Heartworm symptoms in dogs: How to recognize this serious disease

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Heartworm poses a serious – and possibly life-threatening – risk to your dog. Prevention is better than a cure, so treat your dog from June to November inclusively.

A large dog resting his head on a picnic table and being petted lovingly by his owner

All dogs run the risk of contracting heartworm. In Canada, heartworm transmission occurs seasonally in regions of British Columbia, Manitoba, Ontario, Quebec and the Atlantic provinces. Transmitted by infected mosquitoes, heartworm is a serious disease which causes lasting damage to a dog’s lungs, heart and blood vessels, greatly hindering the quality of a dog’s life. Prevention is key, as once a dog contracts heartworm disease it’s is difficult and costly to treat.

If your dog does contract heartworm, early detection is critical to successful treatment.

Here’s how to spot the signs.

Heartworm symptoms in dogs

Heartworm disease is confirmed by a blood test carried out by your veterinarian. There are several physical signs that can indicate the presence of heartworm:


Your dog may cough periodically, especially after increased activity. This symptom could also indicate a less serious condition like kennel cough. If your dog is coughing, it’s important to see your veterinarian to rule out heartworm.

Lethargy or inactivity

Since it attacks the heart and lungs, heartworm can make typical activities, such as walking or playing, challenging for your dog. If you observe a drop in your dog’s stamina or activity level, a trip to the veterinarian is warranted.

Decreased appetite or weight loss

Just like in humans, significant and sudden weight loss or loss of appetite can signal a serious health issue in your dog. This symptom tends to occur as the heartworms mature.

Heart failure

As a pet owner you may not be able to recognize the symptoms of heart failure in your dog. Your veterinarian can, however, look for key symptoms to help make the diagnosis, including a heart murmur or an enlarged stomach due to fluid in the abdomen (known as ascites).

Other signs

Other signs of heartworm include blood in your dog’s urine, fainting spells, anemia, high blood pressure or a rapid heartbeat.

Death from heartworm infection

Heartworm infection can progress from a mild illness where dogs are asymptomatic or have a slight cough, to a severe illness characterized by congestive heart failure, respiratory compromise, fainting spells and potential death. A complication of heartworm infection, known as caval syndrome, leads to shock-like symptoms and sudden death.

If you think your dog has heartworm, act fast

Heartworm disease is a serious and progressive illness in dogs. The sooner it is treated, the better the chance your dog can avoid long-term health complications. If you suspect your dog has heartworm, take your dog to your veterinarian as soon as possible so they can get tested.

Prevention is key

When it comes to heartworm, prevention is the best defense. To avoid heartworm disease altogether, it is important to give your dog a regular heartworm treatment.

Heartworm treatments kill larval heartworms that infect the dog when the mosquito is feeding, before they reach maturity. Most products kill the larval heartworms that have infected the dog in the 30 days since the last treatment. These larvae are then eliminated from the dog’s system, so they do not cause any lasting damage or health concerns. Speak to your veterinarian about a heartworm protection for your dog.

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