Life cycles

The details of the life cycles of parasitic worms vary somewhat by species. But all have at least two phases: one inside the horse and one outside in the environment. Generally, the adult worms inside the horse lay eggs that exit the horse’s body with his manure. When another horse grazes in the area, he may pick up either the parasitic eggs or the hatched larvae (depending on the species), which are swallowed and develop to maturity inside the horse.