Jaydee....yes, im sure it's pinworms. And yes, people get them too.
Egrogan....as I said above, it is VERY important to clean her stall properly as the eggs are probably all over the place in there. That's the main problem. They get picked up by the horse continually when not taken care off. The dewormer, any dewormer, kills only the adults present in the horse. And if your horse has been at the place for two years at least, and had the same stall all the time, it very well could be she's the only one infested.
Ask your vet what he suggests to use for cleaning.
And keep us updated