Bowmanville Veterinary Clinic

2826 HWY 2
Bowmanville, ON L1C 3K5


Heartworm Disease in Dogs


What It Is

Heartworm is a blood parasite that poses a serious health threat to dogs in Canada and the United States.
Heartworms are large roundworms that live in the right side of the heart and the blood vessels that supply the lungs, surviving on nutrients which they steal from the dog's bloodstream. They can grow to a length of 15-30 centimeters and in severe cases a dog may be infested with hundreds of worms.

Damage to the heart, lungs and liver as well as obstruction of blood flow is the result of this infestation. Eventually fluid may build up in the lungs and restrict the dog's breathing. When damage to the internal organs is severe enough, death may be the result.



Heartworms are spread by mosquitoes. When a mosquito bites an infected dog, it will ingest the immature worms (microfilariae) produced by the adults in the heart along with the blood from the dog. The immature worms develop in the mosquito over the next few weeks until they reach an infective stage. When the mosquito bites an uninfected dog it will inject the immature worms into the tissues with its saliva. From here the immature worms develop further and migrate to the heart where they will mature into adults and begin reproducing. This cycle continues unchecked unless treatment is given. From this it is easy to see how one infected dog can infect a whole neighborhood.

The signs of heartworm disease are usually detectable only after the disease has progressed and much damage has already been done to the internal organs. This damage may be irreversible. An advanced case may develop such signs as general listlessness, a chronic soft cough, labored breathing, weight loss, tire easily during exercise and collapse due to heart failure.
Treatment vs Prevention

Treatment for heartworm disease is available, however, the methods are costly and are not without danger themselves. Treatment involves a series of injections to kill the adult worms. During this time period the dog must be kept very quiet as even minimal exercise may result in serious problems from the dead and dying worms. After the adult worms are destroyed, a treatment to kill the immature worms in the bloodstream must be given.

Heartworm may be easily identified in southern Ontario by having your veterinarian examine a sample of your dog's blood for the presence of the immature worms once yearly. If your dog is not infected then a preventative program should be started. The preventative program involves giving the dog a Sentinel pill once monthly. This medication destroys the immature heartworms transmitted by the mosquitoes and stops the cycle of the disease.

We will never be able to completely eliminate heartworm as it is now being found in the wild dog (coyote) and stray dog population which we cannot control.  This will, unfortunately, act as a source of infection for the pet population. 

Prevention programs should not be started before your dog is tested for the presence of heartworm disease by your veterinarian.  The test should be undergone each spring with the Sentinel medication that is given year round to prevent parasites, also covering for the treatment of heartworm. For those of you who vacation in the United States with your dog, consult your veterinarian regarding the best way to provide continual protection against this disease.