Signs & Symptoms of Heartworm Disease in Dogs

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As a loving pet owner, you are no doubt interested in raising a healthy, happy dog. So what happens when a silent disease strikes?

Heartworm is one such “silent disease” which does not manifest in a dog until further development. Caused by a parasitic worm called Dirofilaria immitis, heartworm disease is spread through the bite of a mosquito. The severity of the disease depends on the number of worms present in the body, the duration of infestation, and the response of the host (in this case, the dog).

The worms involved with this disease are called “heartworms” because the adult worms tends to live in the right ventricle of the heart in the pulmonary artery, lungs, and associated blood vessels. Once mature, heartworms in a dog can live for 5 to 7 years. Heartworm cases are mostly found in areas with subtropical climates, like the Atlantic and Gulf Coasts, but are prevalent in all 50 states. In these regions, dogs without proper heartworm prevention are most likely to suffer from infestation. Heartworms can be transmitted to a dog through infected mosquito bites. These subtropical climates are conducive for mosquitos so it’s important to talk to your veterinarian about ways to prevent heartworm infection in your dog. If you think your dog may have experienced infected mosquito bites, you should talk to your veterinarian right away. Keep reading to learn more about ways to prevent heartworm, heartworm symptoms, and heartworm treatment for dogs.

Heartworms in dogs are mostly preventable through preventive medication from a veterinarian. The prognosis for dogs that contract this disease ranges from mild to moderate, with the most mild infections showing little to no symptoms. Dogs with more severe cases, however, can suffer much more serious complications—even to the point of death.

So how can you tell if your dog has heartworm? Read on to discover the symptoms  and signs of heartworm disease in dogs.

Signs and Symptoms of Heartworm Disease in Dogs

signs of heartworm disease in dogs_canna-pet

In the early stages of the heartworm disease, few heartworm symptoms will show. In most cases, symptoms don’t show, at all. The longer the infection is present, however, the more likely it is that symptoms will develop.

Here are some of the signs and symptoms of heartworms in dogs:

A soft, dry cough

A coughing dog is often the first telltale sign of heartworm. As the heartworm parasites enter the lungs, they multiply in the lungs and surrounding veins. With heartworms in dogs, coughing often follows exercise and can result in fainting—even after light exercise.

Rapid breathing, or difficulty breathing

Along with a cough, a dog with heartworm may have trouble breathing when the worms infest his lungs and surrounding veins. In addition, fluid can build around the blood vessels in the lungs, making it difficult for the lungs to pump oxygen into the blood.

Weight loss or anorexia

When a dog has heartworm, even the most minor physical activities can be a chore, such as eating. As a result, you may notice a loss of appetite in your dog.

A bulging chest

heartworm disease in dogs_canna-pet

Ribs may protrude, and the chest appears to bulge out as a result of adult heartworm infection. This symptom can also be a result of weight loss and anorexia caused by heartworms, or by fluid build-up in response to the presence of the parasite.

Fatigue or inactivity

If your dog was once active and energetic but seems more tired than usual, showing no desire to go outside or avoiding physical activity altogether, he may have heartworm. Dogs with the heartworm infection often find it difficult to be active, or even to perform minor tasks without difficulty.

An allergic reaction

Although cats are more prone to allergic reactions from heartworm, it is still possible for dogs to show symptoms of an allergic reaction or asthmatic attack.

Symptoms of an allergic reaction in dogs include:


When enough heartworms infect the heart and block the blood flow (also known as caval, or vena cava, syndrome), the host dog can collapse. Collapse is often accompanied by shock and red blood cell destruction, with death imminent in the days that follow.

Other Symptoms of Heartworm Disease in Dogs

Although the previous symptoms are most common ones in dogs with heartworm, the following symptoms are possible, as well:

  • Excessive sleeping
  • Nosebleeds
  • Secondary pneumonia
  • Seizures
  • Increased blood pressure
  • Blindness
  • Lameness, or difficulty moving

Seizures, lameness, and blindness occur when the parasites get lost and end up in other parts of the body, outside of the heart and lungs. They can even end up in the brain or eyes—though less rare.

Many of these symptoms can also be signs of other conditions, which makes it even more difficult to pinpoint the heartworm infection. If you suspect your dog has heartworm, your vet is equipped to detect—and treat—the condition using a variety of tools.

Heartworm Treatment for Dogs

Once you go to the vet and have a heartworm test confirm the presence of heartworms, you will need to consult with your veterinarian about treatment options. The American Heartworm Society has a suggested treatment plan to follow. Make sure to consult with your vet to come up with the best treatment plan for your pet.

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