Many Canadian pet owners believe their pets aren’t at risk for heartworms, but studies have demonstrated that heartworm infections are increasing in Canada. Our team at North Waterloo Veterinary Hospital wants to debunk some commonly held beliefs about heartworm disease in pets, to ensure your pet is protected.

Myth #1: Heartworm disease affects only pets in the southern United States

Fact: Heartworms have been diagnosed in all 50 U.S. states, and are now slowly making their way into Canada. Climate change is believed to be partly responsible for the increased cases, because rising seasonal temperatures not only allow mosquito populations to travel further north, but also allow them to remain active for longer. In addition, pets imported from other countries, and Canadian pets traveling to other endemic countries, contribute to the increase. The majority of cases diagnosed in Canada have been in Ontario, with the highest prevalence in the southern regions.

Myth #2: My pet is not sick so they can’t have heartworms

Fact: Most heartworm-infected pets in Canada have a low parasite load, which means they are unlikely to show signs. A low worm load allows for easier treatment, but the parasites can still significantly damage your pet’s heart and lungs, and some pets show no signs before collapsing or dying suddenly. Signs that are visible are usually subtle, such as lethargy, a soft cough, and exercise intolerance. Once a pet is showing significant signs, they likely have a heavier worm burden, making treatment dangerous. 

Myth #3: Cats aren’t at risk from heartworms

Fact: Cats aren’t as commonly affected as dogs, but heartworm infections in cats are on the rise. Cats are considered atypical heartworm hosts and respond differently to the parasites. When the larval heartworms enter the dog’s bloodstream, they eventually lodge in the lung vasculature, causing inflammation, which in turn causes scarring and fibrosis in the vessels. The heart then has difficulty pumping efficiently, and eventually fails. In cats, the larval heartworms cause a severe immune response, called heartworm associated respiratory disease (HARD), when they reach the cat’s vasculature, and can cause signs that include wheezing and difficulty breathing. 

Myth #4: My pet lives inside so they are protected from heartworms

Fact: Heartworms are transmitted by mosquitoes. The mosquito ingests baby heartworms (i.e., microfilariae) when they feed from an infected dog or wild canid. The parasites mature to a transmissible stage over a few days, and are then deposited on a pet’s skin when the mosquito takes their next blood meal. Once the larval heartworms enter the bite wound, the pet is infected. Mosquitoes are ubiquitous and can enter your home through numerous avenues, including open doors, torn window screens, and air ducts. In addition, infected mosquitoes can easily bite a pet who escapes outdoors, or who is allowed in protected outdoor areas.

Myth #5: My pet’s screening blood work was normal so they are heartworm free

Fact: Screening blood work includes a complete blood count (CBC) and a biochemistry profile, which offer an overall look at your pet’s health, but don’t detect parasites such as heartworms, particularly heartworm-infected pets in the initial stages. However, some different heartworm tests are available to diagnose the parasites, including:

  • Microfilariae test — Techniques are used to concentrate microfilariae in the blood to detect and identify the baby heartworms. Not all heartworm-infected pets have circulating microfilariae, so these test results can be false negative.
  • Antigen test — The most common tests used to screen dogs, these can detect adult female heartworms. Antigen tests are typically sensitive, but false negatives can occur if the infection has been present for less than five months, the worm load is all males or immature females, or the worm load is extremely low.
  • Antibody test — Antibody tests are especially useful when diagnosing cats, since they rarely have microfilariae and tend to have a low worm load. These tests detect exposure to heartworm larvae. 
  • Imaging — X-rays and ultrasound may also be used to help diagnose heartworm disease.

Myth #6: Heartworm disease in pets is easily treatable

Fact: Treating heartworm disease in dogs is extremely dangerous, because worms that die off too quickly can cause life-threatening consequences. If your dog is diagnosed with heartworms, their activity must be severely restricted, because exercise can exacerbate the worms’ damage to their heart and lungs. Your dog will need to be stabilized before starting the medication to kill the heartworms, and ensuring all parasites are eradicated will take several months. No treatment is approved for heartworm disease in cats, and management focuses on supportive therapy and decreasing the inflammation.

Myth #7: My pet needs heartworm prevention only during the summer months

Fact: The best way to protect your pet from heartworms is with year-round heartworm prevention. Climate change has made seasonal temperatures unpredictable, and mosquitoes are active for longer periods every year. In addition, heartworm prevention retroactively protects your pet from a mosquito bite from a previous month. For instance, if your pet is bitten by an infected mosquito in October, and they don’t receive heartworm prevention in November, the larval parasites will not be killed off, and will mature to cause infection.

Heartworm disease is a concerning issue, but you can protect your pet by providing year-round heartworm prevention. If your pet is due for a heartworm test, contact our team at North Waterloo Veterinary Hospital to schedule an appointment.