Virtually all freshwater fish suffer from parasites at some point during their life span, according to the Wisconsin Department of Natural Resources. Most of the parasites are host specific -- meaning they pose no danger to humans -- but tapeworms, roundworms and flukes can infect humans. Thoroughly cooking fish prior to consuming it adequately kills the parasites and their eggs.
Parasites do not kill freshwater fish unless the fish suffers an underlying health issue. The parasites can weaken the fish and predispose it to other infections or diseases. The parasites do destroy some of the fish's tissue and absorb a bit of its nutrients, but the parasite usually manages not to kill its host. The parasites generally spend only a part of their life within or on the fish.
Internal Parasitic Worms
Anglers may notice flukes attached to the inside of the mouth or ventricle of the fish. These worm-like parasites are white in color and have a flattened body. Grubs form a bump under the skin of the fish, and they are often noticeable around the eye, in the eye and on the liver of the fish. The worms reside within a cyst. When the cyst is dissected, the worm appears approximately 1/2 inch in length and is yellow or white in color. Within the intestines or internal organs of the fish, tapeworms and roundworms are often noticeable as long white worms that sucker their mouth parts to the fish.
External Parasitic Worms
External worm parasites can occur on the gills, skin and fins of freshwater fish. Common external parasites include anchor worms and leeches. A high population of anchor worms on a fish does pose a threat by destroying the scales of the fish, which allows secondary infections to occur. Leeches pose no danger to the fish and only spend a small portion of time feeding on the fish.
Protecting Fish and Humans
Many anglers enjoy catching and releasing fish. Unfortunately, handling the fish roughly can damage the fish's mucus coating that covers its scales and skin. The damaged areas become more susceptible to parasite infestation. Avoid lifting the fish physically out of the water to remove the hook. Keep the fish in the water to minimize damage to the coating. When keeping fish for human consumption, freeze the fish at 0 degrees Fahrenheit for at least 48 hours to effectively kill parasites. Cook the fish for at least five minutes at 140 degrees, according to the University of Minnesota Extension.