Geeze. I really hope you're horse gets better real soon.
First off I wanna say that I am no expert and I know very little compared to many experts.
But I choose whole-istic approaches for myself and my animals. I try to do things the way that nature has designed em. There are natural wormers for living things which nature grows. And the man made mixed chemicals can be destructive. Even when you can't see a reaction right away, like you have been witnessing, those destructive properties stay in the body for years. Even an entire lifetime if the body isn't cleansed of em.
If it were my horse I would find someone else that knew what they were doing. Since the vet doesn't know, I'd be throwing up red flags. Find another vet. And/or find a holistic person with many years experience. And ask them what to do. "Which is not me."
Now with all that said and outta the way........
I would look into bentonite clay.
I just did a quick search and here's a brief description which sums it up pretty well.
"Bentonite Clay is an effective and powerful healing clay used to treat both internal and external maladies. Its greatest power lies in its ability to absorb toxins, impurities, heavy metals and other internal contaminants. When mixed with water, it rapidly swells open like a highly porous sponge. The toxins are drawn in by electrical attraction and are then bound and flushed out of the system."
Truth passes through three stages. First it is ridiculed. Second it is violently opposed. Third it is accepted as self evident.