(I meant to post an update late yesterday afternoon or early evening, but got busy and forgot. Oops!)
It looks like Elvis has made it through his initial heartworm treatment with no trouble. It's now been nearly 36 hours since we gave him the tablet, and we've seen no side effects at all.
I put a call in to Dr. Walthall's clinic yesterday afternoon to give a quick update and also to ask what the dosage would be for him for the heartworm medicine I use. Since it's a cattle wormer and he's a toy breed, I really need to be sure I don't overdose him on it. I think I'm even using a bit too much for my standard dachshund. I suspect that since I've been using it for 15 years, the thinking on the dosage amount has probably changed.
Anyway, starting next month he'll join my other two in the monthly preventive, which I am very happy to see. His little heart has enough going on without adding more of a load to it.