My husband's Rat Terrier had heartworms last year. The symptom he had was severe coughing. He was always on Heartgard treatment.... but the year before he had an eye injury which the vet feels the meds he was taking for the eye (and removal of the eye) may have interfered with the heartgard meds.
The Heartgard company paid for his treatment. All is well now.
The coughing he had was due to him trying to "hack up" worms from his chest/heart.
Hope you don't have to deal with heartworms.