DE works fairly well. One thing I've seen our donkey eat is poison ivy and poison oak, the one time we tested her she was clear of any parasite load, so that might be a way.
My goat owning friends have told me that the poisonous ivy and oak are used by their goats as a natural, self medicating wormer, they apparently only eat it when they need it.
There used to be some
books at our bookstore that were herbal medicines for animals, hopefully you could find one of those through asking at your bookstore or asking a herbalist.
Redhawk