The American Heartworm Association and Duluth Animal Hospital recommend that all dogs be on heartworm prevention continuously. Heartworm prevention is extremely safe, highly effective, and most brands also protect against a number of internal parasites. Heartworm prevention should be administered year-round. Even indoor pets should be on heartworm prevention, since it is mosquitos that transmit heartworm disease, and they often find their way inside our homes.
Most heartworm prevention products are highly effectice. However, as with any medication there can be variations in absorption, pets spitting the medication out, or failure to be given as prescribed. We have diagnosed heartworm disease in dogs that have been on preventative because of these reasons. Heartworm disease is very serious but shows no early symptoms, and it is usually fatal if left untreated. Heartworm disease can be treated successfully in most cases if diagnosed early.
Duluth Animal Hospital and the American Heartworm Association recommend a simple annual blood test, even if your pet is on preventative. Any dog not on heartworm prevention (or if the prevention has been stopped) MUST test negative before restarting, for the safety of the pet.