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Val Caron Animal Hospital

3055 Highway 69 N
Val Caron, ON P3N 1R8


Heartworm Disease


What is Heartworm Disease?

Heartworm disease is a parasitic infection that occurs in dogs and occasionally cats, which can be fatal if not treated. The parasite is a worm called Dirofilaria immitis and the parasites typically live in the main pulmonary arteries and the right side of the heart.

Clinical signs of Heartworm

The most common symptoms of Heartworm include:

* Chronic (long-term) coughing

* Difficulty breathing / Labored breathing

* Exercise Intolerance

* Lethargy (laziness)


How Does a Dog or Cat Get Heartworm Disease?

A dog becomes infected when a mosquito carrying the infectious development stage of heartworm takes a blood meal from the dog. When the mosquito bites, the developing stage (called larvae) is now under the skin. The larvae migrate through skin and tissue for two months until they become adults and make their way to the heart and pulmonary arteries. It takes approximately six to seven months for the female adults to produce microfilaria into the blood stream. Another mosquito comes along, bites the dog and picks up the microfilaria. The mosquito will shelter the microfilaria until they develop into infectious larvae. When conditions are ideal, the mosquito can infect other dogs with one mosquito bite.

The Life Cycle of a Heartworm

The heartworm larva remains in the tissue surrounding the bite for approximately 2 months where it grows into a immature adult worm. It then enters the dog's blood stream and in 3 or 4 months reaches the heart and surrounding arteries. The worms reach final maturity 6 to 7 months from when the dog was first infected and can now produce Microfilaria offspring, thus starting the cycle over again.

Heartworm Considerations for the Sudbury Area

The Sudbury area Heartworm transmission season is the 3rd week of June to the 3rd week of September. This is based on our climate and weather temperatures. Dogs should be started on Heartworm prevention on June 1 and be continued once a month with the last dose given on Novemer 1. For dogs travelling outside Ontario, it is recommended that dogs be on Heartworm prevention year round.

How to Test for Heartworm Disease

A small sample of blood is taken and tested for the presence of an antigen. The antigen is only produced by an adult female heartworm. If an antigen is detected using the SNAP heartworm test, your pet is positive for heartworm disease. This test can provide results in just eight minutes and is considered the "gold standard" in diagnosing heartworm in dogs. Ideally, testing should be done before the mosquito season starts but it is highly recommended to test prior to using a heartworm prevention, even if it is later in the summer. Puppies born after October 1st of the previous year do not require a test for their first heartworm season. This is because it takes six to seven months for the female adults to reproduce and circulate microfilaria into the blood.


Why Do We Test for Heartworm Disease

By testing for the disease we can ensure that if heartworm disease is present it can be immediately treated.

Ensuring that a dog is heartworm negative greatly reduces the possibility of an adverse reaction to the heartworm preventative medication.


Preventing Heartworm Disease

Fortunately Heartworm is easy to prevent. A preventive pill is administered or topical liquid is applied once a month. Heartworm season is from June until November and dogs and cats should be on prevention during that time. Heartworm prevention can be started once an animal has been tested and show they are negative for Heartworm. Please contact us for more information on our heartworm prevention programs.


Why Prevent when we can Treat?

Prevention is much safer, easier and more cost effective than treatment. In addition, treatment can be dangerous to your pet.

So while Heartworm should be a matter of concern for pet owners, annual checkups with your veterinarian, screening with the proper diagnostic test, and a good preventive program will keep your dog Heartworm-free this season. For more information on heartworm disease, please contact your veterinarian. In addition, the American Heartworm Society is an excellent source of information.   

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