Couple of questions for the more experienced?

I obtained a 5 year old dog who was full of heartworms. I took him to the vet and they gave me a break down of the treatment they proposed (expensive and not really good for the dog)....I talked to several people, read what I could find, and after knowing that the life of an adult heartworm is approx. 2 years, I determined that if I could stop the new heartworms my dog could be heartworm free in approx. 2 years...Anyway, I have been giving him Ivomec for 4 years now and he is still here...Not only that, but he doesn't have that nasty cough anymore either...