They are almost always drain fly larvae, not sewer worms or parasites. Drain flies lay eggs in the slimy film that coats toilet rims, overflow tubes, and drain pipes, and the larvae feed on it. Clean out the film and the larvae lose their food and their home.
Drain fly larvae are thin, dark, and about a quarter inch long. They live in biofilm, the bacterial slime that builds up wherever water sits and organic residue collects. In a toilet, that means under the rim, inside the overflow tube, or in the tank. Bleach poured into the bowl kills the larvae it touches but leaves the film under the rim, so a new batch hatches within days.
Guest bathrooms are the usual source, because a toilet flushed once a week gives the film time to thicken. Long, hot Texas summers also speed up the drain fly life cycle.
If the worms keep coming back after you scrub the rim, tank, and overflow, the source sits further down: a drain line with heavy buildup, or a cracked pipe letting in soil and moisture. That needs a camera, not more bleach. If you see worms in stool rather than in clean bowl water, that is a question for a doctor, not a plumber.
Nuflow DFW is a licensed trenchless sewer contractor serving 25 Dallas-Fort Worth cities, RMP# 46694. Our drain cleaning clears buildup from the pipe wall, and a camera inspection shows what sits further down. You keep the footage. Call (469) 701-0597.