One of our members, Frenchy, has a golden that is heartworm positive. He, too, is being dosed with monthly ivermectin. I think the idea is to keep the meds going all year round for a number of years to kill all the larval heartworms. Eventually, the adult worms die, and since there is no next generation (all of the larvae having been killed by the meds), the infection is cured at that point.
With luck, Frenchy will see your thread tomorrow. She may have some advice for you and your basset!