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Horsehair worms

Jeffrey Hahn

horsehair worm

Photo: Gary Averbeck
U of MN Veterinary Pathobiology

Horsehair or gordian worms are long, slender worms related to nematodes. They get their name because of the mistaken belief that they originated from the long thin hairs of a horse's tail or mane that have fallen into a horse trough. When they are immature, they are parasites of insects, arthropods, and other invertebrate animals. As adults, they are free- living. They are harmless to people in all stages of their lives.


Horsehair worms are long, measuring from several inches to over 14 inches. They are quite thin, ranging from 1/25 inch to 1/16 inch wide (1 mm to 1.5 mm) and are uniform in diameter from front to back. They vary greatly in color from whitish to yellow/tan to brown/black. Horsehair worms are found on the ground or on plants, especially near water.


Horsehair worms mate during spring, early summer or fall. Males coil around females in pools of fresh water or damp soil. It is not uncommon for a number of individuals to be intertwined, forming a loose ball during mating. Eggs are laid in a long, gelatinous string in fresh water.

Once they hatch, immature horsehair worms attempt to infect a host. They are known to attack a wide variety of insects and related animals, such as grasshoppers, crickets, cockroaches, beetles, and katydids, as well as dragonflies, caddisflies, millipedes, centipedes, spiders, crustaceans, leaches, snails, slugs, and other invertebrates.

It is not clear how immature horsehair worms infect hosts. Some researchers believe that they encyst (to become enclosed inside a cyst) on vegetation near water and are ingested by a host. The cyst breaks down and the larva penetrates through the intestinal wall and into the body cavity. Other investigators believe that the larvae opportunistically penetrate the body wall of any arthropod or invertebrate that they encounter.

While they parasitize their host, they store up fats and food reserves. When the horsehair worm is mature and near water or damp soil, it emerges from its host. This emergence usually kills the insect (or other invertebrate host). Once emerged, adults are free-living and do not feed.

Horsehair worms are often seen in puddles of water and other pools of fresh water, swimming pools, water tanks and as well as on plants. They are especially common after a rainfall. Horsehair worms may even be found inside homes in toilets. This can cause considerable concern as people often worry that they may have found some type of human parasite.


Horsehair worms are harmless to people, pets, and plants. In fact, they should be considered beneficial because they can be effective in controlling certain insects. Horsehair worms are nothing more than a curiosity. No control is necessary.

Published in Yard & Garden Brief, November 1999

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