I have heard some interesting things about why coyotes, wolves and wild dogs do not suffer from heartworm the way domestic dogs do. Some theories are that first of all these canines do not live as long as our domestic dogs, but even young pets can have terrible complications from heartworm. There is possibly a natural immunity that the wild animals have developed, and we somewhat prevent in our pets by giving preventative medicine. The most compelling idea, in my opinion, of why pets are affected and wild canines are not, is the overall health of our pets. Most of our pets are fed unnatural diets, live in houses and are less active. They are generally less healthy than wild animals. Also breeding has created breeds with coats that offer less protection to pests.
I thought this was interesting, though probably not too helpful with your decision! I live in an area with mosquitos during 5 months and I don't use heartworm medicine. I do know some who use it just during the summer months. I suggest doing some research and then go with what feels right! But don't just take your vets word - they do make money promoting and selling heartworm medicine! Good luck!