Heartworm (Dirofilaria immitis) is a nasty parasite carried by some species of mosquitoes that can infect both dogs and cats. The adult worm lives in the heart and pulmonary arteries of canines like coyotes, foxes, and our beloved doggies. Dogs are the natural host for heartworm, which means the worms can live, grow, and reproduce inside them. Cats are an atypical host, and most worms don’t live long enough to become adults, although damage from immature worms can still be substantial. This blog post will focus on dogs because they are most likely to be affected. Luckily for us in Edmonton, heartworm is not a common problem because the temperature, humidity, and the species of mosquitoes in this area do not provide the right conditions for heartworm to thrive. Our pets are only at risk if they travel to areas where heartworm is endemic when mosquitoes are active. Heartworm endemic areas have been documented in British Columbia, Manitoba, Ontario, Quebec, every state in the USA, and in Mexico, although it’s impossible to know exactly where the endemic regions will be every year because it changes depending on the local climate, availability of susceptible canines, and how well the mosquitoes adapt to cold.

How does a pet become infected?

Male and female heartworms reproduce when they’re cozily nestled in the heart and pulmonary arteries of their doggie host. The worms produce live offspring called microfilariae, which get released into the bloodstream for a mosquito to pick up when they bite and suck the blood of their victim. Microfilariae can live for two years before they die of old age and can be seen under a microscope, during routine blood tests. After a mosquito sucks up some microfilariae, the immature worms will grow into the infective larval stage if the climate is warm and humid enough. When the mosquito bites another dog, they leave the larval worm in a droplet of saliva, and the worm swims through the puncture made by the mosquito bite and makes itself at home in their new host’s skin, where they develop for another 5 to 7 months. This is the stage that is vulnerable to preventative medications because, after this stage, they enter the bloodstream and find their way to the heart where they settle as adults and start the cycle all over again. Adult worms will not be killed by common deworming medications and require more intense treatment plans.

Why is it bad?

Heartworm disease can cause permanent damage to the heart of an infected dog, even in the early stages of infection. Moderate to heavy infections can slow or block the flow of blood through the heart, causing exercise intolerance, persistent coughing, decreased appetite, heart failure, and death. In some cases, the only way to save the dog’s life is to perform a risky surgery to remove the worms from the heart.

How do you treat it?

Treatment is available for dogs that have adult heartworms, but it can be extremely costly, take multiple veterinary visits, different medications, and the damage from the worms can be permanent, so preventing the infection is always better. The American Heartworm Society advises annual heartworm tests for all dogs, even if they’re on preventative medications, but that may be overkill for dogs that never leave low-risk areas like Edmonton. Ask your veterinarian about when you should test for heartworm if you have or plan to take your dog travelling. Heartworm preventative medications do not kill adult worms and are not always 100% effective because dogs may spit out oral medications or rub off topical treatments, so regular testing is essential to catch an infection in the early stages for the best outcome for your travel companion. There are various heartworm preventative medications available that work by killing the larval stages in the skin of your pet, and they usually target other parasites that your pet may have collected for a complete deworming treatment. Always ask your veterinarian about the best deworming medication for your pets.

This blog post has been written with the help of The American Heartworm Society and Veterinary Partner.

Written by Alicia Naundorf, RAHT