Sorry to hear about your dogs having heart worms, but when I was growing up there was no way to prevent heart worms and the cure was about as bad as the disease; so most dogs were put to sleep when they got too sick. Having said that, most dogs had the disease for years and years before needing to be put to sleep. Our neighbor’s dog lived seven years between diagnosis and death. Since it was so common, no one thought much about it.
The shelter’s vet prescribed heartworm preventative for 12-18 months, as that’s the lifespan of heartworms. This is to kill any new ones while the existing ones die off. If the dog survives those already in her, she’ll likely be fine though her lifespan may be shortened. If those inside her do too much damage and cause her too much pain, however, I’ll have a very unpleasant duty to face.
She’s young, though, and has lots of room here to run and play and stay fit, so I’m hopeful.