Hi Dr. Dan, 
My 8 year old quarter horse gelding popped out with hives right before Labor Day weekend.  After a bath they went away only to return a week later. He was given trihist for 2 days and they went away again, but then came back and the trihist did nothing for them.  They are not causing the horse any discomfort.  The hives are also strangely shaped not just raised round bumps. They are mostly on his neck and lower belly.  What could be causing these hives and what should I do? 
Margo O.

There are many  potential causes but allergies are most likely.

These can be triggered by just about anything but commercial feed/flies/bites of other kinds are the most likely. The key generally is changing feed regardless of cause (see audio- “what to feed your horse”), not putting more potential allergens in (less vaccination, chemical dewormers, drugs etc. etc.) , helping the body to get the “junk” out (Health or Joint Check supplements) and boosting the immune system (Aller Check supplement)…

also see the article on allergies in the magazine and the in the library section there is a video as well. thanks for asking! Enjoy the info!