We're in the same situation right now. We switched our dog to a hydrolyzed protein diet temporarily to give his immune system a rest. It will also result in better results should we do allergy testing once this food is done. Day 10 and we aren't impressed but it can take 3 months for allergies to calm down after exposure. This was done upon the recommendation of a vet that's fine with us feeding raw, a tech friend that has a keen interest in canine nutrition, and a canine nutritionist. None of them have made a penny off of us switching to kibble so I'm not worried that they made that suggestion for financial reasons. I expect to be criticized for feeding this food however we're at the end of our rope and are doing whatever we can to get our dog comfortable.
The issue is the allergies could also be environmental. Dogs can definitely develop allergies to things like dust mites and human dander which would explain why the allergies aren't going away in the winter.
I would be careful about the horse meat though. The standards on horse meat are pretty lax and you have no idea what contaminants could be in that meat.