Common side effects from oral heartworm treatments include diarrhea, vomiting and lack of muscle coordination. Side effects from topical heartworm treatments include hair loss at the application site, diarrhea, vomiting, trembling, panting and drooling. Injectable heartworm treatment side effects include seizure, shock, swelling, skin irritation or itching, diarrhea and vomiting. It is rare for dogs to experience side effects from heartworm treatment.
In addition to experiencing side effects from the heartworm treatment, some dogs may also experience allergic reactions to the ingredients in the heartworm medication. A dog experiencing an allergic reaction may show symptoms such as seizures, shock, hives, itching and swelling, particularly in the facial area. Dogs who have a severe heartworm infestation may be more susceptible to adverse reactions to treatment.
Some dog breeds are particularly sensitive to the active ingredients in most heartworm treatments, ivermectin and milbemycin. Many collies, whippets, Australian shepherds and sheepdogs have a genetic mutation that makes them unable to clear heartworm medications from their brains. This abnormality can increase the odds of these dogs suffering from seizures or even dying as a result of taking a heartworm treatment.
Heartworm is a serious disease that is considered endemic in many areas. Most veterinarians feel that the benefits of heartworm preventative treatment outweigh the risks of side effects.