And for anybody wondering: heartworm treatment runs about $1500 in my area.

The SHOTS are $775. The rest of the charges are the bloodwork, tests (antigen and microfilarae and then antigen again), doxycycline, chest x-rays of the lungs, physical exam, Benadryl for pre-injection, and Carprofen for pain afterwards.
There were three heartworm tests, here's that sequence -- I wanted to correct several people on FB who said 'maybe he has microfilarae only' but I didn't.
The annual IDEXX Snap4 test is an antigen test. It screens for adult worms. If a dog throws a positive on that test, you test again (using the same blood sample) for microfilarae.
If the microfilarae test is positive, you've got adult worms that are producing immature worms -- you're far enough into the life cycle of the heartworm to have both. You plan the treatment accordingly: the dog takes Heartgard for at least one month and also takes doxy for a month, then you stage the Immiticide shots.
If the microfilarae test is negative, and Rowley's was, you have to draw more blood and run another antigen test, to be sure that the first antigen positive was not a false positive. They really don't want to give Immiticide shots to a dog that doesn't need them.
Of course, when Rowley had no microfilarae, I was all 'yeah, false positive, we're outta here!' but my vet immediately ran another antigen test and it was positive too. It's not unusual to have positive antigen and negative microfilarae, it just means the worms are adult but not yet reproducing, so a relatively recent infection.
That confirms the heartworm diagnosis and then you do a physical exam to see how the heart sounds, and take film to see how the lungs look. Rowley was excellent on both, so his very good physical condition will help him recover, we think, with no problems.
I requested the rabies waiver because Immiticide, arsenic-based, is a carcinogen; I'm not gonna put that in my dog and then be required to inject him with killed rabies virus, EVER. Let's not stack the deck against him!
Because he is very fit and tolerated the shot so well, he didn't need Pred or Gabapentin afterwards. The Immiticide will clear his body in about 72 hours, so he'll get liver support twice a day for three days; he'll get CoQ10 twice a day for the full two months of treatment. (First shot, 30 days elapse, second shot, another 30 days.)
Four months after the end of the treatment period, he has to pass another antigen test, it has to come back negative. If it doesn't, we test again after 60 days; if that's still not negative, he may need to repeat the treatment. That's not something I'm thinking about right now. In the meantime, he takes Heartgard every 30 days just in case there are any microfilarae from any source; it will prevent any more heartworms from growing to maturity.
Interesting, in a gruesome way.