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By November 2, 2011 Uncategorized

Heartworm is a blood parasite that can potentially pose 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 centimetres, 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 may result from 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 inject 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 it’s saliva. From here, the immature worms develop further and migrate to the lungs and potentially the heart where they will mature into adults and begin reproducing. This cycle continues unchecked unless treatment is given.


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 cough, laboured breathing, and weight loss. The animal may also tire easily during exercise and collapse due to heart failure. Many cases in Ontario have no clinical signs due to low parasite burdens.


Treatment for heartworm disease is available. However, the method is costly and not without dangers. 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. However, if the dog is receiving monthly heartworm preventatives, these typically eliminate the immature parasites after a number of treatments.


Prevention is the Key Heartworm may be easily prevented in southern Ontario. Start by having your veterinarian examine a sample of your dog’s blood for the presence of adult parasites. The appropriate testing frequency will be decided by your veterinarian. If your dog is not infected and depending on the risk of infection, a preventative program should be started. The preventative program involves giving the dog a pill or applying a solution to the skin once a month during mosquito season. This medication destroys the immature heartworms transmitted by the mosquitoes and stops the cycle of the parasites. 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. Preventive programs should not be started before your dog has been tested for the presence of heartworm infection by your veterinarian. The test and the preventive program should be undergone each spring with the medication continuing throughout the summer until November. 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 easily preventable disease.

Common Questions About Heartworm

As veterinarians, we get asked many questions about this parasite and how to prevent it. The most common questions and improper assumptions are reviewed below.

Q. My pet was on heartworm medication throughout the last mosquito season. Why do I need to have my pet tested for heartworm this year? Of the 268 dogs with heartworm in Ontario in 2002, 30 had been on preventive medication (89% had received no preventative medication). Most failures could be traced to a lack of either owner or pet compliance on giving or taking the medication respectively. No medication is foolproof. Also, as mentioned earlier, the earlier a heartworm infection is detected and treated, the less damage is done to your pet’s heart. Also, depending on the type of heartworm preventive medication used, if given to a dog already infected with heartworm, a mild to severe allergic reaction may occur. For this reason, the heartworm preventive medication is a prescription product.

Q. I’ve heard that heartworm is on the decrease. Do I really need to worry about it in my pet? While it is true that the number of dogs testing positive for heartworm in Ontario overall is down, this is likely attributed to the fact that most dogs are on preventive heartworm medication. The decline in the number of positive cases is a testament to how effective a preventive program is. If one were to stop, especially with the presence of this parasite in the stray and wild dog population we would needlessly increase the risk of heartworm infection in our pets.

Q. If my dog is always indoors and never gets bitten by a mosquito, does it need a heartworm prevention program? Mosquitos are a fact of life during the summer months in Ontario, and we have yet to see a foolproof method of avoiding contact with the insect. Remember – it only takes one mosquito bite to infect a dog with heartworm.

Consult your veterinarian about diagnosis and prevention of canine heartworm disease.