Red Worm


This roundworm (nematode), commonly called the red worm, is abundant in yellow perch of Lake Huron and western Lake Erie, but is also found in other localities. As high as 86% of fish examined from areas of Lake Huron have been found infested with as many as 78 worms per fish, although it is usually found in lesser abundance.

This parasite is pink to red in color, very slender, and may reach 2 inches (5 cm) in length. It is usually found encysted within the body cavity of the fish, but in very heavy infestations, or in fish not dressed quickly upon capture, the worms may move out of the cysts and be found free in the body cavity or even in the flesh.

The life cycle of the red worm of perch is not known but it is believed to be similar to the closely related species (P. nodulosa) found in the common sucker which use various species of a crustacean (Cyclops) as an intermediate host


Red worm encysted in the body cavity of a perch. Note the reddish worm at the tip of the pointer.

In this case, larvae developing from eggs of the adult female worms in fish escape into the water and are eaten by species of Cyclops. Within the copepod the larvae attain a certain growth stage, and remain in this state until eaten. When a fish feeds on the colpepod, the larval worm leaves the crustacean, grows, and migrates through the tissues of the fish, eventually encysting in the body cavity.


Enlarged view of an encysted worm in the fatty tissue of a fish.

Although this parasite has not been implicated in mortalities of fish, heavy infestation must certainly affect the normal growth and vitality of such fish. These nematodes are incapable of infesting man, regardless of how objectionable a parasitized fish might be aesthetically. As with all parasites, thorough cooking kills this worm.


Red worm leaving its cyst and penetrating the body wall of a perch.