No, there is not an immunity or if there is it's not long-term. Actually, the session about EPM I attended at the Veterinary conference said that horses who have been infected once are more prone to re-infection.

We've dealt with a number of EPM horses in the practice and there have been a number of them who have been reinfected. Most have been after about two years or so and all have been when changes have not been made for prevention. The major issue is usually hay stored in a manner where opossums can access......we've actually found opossums living in the hay of a farm where a number of horses were infected more than once.

"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