Critters & Creatures

Heartworm in Dogs and Cats

What Every Pet Owner Should Know

By Heidi Wagg

Heartworm is a potentially deadly—yet highly preventable—disease that most commonly attacks dogs, but occasionally strikes cats as well.

The pesky mosquito is responsible for spreading the illness by depositing microscopic heartworm larvae into the bloodstream of animals when it strikes. Once inside, a slow journey begins to the victim’s heart—a trip that could take several months.

Like some other pet worms, heartworms can grow exceptionally large—up to 30 cm or 12 inches long! As the worms mature, their living quarters (within the heart chambers and lung vessels) become increasingly cramped. This restricts blood flow to and from the heart and wreaks havoc on the lungs, kidneys and liver. Coughing, fever, dull fur, heavy breathing, abdominal swelling, weight-loss and lethargy are telltale signs the disease is finally taking its toll on a heavily infested cat or dog.

Left untreated, heartworm disease can be fatal.

Heartworm is tricky to detect in the early stages. Veterinarians may use several methods to check for the disease in a suspected case. A simple blood test is the most popular means; however, a negative test does not necessarily mean the animal is in the clear. If the worms have not yet made it to their final destination—the heart or lungs—or worm numbers are too low, a negative test often results.

Sometimes a chest radiograph is taken, especially with a coughing pet. This enables the veterinarian to assess the impact of the illness on the animal’s cardio-vascular system.

Oral medication is commonly prescribed for heartworm, but when to take it and how frequently depends on the severity of the disease.

In severe cases, the veterinarian may decide to also physically remove the worms by passing an endoscope down the jugular vein into the heart and drawing the worms out. Unfortunately, treatment of advanced heartworm disease is expensive, time-consuming, and not always successful.

However, heartworm disease is easily prevented. The American Heartworm Society suggests that preventive meds begin one month before mosquito season and continue for one month following the end of the “heartworm” season. In Canada, spring and summer is heartworm season.

However not all pet owners in Canada need to be concerned. Heartworms typically habitat in warmer regions where temperatures get high enough for the infant worms to survive inside the carrier mosquitoes. The high-risk areas here are southern Ontario, southern Quebec and the Okanagan Valley of British Columbia. Due to prevailing temperatures, heartworm is not a problem in Alberta or Saskatchewan.

The best way to determine whether heartworm is problematic in your area is to consult your veterinarian. If there is a risk, get your dog or cat on a preventive program.
Remember, prevention is your pet’s best defense against heartworm disease; take it seriously!


“Heartworm Disease: The Key is Prevention”. Alberta Veterinary Medical Association (AB-VMA): 2007. May 07, 2010.

“ProHeart6 Injectable Heartworm Product for Dogs”. (Health Canada): 2006. May 07, 2010.

Grognet, Jeff, D.V.M., B.Sc. “Heartworm: A Bloody Parasite”. Dogs in 2009. May 07, 2007.

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