Brand Names: Ivomec, Heartgard & Heartgard Plus, Iverhart Plus & Iverhart Max, Pet Trust, Tri-Heart Plus & Acarexx

Available in tablets and chewables for heartworm prevention, topical solution for ear mite treatment, or as an oral or injectable solution for other parasite problems      

How this Medication is Used

In the mid-1980s, ivermectin was introduced as probably the most broad-spectrum anti-parasite medication ever. It was an absolute revolution in parasite control for livestock and horses, and for dogs, it forever changed heartworm prevention from a daily pill to a monthly one.

Ivermectin is effective against most common intestinal worms except tapeworms, most mites, and some lice. It is not effective against fleas, ticks, flies, or flukes. It is effective in killing larval heartworms (the microfilariae that circulate in the blood) but does not kill adult heartworms that live in the heart and pulmonary arteries, though technically, it can shorten their lifespan.

The most common uses in small animal practice for ivermectin include:

• Monthly prevention of heartworm infection
• Treatment of ear mite situations
• Clearing heartworm larvae in active heartworm infection
• Treatment of sarcoptic, notoedric, or demodectic mange.

Ivermectin is given monthly for heartworm prevention, daily or every other day for demodectic mange treatment, and every week or couple of weeks for most mites.

If a dose of heartworm preventive is accidentally forgotten, it is important to give it as soon as it is remembered. If the dose is more than two weeks late, heartworm protection has been compromised. 

If a dose is accidentally skipped in one of the other parasite protocols, simply pick it up with the next dose. Do not double up.

It should be noted that doses of ivermectin used for the prevention and treatment of heartworm disease are approximately 50 times lower than doses used for other parasites, a fact that has allowed for FDA approval of ivermectin products for the prevention of heartworm but not necessarily for other small animal anti-parasite uses. (Acarexx® for ear mite treatment is FDA approved, and assorted heartworm preventives are FDA approved, but other small animal uses of ivermectin are off-label.)

Side Effects

Side effects are not a concern with the extremely low doses used in commercially marketed heartworm preventives.

When higher doses are used, as in mange or mite treatment, problems can arise if the patient has an undiagnosed P-glycoprotein gene mutation. In normal patients, the P-glycoprotein is involved in keeping drugs out of certain tissues and is important in keeping ivermectin out of the patient's nervous system. A healthy P-glycoprotein system is what allows ivermectin to be safe for mammals, even in very high doses. Unfortunately, collie-related breeds (and some other breeds) commonly have a mutation in the genes that make P-glycoprotein. (This has been called the MDR1- mutation but has recently been renamed the "ABCB1-1" mutation.) This mutation can create dangerous ivermectin sensitivity. Normal commercial heartworm preventives do not use high enough doses for this issue to come into play; it is usually during treatment of demodectic or sarcoptic mange when the issue comes up.

Because of the prevalence of the P-glycoprotein gene mutation, genetic testing is recommended for dogs of the following breeds: collie, Shetland sheepdog, Australian shepherd, Old English sheepdog, long-haired whippet, and possibly other herding breeds. This is a DNA test using an oral swab. Test kits, which use a simple cheek swab, can be ordered directly from the Washington State University Veterinary School.

Without a DNA test, a low-dose test protocol can be used. In this protocol, a low dose is started, and the patient is observed for dilated pupils or drunken gait. If no problems are seen, the dose can be raised to the therapeutic dose with less concern.

Heartworm preventive doses are so low that side effects are not produced even in ivermectin-sensitive individuals.

Side effects of concern are dilated pupils and drunken gait that can progress to respiratory paralysis and death if medication is not withdrawn and supportive care is not initiated.

Interactions with Other Drugs

Ivermectin should not be used in combination with spinosad (Comfortis or Trifexis) as the potential for ivermectin side effects will be increased. Again, the small doses of ivermectin used in heartworm prevention are not included in this cautionary statement; this only applies to the high-dose protocols used to treat skin parasites.

Concerns and Cautions

Ivermectin use in pregnancy and lactation is not felt to be a problem.

Ivermectin has an extremely bitter taste. Some animals may object.

Again, the breeds classically considered at high risk for ivermectin toxicity are collies, Shetland sheepdogs, Australian shepherds, merle-colored Pomeranians, and Old English sheepdogs. This list is not exhaustive, and many consider any dog with white feet to be potentially affected by the MDR1 mutation. Not every individual dog from these breeds is sensitive to ivermectin. It is possible to test an individual using a low dose of ivermectin. These breeds are not at risk for trouble when using the low-dose heartworm preventive products, but only when using the off-label skin parasite protocols.

Topical ivermectin for ears (Acarexx®) is FDA-approved for cats and kittens over four weeks of age.

Oral or injectable ivermectin is not recommended for patients under age six weeks.