The Big Red Worm

Eustrongylides ignotus and Eustrongylides tubifex

Ohio State University

The Big Red Worm is actually a nematode. Nematodes, also called "roundworms," are found all over the world and there are many different species. Most are tiny, and a microscope is needed to see them well, but this one gets is nickname from its size.

The Big Red Worm (Eustrongylides ignotus) and its close cousin (Eustrogylides tubifex) are parasites. They have interesting and complicated life cycles.

Dr. Lena Measures, Maurice Lamontague I nstitute, Fisheries and Oceans Canada, Mont-Joli, QC, Canada

We'll start the life cycle with the egg stage (there's a total of four stages). Eggs are always inside bird poop. They're not just in any bird poop, but only in the poop of certain species of water birds. The eggs must fall in water to continue their life cycle.

Once in water, aquatic worms (cousins of earthworms, called "oligochaetes") eat the bird poop, along with the eggs of The Big Red Worm. Inside the body of the aquatic worm, each egg changes into the second stage. A larva forms inside the egg. Later, still inside the aquatic worm, the larva hatches out of the egg into its third stage.

Still with us? To continue its life cycle, The Big Red Worm needs a fish to eat the aquatic worm. It is usually the Eastern Mosquitofish or another small fish. Aquatic worms are one of the mosquitofish's favorite foods. When they eat the aquatic worm, then The Big Red Worm larvae are now inside the fish's body.

Field Manual of Wildlife Diseases: Birds, USGS

Big Red Worm third-stage larvae live just under the surface of the fish. If you saw an infected fish, you could see tiny black spots. Larvae are still pretty small. One of two things happens next. Either a bird eats the Eastern Mosquitofish; or, another fish, reptile, or amphibian eats the fish. If this happens, the larger animal becomes a "transport host" for The Big Red Worm. This is not where The Big Red Worm needs to end up, but it can get lucky if a bird then eats the transport host.

Once inside a bird, The Big Red Worm can finish its life cycle. Birds which host these nematodes include herons, egrets, cormorants, mergansers, mallards, and sometimes even eagles or ospreys. Most of the time it is wading birds (herons and egrets). As soon as the bird eats the mosquitofish, or transport host, The Big Red Worm burrows into its stomach. In the bird's stomach, the larva becomes an adult (fourth stage). The nematode grows for about 10 days. Then it mates and lays eggs, which pass out of the bird's stomach and end up in poop. The cycle can start again.

Altered Graphic from Field Manual of Wildlife Diseases: Birds, USGS

Adult Big Red Worms grow over 15 centimeters long. This is very large for a nematode. They are skinny (only 4 milimeters wide).

Most healthy, adult birds can handle a nematode infection, but Big Red Worms can kill nestlings or young birds.

The Big Red Worm is patient. Eggs can live for 2 1/2 years before being eaten by an aquatic worm. Larvae can survive inside an aquatic worm or fish for more than a year.

Big Red Worms do very well in polluted water.

Occasionally, larger animals, such as mammals, can be infected with Big Red Worms when they eat fish. This can cause them great discomfort, but the nematode can not complete its life cycle unless it is inside a bird.

Relationships in Nature:


Great Blue Heron

Great Blue Heron

Great Blue Heron H


Eastern Mosquitofish

Eastern Mosquitofish H

Aquatic Worm

Aquatic Worm H

Black Crappie

Bluegill H

Largemouth Bass

Black Crappie H

Channel Catfish

Bullfrog H

Yellow Perch

Creek Chub H

Creek Chub

Largemouth Bass H

Southern Leopard Frog

Yellow Perch H


Northern Water Snake H

Northern Water Snake

Channel Catfish H


Southern Leopard Frog H


Relationship to Humans:

The Big Red Worm is not usually a problem for humans, but they can be infected. There is a case where three fishermen became infected after eating their bait (minnows). They had to have surgery to remove the nematodes.

Eustrongylides ignotus and Eustrongylides tubifex

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