Is it possible that someone nearby is feeding your horses something inappropriate?
(Like a little kid, sharing a bunch of apples or something - or someone else sharing something not so innocent?)
Is it possible that you have poisonous weeds or trees in the pasture where the horses are?
Is the pasture very grassy, or mostly dirt? Is it possible the horses are taking in a lot of sand while trying to eat?
It concerns me that you've lost multiple horses, and are having an issue now that is not easily explained. I would most definitely keep a vet's number on speed dial.