OK here we go Sweet Itch 101.
Cause - allergic reaction to the bite from Culicoides midges.
Usually start in spring as it gets warmer and goes on until weather cools off in autumn.
Midges are worse early morning and early evening so try and avoid turn out at these times if you can (not always practical I know).
Benzyl benzoate daily is the first order of attack (and the cheapest), problem is it washes off when it rains. Needs to be put on the base of the mane, top of the tail AND under the belly along the middle (this is where the little beggars like to bite). For rain-resistance you can mix it 50:50 with liquid paraffin - makes it really messy but it is more effective. Make sure to wash your hands after putting it on or after handling treated areas - it stings like mad if you get it near your eyes.
Itchnomore by Equine America has very good results but is much more expensive. Cheapest source is via
www.horseandponysales.co.uk/gift/equine
Most effective I've found is Permethrin, very expensive from the vets but it is in Lyclear lotion (for headlice), still expensive but dilute 50:50 with 70% IMS and only needs applying every other day.
Feed supplements - garlic may help, various proprietary products eg NAF D-itch. Most effective one is American product called Bug-Check, I think you can get it over the internet.
Homeopathic remedies: Arsen Alb 1M twice daily for 5 days if you can't get the 1M try the 6c strength) , Sulphur 6c three times daily until sore patches heal, Culicoides midge 30c four time daily for 10 days then once daily through the season. Abrotanum 30c four times daily for 4 - 5 weeks. You may get an initial 'flare-up' when you start them but that will settle within a few days.
Cover-up: Boett rug designed especially for sweet itch sufferers, expensive but very good. Combo fly sheet from Weatherbeeta is not as effective but much cheaper (£45 i think).
Field: as open as possible, preferably an area where it tends to be breezy, avoid trees if possible.
If none of the above help and the horse is not laminitis prone they can have monthly steroid injections through the vet.