Dale Hodgins wrote:Fungal and bacterial action will degrade any contaminants given some time. John Elliott will be along soon to advise on the best way to proceed. 
Thanks, Dale.
Unlike pigs and chickens, we don't share very many common pathogens with horses. And if it isn't fresh (i.e. has sat out in the sun and dried out), it will have even less active bacteria. I've had great results with it in dry desert soil. Because the horse is not a ruminant, the manure is less digested, leaving more nutrients available to build the soil.