For as long as I've known "my" (he is really my trainers horse, but I consider him mine) thoroughbred gelding, he has had the WORST itchy-ness I've ever seen on a horse. There would be times where he would literally have NO hair on his belly and would have open sores from itching himself raw on his tummy. My trainer and I tried MANY different things, and nothing had ever worked (swat, medicated shampoo, skin so soft, microtek spray/shampoo, MTG, etc). We took him to the vet, and the vet never really gave a straight answer as to what it was. I'm moving soon and am going to be taking him with me, so I figured it was time to put a stop to his crazy itchiness and figure something out.
After years of trying, I FINALLY got him to stop itching AND his ENTIRE belly is (almost) completely covered in hair! I'm posting this just in case any of you have had problems with itchy horses and can't figure out what to do about it. What I do is spray him with diluted listerine (it sounds weird but it works!) and rub it into his itchy spots. Then I put hydrocortisone cream (the kind for people) and rubbed it into the spots that were hairless (these gradually decreased with time). He also has been receiving wheat germ oil and 3 scoops of ground flax in his nightly grain to help improve skin and coat conditions. Once a week I wash his tummy (I've been using Fungasol shampoo because it has tea tree oil in it) just because I put stuff on it every day and it builds up and can probably become irritating over time. And now the itchi-ness is gone! Success! If anybody else has a problem with itchy horses they should give this a try. Putting it on every day is the key part, I think. Now that he hasn't been itching as much, I've just been spraying the listerine on him.