Everything You Need to Know About Heartworm in Ontario

How can my dog even get heartworm?

Heartworm disease is transmitted by mosquitoes that carry inside them an organism called Dirofilaria immitis (the larvae of heartworm). As the mosquito bites your dog, it can deposit the larvae into the skin. The heartworms mature and eventually migrate from the skin tissues into the bloodstream over a period of about 50-70 days, by then, they have developed into juvenile adults. Those juvenile adults then migrate to the blood vessels of the lungs, where they become fully sexually mature adults. It can take 6-9 months from the time of the first infection by a mosquito for the larvae to become adults, breed, and then release immature heartworms into the bloodstream. This infected dog can now be bitten by a mosquito who can pick up the baby heartworms and transmit them to another dog – continuing the cycle of spreading heartworm from dog to dog.

What are the symptoms of a dog with heartworm?

Since adult heartworms live in the blood vessels of the lungs and heart, some of the clinical signs can be coughing, exercise intolerance, lethargy, fainting, weight loss, not breathing well, etc. Without treatment to slowly kill the heartworms a pet will die from heartworm disease.

How does the heartworm test work?

Ideally, veterinarians would like to do a blood test for every dog in the springtime to see if there was any chance they could have contracted heartworm the season before. A small amount of blood is drawn, and the lab will then run a test to see if your dog is positive or not. A negative test is recommended before your dog starts on preventative medication, as a dog positive for heartworm requires a much more complicated medication protocol and needs to be closely monitored by a veterinarian.

Where is heartworm and what is the risk factor for my dog?

Although heartworm can be found in other provinces, the majority of Canadian cases are found in Ontario, specifically southern Ontario. Idexx is one of two veterinary labs in Ontario that test for heartworm, and from 2013-2017 they saw an average of 225 cases per year. Keep in mind this is only a representation of dogs that came into vet clinics for blood tests and from only one laboratory. These numbers also do not take into consideration wildlife that we know harbour the disease, so heartworm is much more prevalent than a few hundred cases per year.

If the risk factor is low why should I use monthly preventatives?

The number of positive cases is still a low ratio per dog (considering how many dogs live in Ontario) – which is a great thing! Every dog that takes preventative medication is not only one less dog that can get sick from heartworm, but they are also not spreading it around their neighbourhood by being a host for mosquitos. Preventing heartworm disease is much cheaper ($8-16 per month depending on the size of your dog) than having to treat it should your pet become infected. Treatment for a positive dog is quite expensive and involves many steps including antibiotics, monthly heartworm preventatives, and injections of an adulticide (that will kill the adult worms). Steroids and antihistamines may also be needed as these worms affect other body systems as they die. Contracting heartworm is very serious and a risk that your dog does not need to take!

The bonus of heartworm preventative medications is that they also contain a deworming medication which will protect your dog from contracting other parasites during our warm weather months. Deworming adult dogs for common parasites such as roundworms and hookworms is often overlooked by owners, so giving a monthly “all in one” medication can protect your pet from all the common internal parasites they may come in contact with.

If you have any questions about heartworm disease or medications, please call us today!

Written by McLean Animal Hospital