Unfortunately, if you were late or missed a dose even once, your pet could have become infected if he or she was exposed during that time. Call your veterinarian, and explain the situation. Depending on how many doses have been late, they may recommend that you have your pet tested for heartworm infection, then put your pet on a regular preventive schedule. You should also have your pet retested in seven months, as recommended by the American Heartworm Society. (For heartworms to be detected, they need to be five to seven months old.)