Threadworms are very thin like pieces of sewing thread.
When you are infested with a lot of active threadworms, they cause itching at night when you are still, moreso than the daytime. They are very active at this time and crawl out of the rectum and into the closed cheeks. When they crawl out enough to reach the air, they retract back away from the open air.
This movement then causes itching, and causes the desire to scratch your butt, and the microscopic eggs can get under your nails. Since you are taking humaworm, it also contains cloves which prevents parasite eggs from hatching if they are exposed to it. ;)
An itchy anus at any time can be a symptom of parasites.