Heartworm Is on the Rise: What Waterloo Region Pet Owners Need to Know

Many pet owners are surprised to learn that heartworm disease isn’t just a problem in the southern United States. In fact, Southern Ontario is considered the heartworm hotspot of Canada, and cases have been increasing over time.

While heartworm disease remains relatively uncommon, it is a serious and potentially life-threatening condition—and prevention is far easier, safer, and less expensive than treatment.

What Is Heartworm?

Heartworm disease is caused by a parasite called Dirofilaria immitis. The immature worms are spread through mosquito bites. Over time, these parasites mature and can live in the heart, lungs, and blood vessels, causing significant damage.

Heartworm disease is most common in dogs, although cats can also become infected.

Is Heartworm Really Present in Ontario?

Yes.

According to the Canadian Parasitology Expert Panel, the prevalence of heartworm infection in Ontario has increased over the past two decades. Ontario accounts for the majority of reported heartworm cases in Canada, and over half of infected dogs identified in one study had never left the province.

Researchers from the University of Guelph have also documented an increase in the number of regions in Ontario where heartworm infections have been detected.

Why Is Heartworm Becoming More Common?

Several factors are thought to be contributing to the rise in cases:

🦟 Mosquitoes

Heartworm is transmitted by mosquitoes, which are abundant throughout Waterloo Region during the warmer months.

🌎 Imported Rescue Dogs

Many wonderful rescue dogs arrive in Ontario from areas where heartworm is much more common. These dogs may unknowingly bring the parasite with them.

🐺 Wildlife Reservoirs

Coyotes and foxes can carry heartworm and help maintain the parasite in the environment.

☀️ Climate Change

Warmer temperatures may be extending mosquito seasons and allowing heartworm transmission over a wider geographic area.

Is Waterloo Region at Risk?

Absolutely.

Southern Ontario represents the highest-risk area in Canada for heartworm transmission. Studies have shown that heartworm cases are concentrated in Southwestern Ontario, and researchers from the University of Guelph have found that the geographic distribution of heartworm has expanded over time.

Although the risk to any individual dog remains relatively low, the consequences of infection can be severe.

What Are the Signs of Heartworm Disease?

Many infected dogs show no symptoms initially. As the disease progresses, signs may include:

• Persistent cough

• Exercise intolerance

• Fatigue

• Weight loss

• Difficulty breathing

• Reduced appetite

• 
  

Unfortunately, advanced heartworm disease can become life-threatening.

The Good News: Heartworm Is Highly Preventable

Modern heartworm preventives are extremely effective.

Most dogs in Waterloo Region benefit from:

✔ Annual heartworm testing

✔ Monthly parasite prevention during mosquito season

✔ Regular wellness exams

✔ Year-round parasite control discussions tailored to their lifestyle

Prevention is far easier and less costly than treating an established infection.

Don’t Forget About Cats

Although cats are less commonly infected, they are not immune. Even a small number of heartworms can cause serious disease in cats, and many feline infections go undiagnosed.

We’re Here to Help

At Bridge Street Veterinary Hospital, we believe prevention is one of the greatest gifts we can give our pets.

If you have questions about heartworm prevention, parasite control, or which products are right for your dog or cat, our team would be happy to help.

Bridge Street Veterinary Hospital proudly serves pets and families in Waterloo, Kitchener, Cambridge, St. Jacobs, Elmira, Conestogo, and surrounding communities.

Sources

The information in this article was compiled from the following sources:

1. Canadian Parasitology Expert Panel (CPEP) – Ontario Parasite Guidelines

   
   

   https://research-groups.usask.ca/cpep/parasitedata/ontario.php

   • Notes that heartworm prevalence in Ontario has increased since 2002.

   • Reports that approximately 80% of Ontario heartworm cases occur in Southern Ontario.

   • States that half of dogs diagnosed with heartworm in 2010 had never left their local area.

     
     

2. Canadian Parasitology Expert Panel – Heartworm Supplementary Information

   
   

   https://research-groups.usask.ca/cpep/parasites/heartworm-supplementary.php

   • Reports that nearly 80% of heartworm cases diagnosed in Canada occurred in Ontario.

   • Discusses the influence of climate change and mosquito transmission.

     
     

3. University of Guelph Master’s Thesis: Heartworm Infection Among Domestic Dogs in Canada with a Focus on Ontario

   
   

   https://atrium.lib.uoguelph.ca/handle/10214/14250

   • Examined heartworm trends and geographic distribution from 1977–2016.

   • Found evidence of changing prevalence and expansion of heartworm distribution in Southern Ontario.

     
     

4. Geospatial Health – Epidemiology of Canine Heartworm Infection in Ontario

   
   

   https://www.geospatialhealth.net/gh/article/download/741/758/5347

   • Investigated the effects of climate and geography on heartworm risk in Ontario.

     
     

5. Ontario Animal Health Network (OAHN) Public Health Update

   
   

   https://www.oahn.ca/resources/2022-oahn-public-health-update/

   • Highlights the role of imported rescue dogs and emerging infectious disease concerns in Ontario.