Protecting Your Pet from Heartworm: What Ontario Pet Owners Need to Know

Heartworm may not be a common diagnosis in Ontario, but it is a serious one—and it is possible. While not as widespread as other parasites, heartworm is among the most dangerous. The good news? It’s completely preventable with consistent, veterinarian-recommended care.

Even though the overall risk remains low, heartworm is still present in our province. It’s often diagnosed in rescue animals brought in from the southern United States, but local cases do happen. In fact, we recently treated a dog who had never left Ontario. He disliked the taste of his preventative medication, and after a few months of inconsistent dosing, he developed an active heartworm infection. While such cases are rare, they underscore a critical point: prevention matters, even when risk feels low.

Over my 26-year career, I’ve spoken with countless clients about heartworm. My message has remained consistent throughout the years: heartworm is uncommon, but protecting your pet from Heartworm is essential. So today, let’s take a closer look at what heartworm is, why it matters, and how you can keep your pet protected.

What Is Heartworm?

Heartworm disease is caused by a type of roundworm called Dirofilaria immitis. Immature larvae of the parasite are spread through mosquito bites. Once inside the body, these larvae travel through the bloodstream, maturing into adult worms that lodge in the heart, lungs, and surrounding blood vessels.

Treatment in dogs is complex and costly. It involves a series of injections to kill the adult worms, medications to manage inflammation, and strict activity restriction to reduce the risk of complications. Recovery can be hard on both your pet and your wallet, which is why prevention is always the better choice.

Heartworm in Ontario

According to the Canadian Parasitology Expert Panel (CPEP), Ontario has a relatively low heartworm prevalence with only about 0.5% of dogs testing positive. However, in high-risk areas like the southern United States, that number can reach 5–10%.

Most Ontario cases are concentrated in Southern Ontario, due to several contributing factors:

• Importation of rescue dogs from heartworm-endemic regions
• Local wildlife reservoirs, such as foxes and coyotes, that carry and spread the disease
• Climate conditions that allow mosquitoes to thrive

Importantly, some Ontario dogs who test positive have no history of travel, indicating that local transmission is possible.

When Is Heartworm Season?

Heartworm transmission depends on temperature. The risk increases once ambient temperatures consistently exceed 14°C (57°F)the point at which mosquito larvae can develop within their hosts. Based on three decades of climate data, Southern Ontario’s heartworm season typically runs from June 1st to November 1st.

Why Heartworm Testing Matters

Annual heartworm testing is recommended, ideally in spring, about six to seven months after the last potential exposure. This helps detect any infections acquired during the previous season.

While some low-risk pets may be tested every two years, yearly testing remains the gold standard, especially if there’s ever been a lapse in preventatives.

The 4Dx test is quick, minimally invasive (just a few drops of blood), and informative. Along with heartworm, it screens for three major tick-borne illnesses,—Lyme disease (Borrelia), Anaplasma, and Ehrlichia,—offering excellent value and peace of mind.

Protecting Your Pet from Heartworm with Heartworm Protection

There are several effective veterinary products designed to protect pets from heartworm.

At Westway Animal Clinic, we recommend: 

• NexGard Spectra for dogs – a tasty, chewable tablet that provides broad-spectrum protection against heartworm, fleas, ticks, and intestinal parasites.
• NexGard Combo for cats – a monthly topical treatment that offers similar coverage in a format that’s easy to apply.

For pets with medical conditions, sensitivities, or unique weight requirements other options are available. Our team can help you choose the safest and most effective product for your pet.

Currently, no natural remedy provides the same proven protection as veterinary prescription products. While prescription preventatives may seem costly or intimidating, they are the most reliable and cost-effective option in the long run, especially when compared to the expense and difficulty of treating a heartworm infection.

Always ensure any product you purchase has a Drug Identification Number (DIN) issued by Health Canada. This number confirms the product has been evaluated and approved for safety, quality, and efficacy.

Why You Should Never Skip a Dose of Heartworm Prevention

Heartworm preventatives work retroactively by killing the immature larvae introduced during mosquito bites before they can mature into adults. Missing even one monthly dose may allow those larvae to survive and develop, leading to infection. That’s why consistency is so crucial in prevention of this disease.

Final Thoughts on Protecting Your Pet from Heartworm

While the overall risk of heartworm in Ontario remains low, it’s a disease that’s simply too serious to ignore. The cost, complexity, and danger of treating an active infection far outweigh the simplicity of regular prevention. With annual testing, consistent medication, and guidance from your veterinary team, you can protect your pet from this entirely preventable disease.

If you have any questions about heartworm or your pet’s prevention plan, don’t hesitate to reach out to us at Westway Animal Clinic. We’re here to help you keep your furry family members safe, healthy, and heartworm-free.