I think it is more about managing the condition however there is product called Cavalesse which I think is in its second year now and is proving to be very sucessful.  It helps balance the natural immune response of a horse or pony and reduces the production of histamine and improves the skin barrier hense greatly reducing the symptoms.  Best price I have found it is at www.petdispensary.co.uk and further details are on that website also.  My friend used it on her horse last year and is using it again now the difference has been fantastic.