Treating heartworm in dogs

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Heartworm is a particularly nasty parasite that is present in Canada. Learn how you can protect your dog from this disease. And don’t forget that this parasite can affect cats too!

Dogs get heartworm from mosquitoes. A bite from an infected mosquito will transmit heartworm larvae into your dog. These larvae grow and develop inside your dog over a period of several months. Eventually these larvae become adult heartworms living in the major vessels of your dog’s heart and sometimes in the heart itself. Heartworm is an extremely serious and difficult to treat disease, so taking steps to prevent it is crucial.

Big dog chewing on a bone in the grass

In which regions is heartworm found?

In Canada, heartworm transmission occurs seasonally in regions of British Columbia, Manitoba, Ontario, Quebec and Atlantic Canada. Heartworm is found in many other countries across the world as well. It is generally found in tropical and temperate regions, with the highest known prevalence in the USA, South America, Japan, Australia and Italy. It is also found in some southern European countries such as Spain, Portugal, France, Greece and Turkey. If you are planning to travel with your dog, check with your veterinarian first.

Heartworm symptoms in dogs

Symptoms can vary depending on the number of worms present in the dog. Dogs with a low number of worms may show fewer symptoms, but high worm burdens can lead to coughing, tiring easily when exercising, and even heart failure.


Veterinarians can diagnose heartworm in a couple of ways. One method is by testing the dog’s blood and looking for microfilariae (microfilariae are the early stage of the heartworm lifecycle produced by the adult heartworms and circulate in the animal’s bloodstream). Another blood test can detect the presence of female adult worms. Chest X-rays can also help to determine the extent of any heart and lung damage.

Treating heartworm in dogs

This condition is difficult to treat, largely because killing the worms can cause the dead worms to become lodged in the dog’s and some dogs can have severe reactions to the dying worms.

One step of dog heartworm treatment involves giving a drug to kill the adult heartworms. Dogs need restricted exercise and supportive therapy during this time to avoid complications. A different drug is given that targets the stage of the heartworm that is circulating in the bloodstream (the microfilariae). This part of the treatment is equally challenging, as these dying microfilariae can lead to anaphylactic (allergic) reactions.

A range of additional therapies may be required, depending on the symptoms the dog is exhibiting. Surgical extraction of the worms is sometimes necessary in cases where the worms block blood flow from the heart.

Microscopic close up of heartworm

Heartworm prevention

Heartworm is an incredibly serious condition and treatment can be costly, lengthy and difficult, with considerable risks to your dog. If you live in an area where heartworm is found or are taking your dog to areas where this parasite is a problem, it is essential to take preventative measures to avoid your dog becoming infected in the first place.

Speak to your veterinarian about the necessary steps you need to take to make sure your dog stays protected.

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