I'm on my phone so can't post the link right now, but search on here for a thread called "worm woes."
I had a nasty bout of pinworms in my mare last year. They won't show in a fecal. Not all dewormers will get them. If you can pick an adult worm and bring to your vet, they should be able to visually confirm you have pinworms.
You should talk with your vet, but I decided to use Panacur Powerpac (not the same as regular Panacur) for my mare, and that cleared her up. I also did a thorough scrub/disinfect of her stall at the same time, as she was rubbing her tail like mad in anything she could reach in her stall. Poor girl, still makes me cringe thinking about those nasty things crawling out if her...
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