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Heart Worm Disease – What You Need to Know

Yes, heartworm disease in Canada is real.  Yes, heartworm disease can kill your dog.  Yes, cats can develop heartworm disease too.  No, not all dogs and cats in Canada are at risk.  Let’s talk about heartworm disease and how you and your veterinarian decide if your pet requires regular testing and prevention.

The Ontario Veterinary College published a paper in 2010 which showed that 564 dogs tested positive for heartworm in Canada.  Over 75 percent (431 in total) of these dogs lived in Ontario.  There are also pockets of heartworm disease in southern Manitoba, southern Quebec, and southern mainland British Columbia.  This study showed the presence of heartworm infection in dogs increased by nearly 60% since the last study of its kind in 2002.  Researchers speculate that one factor may be the adoption of heartworm-positive rescue dogs from the southern United States after Hurricane Katrina or through other rescue programs from American states where there is a higher prevalence of heartworm disease.   Other studies suggest that cats may be infected at approximately 1/10 the rate of dogs in heartworm prevalent areas.

Let’s look at the life cycle of heartworms in the dog.  Dogs become infected with heartworms through mosquito bites.  Mosquitoes “suck up” microfilariae, microscopic larvae, when they bite and feed from dogs, coyotes, and foxes who already have contracted heartworm disease.  The larvae mature in the salivary glands of the mosquitoes who then feed from an uninfected dog.  The larvae are then transferred into the tissues of the new dog and go through a maturation process in the dog’s body.  The larvae eventually become adult worms who travel through the lungs and bloodstream and end up in the pulmonary blood vessels and the heart.  Once they arrive here, they can mate and produce thousands of the tiny microfilariae.  It takes between 5 to 7 months for one female heartworm to mature and begin producing these tiny larvae.  The microfilariae travel through the bloodstream where they are available for the next mosquito to come along and feed.    Thus the cycle repeats itself.

The adult heartworms can grow to be 15 to 30 centimetres in length.  Severely affected dogs may carry 30 to 100 worms within their hearts and lungs.  The presence of these parasites over time can lead to lung and liver disease and heart failure.  While the worms themselves don’t necessarily kill a dog, these conditions can certainly be fatal.  Treatment often involves the killing of the adult worms with an arsenic-based compound which can result in severe complications.  Prevention on the other hand is quite easy and with minimal adverse effects.  Preventive medication is aimed at killing the microscopic larvae in the dog’s system before they have a chance to reach the heart and the lungs.  Prevention may come in the form of a tablet, a chew treat, or a spot-on product that you apply over your dog’s shoulders and back.  Normally these medications are prescribed on a monthly basis and are given during the summer months, according to a local area’s “mosquito season”.

The American Heartworm Society recommends annual testing and preventive medication for dogs that live in heartworm prevalent areas of the country.  While we pet parents who routinely give our dogs heartworm medication may wonder why we need to re-test annually, there are a couple of very good reasons.  Some studies suggest that 1 in 10 positive dogs may have been given a preventive in the year prior to having a positive test.  While the medications themselves are very effective, some of us may forget to give the pill on time or may miss a month or two at the end of the mosquito season.  Some dogs are quite wily and will appear to have swallowed their tablet but may spit it out in the backyard or behind the sofa.  A dog who eats grass “just because she likes it” may accidentally vomit the pill in the backyard without your ever realizing it.  These dogs may be unprotected for a whole month of heartworm-infecting mosquito bites.

Mosquitoes do not discriminate against small dogs and dogs who spend much of their time indoors.  These dogs may be at risk for heartworm disease depending on where you live and will benefit from preventive medication.  Your veterinarian will be able to offer you the specific statistics for heartworm prevalence in your area to help you determine if prevention is a good idea for your dog and how often your dog should be tested.  Also remember that dogs who travel to the United States or areas of Canada with a higher prevalence for heartworm disease should be prescribed a preventive medication during travel and in the month afterward.  These dogs will need to be re-tested 6 to 12 months after they return home.

Cats are not considered a natural reservoir host for heartworms, but we can still see the disease in this species.  While the life cycle is similar, cats tend to develop infections with smaller numbers of worms and are as likely to develop breathing problems or an asthma-like condition as true heart failure.  Cats most at risk are those who spend time outdoors at dusk and dawn.  Heartworm testing is more complicated in feline patients but preventive medication is available.  Please ask your veterinarian if your cat would benefit from a heartworm prevention program.

Article by Petsecure Pet Health Insurance

Dr. Colleen Skavinsky
Chief Veterinary Officer
Petsecure pet health insurance

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