If treated with Marquis the "flare ups" aren't flare-ups but rather a re-infection. If the horse has been infected once the likelihood of re-infection is great unless measures have been taken to change the sources of infection in the environment.

Marquis kills the protozoa. How affected the horse is after treatment will be a matter of how quickly the diagnosis was made and how much damage was done to the spinal cord before the protozoa was killed. If caught early the horse could recover 100%.

The horse is an end host for EPM meaning that it can not reproduce once in the horse.

"Oh, we are but soft and squishy bags of mortality rolling in a bin of sharp circumstance, leaking life until we collapse, flaccid, into our own despair." --"Fool" by Christopher Moore