A: I agree with your veterinarian and the American Heartworm Society that all dogs in this area should be screened for heartworm infections every year. There are four major reasons why.
The first is that the test used to detect heartworm infections is imperfect. The most accurate and most common test is called an ELISA. It is able to detect only female worms that are six months old or over. If a test is negative today, it tells us nothing about what your dog has been exposed to in the past six months. Only at least six months from now can I tell if your dog has heartworms today. In addition, infections that start only with male worms will not be detected unless and until females arrive.
The second is that no preventive program is 100% effective. I have diagnosed several heartworm positive dogs that surprised their owners. Fortunately, if you have purchased your heartworm preventive medications through your veterinarian, the manufacturer will usually pay for the costs of treating the infection. The reasons for preventive failure vary. Some are absorbed properly because they are not given with a meal. Some dogs occasionally vomit their dose without an owner knowing. Topical forms may not be applied properly or may be rubbed or washed off too soon. Some may not be given exactly once every four weeks. Sometimes the medication may just not work the way it should have due to improper storage or shipping. No matter what the reason, no regimen is 100% effective.
The third reason to test annually is the emergence of resistant strains of heartworms. Fortunately, these resistant strains have been geographically limited for now. However, it is likely that they will eventually spread throughout the country. It’s also helpful to test before and six months after changing heartworm prevention brands. That will help the veterinary profession determine which preventives are more or less effective against the strains of heartworm in your area. Dog owners should also be aware that not administering prevention every month all year may help to contribute to resistant strains – so be sure to give your dog twelve monthly doses each and every year.
In my opinion, the best reason to check annually is to follow other important health information from year to year. Since a heartworm test involves drawing a blood sample, many veterinarians offer additional screening tests at little to no additional charge. In my practice we check a CBC (complete blood count) and basic serum chemistry panel every year with the heartworm test. Other practices use a test that screens for three common tick-borne diseases, including Lyme. These tests have helped veterinarians catch serious problems long before owners ever see symptoms. That’s great news for those dogs.