OP, you are wonderful for going the extra mile for your horse.
I do not know very much about treating for EPM, expected outcomes and so forth. I do know that Marquis is very expensive.
There is an alternative treatment that is much less costly. Hopefully those with more experience can comment on Rebalance vs. Marquis. I wonder if Rebalance would be enough for a possible marginal case. What is it were given for only a month?
Rebalance: (can be had from Valley Vet or Chewy’s with prescription
https://www.valleyvet.com/ct_detail.html?pgguid=2187cad3-8cf3-410e-aaeb-d2de2f608af5&grp=UUUU&grpc=UUUU&grpsc=UUUU&ccd=IGO057&gclid=EAIaIQobChMI6oHf9tT27QIVRlXVCh3soQ76EAAYAiAAEgKFf_D_BwE
Marquis: (Valley Vet & Chewys)
https://www.valleyvet.com/ct_detail.html?pgguid=704FDA78-5D4B-4131-926E-9E9DEF9802A9&q=marquis
I’m glad to hear that you did well by treating borderline cases. I think I may have one now. If it is EPM, I’d prefer to knock it down before it goes further.
This. ^^^ You need a vet who has deep experience with EPM in your area.
I would not spinal tap for EPM. It is extremely painful. Plus how many vets have experience with spinal taps? It’s not for someone without considerable experience to do alone.