When you look out across your pasture to watch your horse grazing, you might only see the obvious–a 1,000-pound friend and teammate. But if you were to look beyond the surface, you’d discover that he is really an ecosystem unto himself.

Your horse’s body (and that of every other horse) is host to thousands, perhaps millions, of microscopic creatures that use a share of his oxygen and nutrients, and enjoy the body heat he generates and the shelter of his internal recesses. At best, these hitchhikers live symbiotically, providing benefits of their own in exchange for what they take from him. The microbial populations that live in his cecum and help him digest plant fibers are a good example.

At worst, they are parasites–freeloading organisms that need their hosts to provide a living environment and nutrients while contributing nothing to the relationship, and usually leaving their "dwellings" in worse shape than how they found them.

To some extent, harboring parasites is a natural and normal condition for equines. In the wild, however, parasites rarely build up to the point of seriously damaging their hosts’ health. (After all, it’s not to a parasite’s advantage to kill its host, as that would guarantee the death of the parasite as well!) Not only does a feral horse’s wandering, grazing lifestyle limit the amount of parasites he picks up from his environment, but if he’s repeatedly exposed to low levels of a parasitic invader, he usually develops some natural immunity.

Domestic horses, on the other hand, find parasites a potentially devastating threat to their health, largely bec