Horses typically get worms when turned out with previously infected horses or when they are turned out in a contaminated pasture. In both situations, it is highly likely the horse will become infected as well. Pastures become contaminated with the eggs and larvae or parasitic worms through the manure of an infected horse’s manure, which then mixes in the grass of the pasture. As your horse grazes, the eggs and larvae are ingested. A pasture can stay infected for a considerable amount of time, so always keep the threat of horse worms in mind. Traditionally, veterinarians recommend worming your horse every two months. However, there is a lively debate about the effectiveness of repeated use of the same wormers. Before beginning a worming schedule, it is wise to have a serious discussion with your vet about the best possible worming schedule for your horse.