I also noticed a bald patch that looked exactly like this on her belly that hasn't gone away:
I haven't seen her itch it but she doesn't lay down (arthritis/fusing hocks) so it's not like she's laying on anything.
I found this photo on a site dedicated to neck worms...
Anywho, I've always wormed evey other month, but she's been on free-lease for a year (clean grounds, only a mule on the property), though I've had her back for two months.
She's in a rural area with minimal access to vetcare for parasite testing, and some types of worms don't even show up on the tests anyway. I'd rather not do a Powerpak, though I've heard Quest is pretty strong.