My girlfreind's dad has 8 bird dogs that were all diagnosed with hearworms several years ago. The vet gave them a shot I think, then treatment once a month, I don't remember if it was a shot or pill. The last I checked with him, none had died of the heartworms. You know you can't give them the heartworm medicine after they have them, because the worms will die and the dog will bleed to death from holes in the heart. The way the treatment works after they have worms, is the medicine kills the eggs so no more worms will be hatched. Eventually, the live worms die off naturally a few at a time and are passed from the dog. The vet won't tell you, but after a couple years, the worms have died off, and you can put them back on heart medicine. My gf's dad did it. They catch the worms from mosquitos, which are not in short supply down here in Florida, and since I use my dogs to hunt wild hogs, I am sure to keep them on the med. They give you a little sticker to put on your calender to remind you when to give them.
So take heart, there is treatment for your dog, but you have to keep up with it. I think her dad told me it was $1100 when he took all 8 dogs in for treatment the first time.