Is 'scratches' mud fever? Because I definitely agree that health and management are a big part of it, especially here in Scotland where it never stops raining! My last horse was known for mud fever, and had it even in summer when I got him! After months of anal treatment from me, he didn't get it worse than a single scab again - I was on the warpath with the barrier creams and mud boots whenever it rained!
My new horse has a history of mud fever too, and I've been uber prepared for it with my mud boots and barrier cream... and no signs of it at all as yet, despite knee high mud. Fingers crossed!!
But feather removal would definitely help, just so you can get at the skin and treat it... one of the Clydesdales I worked with before had to have his shaved off to treat mites, couldn't get close enough to his skin with them on!!