Annual testing is recommended for several important reasons. First, many of us do not take our own medications as directed let alone medicate our pets. We're busy; we forget; we miss a dose here and there. Second, even if you never miss a dose there is nothing to prevent your dog from eating some grass and vomiting up the medication you just gave. Your pet would be without protection for an entire month. Third, if your pet accidentally became infected with heartworms, your veterinarian needs to detect it as soon as possible before irreversible heart and lung damage occur. Early detection and treatment are always best. Annual testing provides peace of mind in knowing that your pet is free of heartworms, and should your pet be infected, it assures you of early diagnosis.