If you notice small brown worms in your toilet tank or bowl, they are likely one of two creatures: the horsehair worm or the moth fly larva, which isn't actually a worm. Neither critter will bite, spread disease or harm humans in any way, but the presence of worms in the toilet can be frightening and embarrassing. Learn to identify these two different creatures to determine the appropriate course of action for preventing worms and larvae in your toilet.
Horsehair worms are thin roundworms 10 to 35 cm (4 to 14 inches) long and named for their resemblance to the hairs in a horse's mane. These parasitic worms feed inside and kill insects such as centipedes and cockroaches, emerging and turning brown once the host is dead. Horsehair worms never infest humans, and are considered beneficial in controlling pest insects in toilets.
Moth fly larvae
The larvae of moth flies, also called drain flies, live in wet areas such as drains and toilet tanks. After you flush the toilet, the translucent tan-to-light-brown larvae may appear in the toilet bowl. Larvae feed on the bacteria-rich coating of drains and tanks. In their adult stage, moth flies may be annoying but cause no physical harm to humans or pets.
How did they get there?
Horsehair worms occur in toilet tanks when humans throw dead, infested insects into the toilet or when insects fall into the bowl and drown. Moth flies may lay their eggs in toilet tanks, where larvae hatch and remain to feed until they pupate and turn into adult flies.
To remove horsehair worms from the toilet, flush the toilet or put on rubber gloves and remove the worms by hand. There is no reason to kill the beneficial worms. To prevent horsehair worms in your toilet, dispose of dead insects outside or in the rubbish, or treat your home with an insecticide or insect repellent to prevent insects from invading your home. To eliminate moth fly larvae, pour boiling water in the toilet tank or bowl. Clean the tank regularly to remove the gelatinous coating that feeds the larvae.