Macrocyclic Lactones: Ivermectin for Dogs

In recent years veterinarians have made greater use of a group of chemicals known as macrocyclic lactones. Chances are your dog has been prescribed one or another drug from this group possibly by way of a monthly preventative. You may be giving these drugs to your dog or cat every month. They are available in a number of products but what are they? Are they safe?

To answer those questions I’ll focus on a popular commercial macrocyclic lactone called ivermectin.

What is ivermectin?
Animals can absorb ivermectin through oral or topical exposures as well as via injection.

Ivermectin is effective against heartworms and many gastrointestinal worms. It has a varying efficacy against external parasites. It is sometimes combined with other medications to treat a broad spectrum of animal parasites.

What is ivermectin used for?
Ivermectin is one of the most widely used medications for preventing heartworm infection in dogs. It is also used in cats for the same purpose but to a lesser extent. Some of the most well-known brands of heartworm prevention medications that contain ivermectin include Heartgard Plus®, Iverhart Plus®, Iverhart Max® and Tri-Heart®.  Besides preventing heartworm disease, ivermectin is also commonly used to treat other types of parasites including the following:

  • Demodex, the mite responsible for causing demodectic mange in both dogs and cats
  • Sarcoptes, the mite responsible for causing sarcoptic mange or scabies in dogs and in people
  • Otodectes cynotis, ear mites in dogs and cats
  • Capillaria, a lungworm in dogs and cats
  • Oslerus (Filaroides) osleri, another lungworm in dogs
  • Eucoleus boehmi, a nasal worm seen in dogs
  • Pneumonyssoides caninum, a nasal mite found in dogs
  • Potential toxicity

Ivermectin toxicity
Many dogs can tolerate the low dose required to prevent heartworms but when ivermectin is used to treat the many other parasitic infections in dogs it is used at much higher doses and the likelihood of toxicity increases.

Additionally, many heartworm preventives are produced in very tasty formulations and there is a possibility of inadvertent exposure to higher levels that in certain breeds can be toxic. I have personally had patients ingest a 12 month supply of heartworm preventive. Fortunately these dogs suffered no ill effects from the binge.

Ivermectin is also available as a parasite control for farm animals. Some dog owners use this formulation for their dogs as a heartworm preventive. I strongly discourage this practice unless under the direction of your veterinarian. The required dose for heartworm prevention is very low. These products -- intended for agricultural applications -- are produced in much higher concentrations making accurate dosing tricky and increasing the chance of overdosing your dog. This is of particular concern for specific breeds.

Questions to ask your veterinarian about ivermectin:

  • Why have you specifically recommended ivermectin instead of one of the other related compounds?
  • What impact does ivermectin have on the environment?
  • Can we test my dog to learn if there is a risk of toxicity from ivermectin?
  • Why don’t you use diethylcarbamazine instead of ivermectin to prevent heartworms? It was used for many years.

When used under the guidance of a veterinarian and with appropriate precautions in at risk breeds, ivermectin and all macrocytic lactones are safe and effective. All dogs should receive heartworm preventive drugs year round.

Macrocytic lactones including ivermectin are given as a part of the treatment of heartworm disease but only in conjunction with more specific heartworm therapy. In the past, some veterinarians have advocated using ivermectin as a “slow kill” treatment for heartworm infection rather than prevention. Recent statements from the Companion Animal Parasite Council indicate that this may result in resistance of heartworms to this vital drug. The “slow kill” therapy sometimes prescribed by veterinarians is never appropriate, as it has been demonstrated that using this modality—repeated macrocyclic lactone administration over a period of time — increases the proportion of circulating microfilariae that possess resistance markers. This approach to treatment should not be used. Generally, dogs should be treated with Immiticide® to kill the adult heartworms, although ivermectin often along with the antibiotic doxycycline is still used to kill the larva and prevent additional infections.

If you have any questions or concerns, you should always visit or call your veterinarian -- they are your best resource to ensure the health and well-being of your pets.

The opinions and views expressed in this post are those of the author's and do not necessarily represent the beliefs, policies or positions of, IDEXX Laboratories, Inc. or its affiliates and partner companies.