Have the horse as clean as possible. Ideally, bathe the horse with soap and water, and scrub with a brush to get the deep dirt out. Rinse thoroughly. If it is too cold to give a bath, try vacuuming. If these are not options, use a curry comb or a long nylon mane comb (like a curry comb) to get down to the skin and loosen up the dirt and dander. Then brush the horse to remove the loose dirt.