My 3 year old, Spencer, started having Grand Mal seizures in July 2008 and was diagnosed with canine epilepsy. We did bloodwork and started him on Kbr. He continued to have seizures, so we added PB. Then he started clustering. We added Zonisimide and this combo has kept him seizure-free since mid-December 2008. We did have him checked out by a Neurologist (who did not push for the expensive tests since with his symptoms and lack of other symptoms, she felt his diagnosis was correct).
I'm a bit surprised that a Vet would recommend anything with Ivermectin in it for a Sheltie since it has been known to have caused problems. Same with the Lepto vaccination. I did not have Spencer tested for the MDR1 mutation, but I provided his Vets with the list of meds that can affect an MDR1+ dog and the list is prominently in his files. I truly hope that your boy's seizures were, in fact, brought on by the heartwork meds and that the change will do the trick.