Symptoms of Heartworm

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 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. Your lively, athletic dog may simply have less “get up and go.”
2. Next, a mild cough without any nasal or ocular discharge may appear.
3. The cough is often accompanied by weight loss and a decrease in appetite.
4. Heart failure progresses and weight loss becomes severe (called cardiac cachexia).
5. You might notice a distended abdomen as fluid backs up from the stressed heart.
6. The cough will become more frequent, and exercise intolerance will be more pronounced.
7. The dog begins to struggle to breathe as the heart and lungs try to get oxygen out to all his tissues.
8. Rarely, a large number of heartworms will disrupt blood flow within the heart causing a cardiac crisis known as caval syndrome.
9. Without treatment, your dog will die of heart failure.

Heartworm Treatment

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.

Heartworm can be treated, but damage already done to the heart tends to remain. Treatment is not without risk. Clearly, this is a disease it is best to prevent.

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 macrocyclic lactones and, as noted by the American Heartworm Society, are effective 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.