Change doesn't necessarily precipitate allergic reaction ;) I used to know someone who had been stung by bees loads of times, and then this one time they got stung and had a massive anaphylactic reaction. They were fine but it really scared them.
Also have heard a story of someone who suddenly and inexplicably developed a peanut allergy, they had eaten peanuts plenty of times before with no ill effects, and then one day nearly died of an allergic reaction. They had the scratch test done as they couldn't identify a possible allergen... and reacted to peanuts.
Mum's old horse developed an oat allergy when he was 5, and feeding him oats caused eye issues very much like night blindness aka uveitis. She was told she would have to put him down within 2 years as he had an autoimmune disease that was attacking his eyes. Eventually she put 2 and 2 together, cut out oats and oat products from his feed, and he lived to be nearly 21 with no further eye problems.
If your mare is totally sound, and it's only YOU that this is bothering, then it doesn't hurt to try a change in diet and see what happens. Food allergies can cause all sorts of out there reactions.
Edit to add; and thrush treatment did absolutely nothing for the other similar case I have seen, in fact that horse went unsound when the thrush treatment was applied as it irritated the sores and made him lame. His were worse, though.
Last edited by blue eyed pony; 08-04-2012 at 04:01 PM.