Heartworm prevention is a monthly dose that is vital to animal health. Heartworms has been found in all 50 states and it is advised to prevent early and continue through the lifetime of the animal. It is their first defense against heartworms. It is crucial that you also have your animal tested by a vet once a year as prevention medications can add to problems if in fact it is found that your animal is heartworm positive. Early prevention with heartworm medication is the key to health as treatment for animals is very expensive.