Horsehair Worms
Horsehair worms belong to the group Gordiacea, which is closely related to nematodes. They often are seen in knotted masses or as single worms in water sources such as ponds, rain puddles, swimming pools, animal drinking troughs, and even domestic water supplies. Adult worms measure 1/25 inch in diameter and may reach 1 foot in length. An old and still common misconception is that these long, thin, blackish worms develop from horsehairs that fall into water. The worms are commonly encountered in agricultural areas, particularly those having water-impoundment and irrigation facilities.
If adult worms are accidentally swallowed with drinking water by people or domesticated animals, they are either promptly vomited or discharged via the intestinal tract. Their movements or secretions may cause mild intestinal disturbances in humans, but the worms are not parasites of humans or other vertebrates; they are parasites of certain insects and other invertebrates.
LIFE CYCLE
There are four stages in the life of a horsehair worm: the egg, the pre-parasitic larva that hatches from the egg, the parasitic larva that develops within an invertebrate (its host), and the free-living aquatic adult. The worms spend the winter in water. After mating in spring, the female worm deposits a string of eggs 12 to 24 inches long in the water. About 3 weeks to 1 month later, minute immature larvae hatch. These larvae must infect an invertebrate host to develop. Suitable hosts for different species of horsehair worms include larger predaceous arthropods (often mantids, water beetles, carabid beetles, or dragonflies) or omnivores (such as crickets and other closely related insects or millipedes).
About 3 months after the horsehair worm parasites a host, the host seeks out water. When the host enters the water, the mature worm emerges. Adult worms are free-living in water and do not feed, but they can live many months. They overwinter in water or mud, and the cycle repeats itself the following spring.
MANAGEMENT
Control of horsehair worms in natural water sources is impractical. Livestock water troughs can be kept sanitary with routine flushing. A fine mesh filter should be used if water is pumped from a surface supply such as a canal or pond. Domestic water supply systems should be filtered and chemically treated and inspected for necessary repairs, especially when the homeowner discovers horsehair worms in wash water, bathtubs, or sinks.