Generally, dogs who get their heartworm prevention on time don't get heartworms. But you should test them annually anyway. Heartworm disease is a potentially fatal condition caused by a parasite -- Dirofilaria immitis -- that's transmitted by mosquitoes. Multiple prevention products are available.
Annual testing is strongly recommended by the American Heartworm Society. Early diagnosis of heartworm disease leads to a better prognosis than waiting for clinical signs of infection to occur does. Signs include lethargy and coughing resulting from heart or lung damage. Even if you administer prevention on a regular schedule, the product can fail. Or, your dog may unknowingly spit out the pill or vomit it after swallowing it. Certain products -- including Heartgard chewables -- must be chewed, or they are not effective. A variety of cause could render your regular heartworm preventive from working, and that's why you should test annually.
If you administer heartworm prevention to an infected dog, the heartworms may rapidly die off and cause shock. Your puppy should start heartworm prevention at 8 weeks and be tested when he is around 8 months. Many heartworm tests indicate tick-borne diseases, such as Lyme disease and Ehrlichiosis. When your dog is tested annually, he can be diagnosed and treated most effectively.