Many dogs have seizures as a characteristic of the breed. Is this truly a genetic predisposition or could it be due to spirit meddler influence as well?

Both are components. There is a genetic predisposition to vulnerability, but the presence of spirit meddler attachments can trigger this to be expressed. And this serves to cause intense energy flow within the animal that serves the onboard spirit attachments, and the fact it causes consternation to the human owner is a double win for their side, so to speak.