I bought a started dog. Immediately took him to the vet and gave him the one year heartworm shot. Vet tells me they check the dog when the shot is administered and again 6 months later because heartworms have an incubation period. I took him back for the sixth month checkup (heartworm test), now he is positive. Does anyone have any experience with this? The breeder has yet to supply an evidence the dog had his heartworm prevention during the time he had the dog. Feeling like im getting screwed.