Horses and Sweet Itch

I was wondering what you would recommend for sweet itch, girth itch, etc, for horses.

Sweet itch is a a form of dermitis due to a reaction to the bite of a midge called Culicoides, active from April to November and most prevelant in wooded or water-based areas. As a preventative, try to keep your horse away from areas as described above and indoors during the worst times of infestation, ie: dawn and dusk as well as on hot, humid days. If this is not possible, try to ensure there is an area of shade where it is cool and free from midges and use an effective fly repellent. Herbs to help strengthen the immune system would include: Garlic (Allium sativum) at 6 to 8 crushed fresh cloves per day in food, Calendula (Calendula officinalis) at 15 to 20 grams of the dried flowers per day in food, Clivers (Galium aparine) at 20 to 30 grams of dried herb per day in food, Nettle (Urtica dioica) at 20 to 30 grams per day of dried herb in food and Echinacea (Echinacea purpurea or E. angustifolia) at 10 to 20 grams of dried, cut root per day in food. (Note: give Echinacea in doses of two weeks on then one week off).
You can also use cider vinegar preventatively: internally at 30 ml per day and as a rinse at a ratio of two tablespoons vinegar to one litre water. Do not use the latter rinse if your horse already has lesions. Should lesions already be apparent, you could treat them with a salve of Calendula or St. John's Wort (Hypericum perforatum).