Shady,,Check your pasture for any broad leaf plant, either smooth or fuzzy, it may or may not have berries this time of year. It is possibly Night Shade, there are over 50 different varieties and they all produce the symptoms you are talking about, plus diarhea. I had a mare last year that had those symptoms, my vet thought it was WN, but all the tests proved negative. She was treated with DMSO the first time due to the possiblity of WN or EEE and then took her back to be oiled 2 days later.
The normally will not eat them,, but some will if their pasture gets sparse or if they are bored. Go pull all the weeds up with broad leaves. you could probably spot spray round up, but you rist the overspray getting on your grass. If you mow, especially if they have berries, you will spread them.
Good luck with your mares. I hope they get better soon.
Angie