Do not ingest the horse paste! Most horse pastes contain trichlorfon. It will be ok on your scalp though. I don't know enough to say whether or not they'd stay in one place... but don't get too worked up over it... take one step at a time and try one thing at a time.
The thought of having a parasite is very anxiety inducing, but if you dwell on it, you will drive yourself crazy and make yourself sick.