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Macrocyclic Lactones: Ivermectin for Dogs

Posted February 07, 2014 in Dog Diseases & Conditions A-Z

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

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Mike has more than 35 years of experience in companion animal veterinary practice and is a valued member of IDEXX’s Pet Health Network team since 2013.