Horses coming off pasture should be treated for bots and tapeworms as well. Pastures should be inspected for toxic plants and have them removed before turning horses out. While many are not highly palatable, intensive grazing may reduce forage to the point that horses will graze toxic plants. White snakeroot, deadly nightshade, wild onion, red maple trees, black walnut, and locust seed pods plus numerous other plants are capable of causing illness and death especially in old established permanent-type pastures with low wet areas and or wood lots. In the Midwest, skunks are the primary carrier of rabies and the horses natural curiosity causes them to lower their heads to investigate strange objects or animals, resulting in their being bitten on the nose.
Rabies is always fatal in any warm blooded animal that contracts it, so there are no options.
Annual vaccination is the only protection. Protection against biting insects is helpful in maintaining the normal health and condition of pastured horses. Fly tags in manes and tails, pour-on materials that provide some residual effect and environmental aspects are all helpful. Face fly masks should be of the break-away variety so if horses get the mask caught, they can get loose. Horses are continuous grazers and do best when pastures are a grass mix with minimal legume.