pike. Its plerocercoids infect whitefish and ciscoes (also known as tullibees), forming cysts filled with yellowish fluid and connective tissue. Netters of whitefish and cisco often encounter cysts when they fillet their catch for pickling or smoking.
Life Cycle: 1 The adult tapeworm lays eggs in the intestine of northern pike. The eggs pass out of the fish with feces and hatch into coracidia.
2 Copepods eat coracidia, which then develop into procercoids.
3 When whitefish or cisco eat the copepods, the procercoids burrow through the fish's gut wall into its muscle tissue, where they form cysts and develop into plerocercoids.
4 If a northern pike eats a whitefish or cisco, the plerocercoids attach to the intestinal wall of the pike. The plerocercoids develop into adult tapeworms to complete the cycle.
Effects on Fish: Infected whitefish or ciscoes are seldom harmed. The whitefish tapeworm may harm northern pike by competing with them for nutrients. Pike infected with tapeworms may become emaciated and unhealthy.
Effects on People: Whitefish tapeworms don?t harm humans, but the yellowish plerocercoid cysts in the flesh of whitefish and ciscoes are often numerous enough to make them unsightly. You can check for cysts by holding a fillet up to a light. A pea-sized shadow indicates the presence of a cyst. You can cut out the cyst before cooking the fish. Cook the fish thoroughly in case other harmful parasites are present.
Heterosporis, a microsporidium, primarily parasitizes yellow perch and walleye. Also found in Thailand, France, Germany, and Japan, this parasite was first discovered in Minnesota in 1999. It is now found in Michigan, Wisconsin, and Ontario as well. Yellow perch, walleye, trout, channel catfish, and fathead minnows appear to be most susceptible to heterosporis.
Some people think this parasite may be spread by infected fathead minnows sold as bait. To reduce the risk of spreading heterosporis, never release unused minnows into lakes. Currently heterosporis infections in Minnesota have been documented in Big Sand, Winnibigoshish, Leech, Clitherall, Vermilion, Mille Lacs, Bear, Moose, Cass, Andrusia, and Gull lakes.
Life Cycle: Heterosporis spreads when fish pick up spores from the water or eat infected fish or carcasses. Little is known about the life cycle.
Effects on Fish: Heterosporis causes muscle degeneration in fish. Areas look like they were cooked or freezer burned—white and opaque rather than translucent as in uninfected flesh. Infections can weaken or kill fish, depending on the severity of the infection and the condition of the fish. Some observers have reported seeing fish fillets where 90 percent of the fillet was infected with heterosporis.
Effects on People: We don't know what, if any, effect the spores of heterosporis have on humans. The Centers for Disease Control and Prevention has tested these spores on human cell cultures and found no infection. It is thought, but not proven, that thoroughly cooking infected fish will destroy spores.
If you see opaque-looking patches when you fillet a fish, report it to the DNR Information Center, listed on page 63. Either cook the fish thoroughly or discard the flesh by burning or burying it. Don?t throw it back into the lake!
John McCue is a retired professor of biology at St. Cloud State University, where he taught courses in general and wildlife parasitology. Tim Brastrup is DNR area fisheries supervisor at Brainerd. He became fascinated with fish parasites while a student of McCue.