There are products in the Jeffers Catalog and in Valley Vet Catalog that will treat ring worm. Sullivans also has a good product. We have also used a povidine shampoo to help prevent reinfecting. This stuff is contagious and will go thru the herd if left untreated, you are also at risk of getting it. Go to the top of the page and click on search and type in "ring worm" and you will get previous posts that appeared on the subject.
You will need to be sure to clean (bleach solution works) halters, leads, brushes, curry combs etc so that you don't reinfect once you have treated.