By Elva Ma, DVM

This is a heartworm larva, found in a blood sample of one of our patients this year!

As temperatures start to climb in the city of Toronto and we enjoy the milder weather with our canine companions, we renew our focus on parasite prevention at Wellesley. In a previous blog post, Dr. Pilar discussed how to protect our pets from the growing threat of ticks and Lyme disease, and in today’s blog, I will review the biology of heartworm disease, treatment and prevention strategies to keep your pet safe and protected all year long.

Heartworm disease is transmitted from animal to animal by mosquitoes. Therefore, whenever mosquitoes are active, dogs are at risk of contracting this deadly disease. Even cats can be at risk if they go outdoors, or are exposed to mosquitoes in the home. In Toronto, heartworm protection is typically recommended from June 1st to November 1st.

A mosquito close-up. They average 3-6mm in size, which is about the size of a popcorn kernel.

When an infected mosquito bites an animal, it transmits young heartworms called microfilariae, which develop into infective larvae in the pet’s body. These larvae will grow into adult heartworms within 6 months. After this time, they travel throughout the body for several more months, eventually reaching the blood vessels of the lungs. In the final stages, the females can grow up to 14 inches, damaging blood vessels and the heart itself. Once this occurs, infected dogs may show signs of severe lung and heart disease, including:

  • coughing
  • exercise intolerance
  • loss of appetite
  • difficulty breathing

Because dogs with recent or mild heartworm infection may not manifest any signs of disease at all, heartworm testing becomes an important tool for veterinarians to detect early infection (as early as 6 months after heartworms enter the body). This allows treatment to occur in a timely manner and gives the pet the best chance of recovery with minimal complications. Heartworm testing consists of a simple blood test, which can be performed with great accuracy within minutes in the hospital.

Treatment for heartworm disease is costly, and carries substantial risk. However, successful treatment is possible with a carefully staged treatment protocol that usually takes several months to a year to complete.

The good news is that heartworm infection is easily preventable in dogs and cats. There are several safe and effective heartworm preventives available in a variety of formulations. Please speak to a team member at Wellesley to discuss the best preventive method based on your pet’s risk factors and lifestyle.

It is important to note that some heartworm preventives are not safe to give to animals already infected with heartworm. Therefore, the American Heartworm Society recommends having your pet tested for heartworm BEFORE starting their heartworm preventive. This is typically done once a year in the spring, before the official heartworm preventive start date of June 1st. It is also important to remember that no preventive medication is 100% effective, which further highlights the importance of annual testing.

In addition to heartworm testing, your dog’s springtime parasite prevention visit will include a full nose to toes examination with the veterinarian. We also encourage pet owners to take this opportunity to discuss the following:

  • how to best protect your pet with a customized parasite prevention plan
  • yearly fecal screening for internal parasites
  • wellness blood testing to check internal organ health

If you haven’t yet scheduled your pet’s parasite prevention visit, it‘s not too late! Please give us a call, email or text to book your appointment today. And as always, please don’t hesitate to contact the team at Wellesley if you have any questions on keeping your pet protected. Wishing you and your furry companion a safe, joyful and parasite-free summer!

For more information on heartworm disease, please visit the American Heartworm Society (https://www.heartwormsociety.org/).