The name of the strongylid that lives in the horse is a Westeri. This particular strongylid passes from mother to foal when the foal is nursing. This is only done if the mother is already infected with the Westeri. If the parasite is living outside of the horses - which the Westeri can do - the eggs hatch in the horse’s poop. When this is the case with the worms, they can borrow through the skin to get into a horse’s system.
This worm is, rather unfortunately, common throughout the entire United States of America. They will infest a small intestine quickly, especially when the horse or foal is well fed. If there are many worms in a horse, the adults may not show signs of an infestation. The foals will always show signs.