Side Effects of a Heartworm Injection

Heartworm treatment is a physically trying process for your dog.
Heartworm treatment is a physically trying process for your dog. (Image: Dog image by Istoria13 from

Heartworms are transmitted through mosquito bites. The worms take up residence in a dog's heart, where they grow up to 6 inches long. Treatment of a dog infested with mature heartworms results in side effects even when the treatment is successful.

Artery Blockage

Partial arterial blockage is an inevitable side effect of heartworm treatment as the dead worms pile up to form an obstruction. Over a period of about three to six weeks, your dog's white blood cells will eat away the blockage. But until then, your dog may have a fever and a cough that may bring up blood.


Caparsolate is administered through a catheter and can cause severe tissue damage if it contacts tissue outside the vein. Most dogs have to remain hospitalized through the treatment. Vomiting is common after caparsolate is administered, and the injection can cause liver and kidney damage. Other side effects include lethargy and appetite loss.


Immiticide is the most common heartworm treatment and has far less severe side effects than caparsolate. The injection site can swell and become tender, and your dog may experience appetite loss, lethargy and coughing. Dogs 8 years and older tend to exhibit these symptoms more frequently than younger dogs.

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