Just did some reading on these nasty little critters.
Due to their lifestyle, adults live in the large intestine, sbd females emerge to deposit up to 60000 eggs at a time around the anus, a fecal will probably not detect them. A piece of clear sticky tape stuck to the yellowish crusts will.
These crusts are the cause for the itching and subsequent tail rubbing, where they end up on the surface the horse rubbed on, or fall to the ground where the horse pics them up when searching for food.
Any type of dewormer kills the adults, according to recent studies.
For relieving the itch, papertowels and warm water will work.
Aside from dewormer, cleaning waterbuckets, feeder, the wall where the horse rubbed against and the stall(fresh bedding) is very important, since that's where the eggs are lurking.
If it was my horse, I'd do the dewormer myself, clean well and have the horse on a regular deworming schedule.
It takes 5 months for the eggs to develop into adults. Your horse probably reinfested herself.
Since she has been dewormed before I doubt there is any danger for colic. But I'd ask my vet, just to be safe