Think most people use Oroquin, Marquis isn’t that popular any more. I’ve seen full blown EPM in very recent imports, I wouldn’t trust an herbal. No, EPM is not a problem in Europe, no Possums and Possums are the most common interim host, But the minute they step off the van and nibble US grass or even eat US produced hay, they can be exposed to the Protozoa and with no natural resistance as native US horses have built up over generations, some imports get awful sick awful fast and go downhill in a big hurry.
Don’t know if it works as a preventative like a vaccine or something but it might slow it down long enough for the horse to build some defenses.