Heartworm medication is a life-saver for dogs. It can prevent heartworms from growing to lethal size, and kills them before they do more damage to the heart and lungs. Some dogs, however, are allergic to certain types of heartworm medicine. The inactive ingredients, such as beef, used in some brands can be a problem for dogs with beef allergies. Other heartworm medications can harm or even kill certain breeds of dogs.
Heartworm disease is spreading, warns the American Heartworm Society, and even if you live in an area considered low-risk for heartworm disease, don't assume your dog is safe. Heartworms are spread by infected mosquitoes. When they bite your dog, they inject immature heartworms or microfilaria into your dog's bloodstream. Once they lodge in the heart, they can grow several feet long, eventually leading to cardiovascular collapse or heart failure.
Several types of medication prevent or cure heartworms. Ivermectin, milbemycin and moxidectin prevent heartworm disease by killing the microfilaria before they have a chance to lodge in the heart and mature. These are sold under several brand names and typically applied topically or given orally to the dog once a month. The only FDA-approved medication for killing adult heartworms in an infected dog is melarsomine dihydrochloride, or Immiticide. Your veterinarian will administer two Immiticide injections several days apart, and it can take over a month for the worms to die, break down and leave the dog's system.
Dogs who have a beef allergy can't take Heartgard Plus, a widely-prescribed monthly heartworm preventative containing ivermectin. Your vet can give you other options for your allergic dog. These may include topically-applied milbemycin (Revolution) or plain ivermectin, available as a liquid formula. The dose is dependent on the dog's weight, so get proper dosing information from a veterinarian.
The mdr1 gene mutation in some collies and other breeds, including Australian shepherds, Shetland sheepdogs, old English sheepdogs and long-haired whippets, causes adverse reactions and even death, if the dogs are given ivermectin. According to the Washington State University veterinary school, affected dogs can't expel the chemical from their brains, and can have neurological problems and seizures.
The mdr1 mutation not only makes affected dogs react to ivermectin, but they may have a sensitivity to several other medications as well, so if you have a susceptible breed, talk to your veterinarian about testing your dog's DNA. The test is simple and involves analyzing a cheek swab.