I bought a mule with sweet itch. I purchased her in early spring when she had a lovely glossy blemish free coat. But this deteriorated as the year went on and by early summer I realised what the problem was. I used various things, most of them natural, to deter the midges. But I had to apply every day, or next morning there would be another bare patch of skin. It kept it under some sort of control but not 100%.
Then I started using pig oil (ie liquid paraffin) with sulphur. I reasoned that if it got rid of feather mites on the legs of hairy cobs, then there was a good chance it might deter midges. And it did! I would trickle it along Ebony's mane and all along her back, and rub it into her sides. She really loved this and I ended up with really soft hands. It used to soak in fairly rapidly so she didn't remain greasy all day, and best of all it would last for about 2 weeks so I didn't have to apply it daily. However, there were numerous accounts in the equine press about pig oil causing extensive burns on horses. I never had a problem with Ebony, even though she was black so her coat used to get really hot in the sun. But even so - I was aware it might cause a problem.
So although the pig oil and sulphur was incredibly effective and easy to apply, I was still looking for something that was 100% fool proof.
I had for many years used homeopathy on my sheep and cattle. That is not to say I didn't call a vet in when necessary eg for physical treatment such as treating a broken leg, or help with a difficult calving or lambing. But I found homeopathy very effective for such as a fertility problem in one cow, a urinary infection in another, and treating sarcoids on my cob pony, amongst others. (I must admit however that I've never found Corpse Reviver particularly effective!)
I rang my homeopathics supplier and asked if they did a Sweet Itch remedy and they did! It was mid summer when I first tried it. I gave Ebony a few drops each day on a carrot and it kept the Sweet Itch at bay for the rest of the season, although you could still see slight bare patches from where she had got bitten in spring as I hadn't started early enough with the pig oil/sulphur.
Ebony's coat grew again in winter and the next Spring I started giving her the homeopathic remedy in March before the midges were about. I gave it her every day on a carrot until about August and then maybe every couple of days till the end of September. In all that time there were no bare patches or itching and so the remedy really was effective. I don't know if it made her unattractive to the midges or if it made her resistant to the midge saliva so she no longer reacted to it. But it certainly worked.
You will read and be told by anyone that has had an animal with Sweet Itch that there is no effective treatment (apart from a Boet rug) and certainly no cure. I would say that either of the treatments above are about as close to a cure as you can get, and I would certainly not be worried about buying another animal that suffered from it.