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As a responsible dog owner, it’s important to be aware of heartworm disease and take the necessary steps to prevent, diagnose and treat it. Heartworm is a serious and potentially fatal disease that can lead to severe lung disease, heart failure and organ damage. Here’s an overview of what you need to know.

What is heartworm?

Heartworms are parasitic roundworms that infect dogs, wild canines and other animals like cats and ferrets. It can be fatal to your dog, as well as other species. Cases of heartworm have been found all across the continental U.S. and in southern regions of Canada. Because the life cycle of these parasites from infection to maturity can be as long as seven months, year-round protection for your dog is essential.

How heartworm spreads

Heartworms are spread by mosquitos. When a mosquito bites an infected animal, it picks up microscopic baby worms called microfilaria that circulate in the bloodstream. The microfilaria mature for two to six weeks inside the mosquito until they reach an infective stage — that’s when they're able to cause heartworm disease.

When an infected mosquito bites your dog, the infective larvae are deposited onto the surface of the skin and enter the bloodstream through the mosquito’s bite wound. Once the larvae are inside your dog, they develop into adult worms that eventually reside in the heart and surrounding blood vessels, leading to heartworm disease.


In the early stages, heartworm disease is often asymptomatic. Your dog may not show any immediate signs of the disease for up to seven months. Yearly testing is recommended so that your vet can detect heartworms before your dog experiences any symptoms.

Sadly, once symptoms start showing, it’s often too late for effective treatment. Your dog may start by having a mild cough, which progresses into shortness of breath, reluctance to exercise, decreased appetite and weight loss. Critical signs of infection, such as a swollen belly due to heart failure, can take years to show up as mature heartworms can live inside an animal for up to seven years.


Treating dogs with heartworm is a complex multi-step process. Before deciding on a course of action, the vet will likely check the health of your dog’s organs to determine the extent of the infection. Treatment typically includes medications in chews form or injections to kill the heartworms in every stage of their life cycle. Topical treatments may also be applied. Some treatments target only heartworm, but others double as protection against other parasites.

Treatment involves multiple visits to the vet, and keeping your dog calm is an important part of its recovery. The treatment process involves breaking up the heartworms, which can cause a blockage in the pulmonary vessels. Preventing your dog’s heart rate from elevating is critical during this period of treatment.


Prevention is the key to avoiding heartworm disease. That’s why routine testing is so important. At Bellamy-Lawrence Animal Hospital, we encourage starting heartworm testing on dogs once they’re seven months old and then once a year after that. The earlier heartworm is detected, the better the chances of addressing it before it progresses.

It’s also recommended to use heartworm preventives, such as oral chews, topical gels and injectable medications. These and many other preventative care products are available to purchase from our online store.

Heartworm prevention services for dogs in Scarborough, Ontario

At Bellamy-Lawrence Animal Hospital near Toronto, we encourage preventive heartworm treatment and testing for your dog. If your pet has contracted heartworm disease, you can rest assured that the veterinarians at our dog hospital are experts in treating the illness and getting your pet back to full health. Contact us to learn more or book an appointment.

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