EPM is caused by a protozoa that infects the horse's cetral nervous system. When the protozoa die, the waste is carried out of the central nervous system by the blood. The blood is cleaned by the liver, and the waste and toxins end up in the manure.
I had had two horses with EPM, neither of which has ever had an abcess on the head.
How was the horse diagnosed? There are currently four blood tests that are recognized as appropriate to diagnose EPM, along with a neurological exam, and ruling out other diseases. All three of parts are necessary for a diagnosis of EPM. I would recommend the IFAT test through UC Davis. If your horse doesn't have EPM, you are wasting a lot of money for treatments that he doesn't need.
The horse's immune system is supressed, and that is why horses get EPM. Products that help the immune system help the horse. BUT... Any treatment for EPM must do two things. First, it must cross the blood-brain barrier to kill the protozoa in the central nervous system. Second, it must be able to enter cells to kill the protozoa that live within the horse's individual cells. Any remedy for EPM that doesn't do those two things will not kill the infection.
I would recommend you visit www.EPMhorse.org
to find the latest information on EPM. Yes, I am biased, I created the site.
The information on the site comes from university clinical trails, and EPM researchers.
Please help your horse by spending some time reading about EPM.