The answer to your question is that it is not presently known how
sporozoites of Sarcocystis neurona migrate to the CNS of horses (and
potentially other animals). Presently, there is no animal model to
examine this question.
I would assume that:1) sporozoites intervacular system or lymphatics.
2) schizongony may or may not take place in endothelial cells, 3)
merozoites make it to CNS, 4) the "cue" to produce sarcocysts is not
present in the horse so these terminal stages are not produced, 5)
continued schizogony causes the CNS lesions.