3484 Mavis Road, Mississauga, ON, L5C 1T8
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Heartworm is caused by the parasite Dirofilaria Immitus, a fairly large worm which can attain lengths of 14 inches.  Adult worms live in the heart and pulmonary arteries of an infected animal.

Animals acquire this infection through mosquito bites.  Dogs are the primary host although cats can also be infected.  Mosquitoes readily pick up a young larval form of heartworms from infected animals (dogs, coyotes) and carry them to uninfected pets.

Some geographic areas have severe heartworm problems while other areas have virtually none.  In order for rthe parasite to establish its presence in an area, the folllowing conditions must be met:

  • the varieties of mosquitoes capable of carrying larval heartworms must be present
  • the weather must be warm enough to allow heartworm larval development within the mosquito.  Generally this requires about 20 degrees C for about 2 weeks
  • there must be infected dogs (or coyotes) in the area as well as vulnerable host dogs in the area

Unfortunately the Mississauga, Brampton and Oakville areas meet the conditions and heartworm is present here.

When a dog or cat is bitten by an infected mosquito it passes on the larvae.  Once inside the host, the larval stage lasts for about 50-70 days in the skin.  During this stage it is susceptible to heartworm preventive and this is why once a month treatment is effective. 

If preventive has not been given to wipe out the larvae the worms enter the circulation going to the heart and lungs where they develop into adult worms and cause signs of disease.  Early in the disease there may be few clinical signs but as the disease progresses symptoms include cough, fatigue after light and moderate exercise and weight loss.  As it progresses signs of severe heart and lung disease develop.  In cats the signs are more subtle; however, the disease can be very severe and often fatal.

Treatment of the disease in dogs can be difficult and intensive.  However, prevention is very easy.  We do a blood test in the spring to make sure your dog is heartworm free and then your pet can go on once a month preventive treatment throughout the heartworm season.  Cats do not need to be tested but can go on the same medication.