Dog Heartworm Symptoms and Prevention

Heartworm in dogs is deadly and spread by the bite of a mosquito infected with Dirofilaria immitis, which is a parasitic roundworm that migrates to the heart and lungs. Early on, symptoms of heartworm infection are difficult to notice. As the disease progresses, your dog will find it difficult to breathe, as his lungs and heart struggle against the heartworm invasion to support his life.

Symptoms of Heartworm in Dogs

The symptoms seen in a dog with heartworm vary with the stage of disease and the severity of the infection. Early on, your dog may not show any signs of heartworm disease. As the disease progresses, signs related to your dog’s heart and lungs will become apparent and deadly.

The symptoms of heartworm often develop like this:

1. Lethargy: Your lively, athletic dog may simply have less “get up and go.”
2. Persistent Cough: Next, a mild cough without any nasal or ocular discharge may appear.
3. Loss of Appetite: The cough is often accompanied by weight loss and a decrease in appetite.
4. Severe Weight Loss: Heart failure progresses and weight loss becomes severe (called cardiac cachexia).
5. Swollen Abdomen: You might notice a distended abdomen as fluid backs up from the stressed heart.
6. Worsening Cough: The cough will become more frequent, and exercise intolerance will be more pronounced.
7. Difficulty Breathing: The dog begins to struggle to breathe as the heart and lungs try to get oxygen out to all his tissues.

In rare cases, a large number of heartworms will disrupt blood flow within the heart causing a cardiac crisis known as caval syndrome. Characterized by symptoms including collapse, pale gums, rapid heart rate, and fast, irregular breathing, caval syndrome requires urgent surgical intervention to remove the worms. The surgery is risky and the prognosis for dogs with caval syndrome is extremely guarded.

Heartworm can be treated, but damage already done to the heart tends to remain. Treatment is not without risk but without treatment, your dog will die of heart failure. Clearly, this is a disease it is best to prevent.

How Do Dogs Get Heartworm?

When an infected mosquito bites a dog, it injects an immature heartworm stage called microfilaria into the dog’s bloodstream. The microfilaria eventually develop into mature heartworms (they look like long, white worms) that usually reside in the dog’s heart.

Heartworms are found in every state, and the Companion Animal Parasite Council tracks where they are. You can track forecasts for heartworm infections here.

Heartworm Prevention

Start prevention following an annual screening test to verify your dog does not have heartworm already. The FDA-approved heartworm preventives are ivermectin, milbemycin oxime, moxidectin, and selamectin. These are all noted by the American Heartworm Society aseffective and safe medications when used as directed. Most of these are given as monthly oral doses. Treatment is recommended year-round.

Note: Some dogs (mainly collies, herding breeds, and mixes with these breeds) react to ivermectin due to a genetic defect known as MDR1. Options without ivermectin are best for those dogs, although the dosage in most monthly preventives is low enough to avoid toxicity.