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October 25, 2001

Indigestible Parasites

Q: Anyone who has cleaned and examined the stomach contents of a wahoo has come across one of these critters. The wahoo I cleaned recently had two of them. What are they and how do they live in a harsh environment like the digestive tract of a fish? Are they harmful to the fish? And how does a pelagic fish like a wahoo encounter an internal parasite to begin with? - Scott Kerrigan, Wilton Manors, FloridaA: These are "giant stomach worms" of the genus Hirudinella, which reach sizes up to 8 inches long. I have seen them also in the stomachs of tunas caught in the Gulf of Mexico.

Q: Anyone who has cleaned and examined the stomach contents of a wahoo has come across one of these critters. The wahoo I cleaned recently had two of them. What are they and how do they live in a harsh environment like the digestive tract of a fish? Are they harmful to the fish? And how does a pelagic fish like a wahoo encounter an internal parasite to begin with? - Scott Kerrigan, Wilton Manors, Florida

A: These are "giant stomach worms" of the genus Hirudinella, which reach sizes up to 8 inches long. I have seen them also in the stomachs of tunas caught in the Gulf of Mexico. They belong to a class of parasites known as Digeneans, of which more than 1,700 species have been found in fish. Most live in the stomachs or intestines of their hosts. They have two suckers, one around the mouth at the front end of the animal and a larger one at either mid-body or the back end. Unlike some other intestinal worms, they lack hooks, clamps or spines, and so are probably less abrasive to their hosts.
Digeneans have complex life cycles, typically including seven stages of metamorphosis and two intermediate hosts. The first intermediate host is a mollusk such as a snail. The second intermediate host and the final host are both fish. In the fifth metamorphic, or cercaria, stage, between the first and second intermediate hosts, the tiny, tailed parasite swims free in the water, where it actively attacks and burrows through the skin of the second host and into the flesh. There it encysts until it is eaten by the final host, such as a tuna or wahoo. It then enters the final host's digestive tract and grows into an adult parasite.
Some marine cercaria can cause severe damage and death to young fish, but there are no reports of any serious effects of the adult Hirudinella worm on marine fish. However, if infected intermediate-host fish are eaten raw, other species of Digenean parasites can survive and reproduce in the human intestine, and some human deaths from this cause have been reported.