Question:
Is it necessary for my dog to have a yearly heartworm test?
Answer”
The Ontario Veterinary Medical Association (www.ovma.org) and the American Heartworm Society (www.heartwormsociety.org) currently recommend annual heartworm testing in dogs.
The heartworm (Dirofilaria immitis) is a parasitic worm that is transmitted to dogs by mosquitoes. The peak months for heartworm transmission in Ontario are July and August. Once a dog is infected, the heartworm larvae migrate to the heart and develop into adult heartworms. This can obviously cause significant cardiopulmonary disease in dogs.
Just recently our veterinary hospital has gone back to recommending annual heartworm testing, after several years of every other year testing. The every other year testing was originally felt to be adequate as the majority of our clients are very compliant with giving heartworm preventives on time every year. Current heartworm preventives are nearly 100% effective and some people question the necessity of testing annually for heartworm.
After doing some research we realized that there were still a number of dogs on heartworm prevention that were testing positive in North America. “As a lack of effectiveness has been reported for all macrocyclic lactones (i.e. heartworm preventives) annual retesting is an integral part of ensuring that prophylaxis is achieved and maintained.” – American Heartworm Society. Last year, two dogs tested positive for heartworm in our practice in Oakville.
The heartworm test is a simple blood test that takes a veterinary technician approximately five minutes to collect. A wellness screen, which is a general test of your dog’s inner health, is usually run from the same blood sample every spring.
The word “wellness” is used frequently in today’s society to help communicate healthy living in pets and people. Many veterinarians recommend annual wellness testing which is essentially a blood test to screen for metabolic disease. The veterinary community realizes that annual wellness screens are important to the health of every dog. Rather than waiting for the signs of illness to occur, many diseases are picked up early in their course, allowing the veterinarian to more successfully manage and treat the disease. This improves the dog’s quality of life and longevity.
The most widely used test for heartworm disease is called an ELISA antigen test. This test looks for a microscopic piece of the mature/adult female heartworm. This microscopic piece is called an antigen.
Every year in southern Ontario, the heartworm/wellness season runs through April and May. Dogs are tested in April and May because it takes at least 5 months before the heartworm antigen can be detected post-infection. Let’s say your dog missed their September dose of heartworm preventive last year and was bitten by a nasty mosquito that was carrying infective heartworm larvae. If an infected mosquito bit your dog last September, and the test is run seven months later in April, the test should reliably diagnose heartworm disease.
Treatment for heartworm disease is not easy on the dog, and can be quite costly to the owner. Fortunately, when you have your dog tested annually for heartworm disease, and heartworm preventives are given appropriately once monthly from June 1 to November 1, the drug company that produced the heartworm preventive will usually compensate you for the cost of the treatment. If the annual heartworm test was missed, if the preventive was not given, or if one dose was missed, the costs of treating your dog for heartworm disease are not usually covered by the drug company.
Other reasons to test your dog annually for heartworm include: if a preventive was missed or was applied late, if the medication was not absorbed properly, or if the previous year’s test was a false negative test. Some heartworm tests yield false negative results when infections are light, when female worms are immature, when there are only male heartworms present, or when there is laboratory error.
At our hospital, puppies that were born after October 15 of the previous year do not require a heartworm test as they were only a twinkle in their parents’ eyes and were not alive to be infected with heartworm before this date.
With our changing climate it is becoming ever more apparent that heartworm and flea preventives should be considered in April and May, especially with dogs travelling to the cottage during these months. Ask your veterinarian which wellness screen they recommend for your dog. I wish you and your dog a safe, fun, and happy Spring.
Dr. Chris Catena, DVM is a companion animal veterinarian in Scarborough ON. Please send your question or topic suggestion to Dr. Catena, c/o Dogs, Dogs, Dogs! at info@dogsx3.com or fax 905-620-0768.
