Ivermectin

(for veterinary information only)

BRAND NAMES: IVOMEC, HEARTGARD & HEARTGARD PLUS,
                         IVERHART PLUS & IVERHART MAX, PET TRUST,
                         TRI-HEART PLUS & ACAREXX

AVAILABLE IN TABLETS / CHEWABLES    
FOR HEARTWORM PREVENTION, TOPICAL
SOLUTION FOR EAR MITE TREATMENT,    
OR AS ORAL OR INJECTABLE SOLUTION   
FOR OTHER PARASITE PROBLEMS         

HOW THIS MEDICATION IS USED

In the mid-1980's, ivermectin was introduced as probably the most broad-spectrum anti-parasite medication ever. It 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 against larval heartworms (the "microfilariae" that circulate in the blood) but not against 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 would include:

It should be noted that doses of ivermectin used for 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 unknown 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 in the 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. Genetic testing can be done . This is a DNA test using an oral swab. Test kits can be ordered directly from the Washington State University Veterinary School via

www.vetmed.wsu.edu/depts-vcpl

Ivermectin sensitivity is not always predictable so another method that can be employed is the use of a test dose. 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.

Side effects of concern are: dilated pupils and drunken gait which 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 conjunction with spinosad (Comfortis® or Trifexis®) as the potential for ivermectin side effects will be increased. Again, the very 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 considered at high risk for ivermectin toxicity are Collies, Shetland sheepdogs, Australian shepherds, Merle colored Pomeranians and Old English sheepdogs. 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; only when using the off-label skin parasite protocols.

Topical ivermectin for ears (Acarexx®) is FDA approved for cats and kittens over 4 weeks of age. Oral or injectable ivermectin is not recommended for patients under age 6 weeks.

WHILE WE RECOGNIZE THAT IT IS POSSIBLE TO BUY
LARGE ANIMAL FORMULATIONS OF IVERMECTIN (SUCH AS IVOMEC®)
THROUGH CATALOGS, WE STRONGLY DISCOURAGE THIS PRACTICE
BECAUSE OF THE POTENTIAL TO EASILY GIVE A TOXIC DOSE
IF THE PRODUCT IS INCORRECTLY USED.

LARGE ANIMAL FORMULATIONS ARE MUCH MORE CONCENTRATED
AND IT IS EXTREMELY DIFFICULT TO MEASURE A DOSE APPROPRIATELY
FOR A SMALL ANIMAL ESPECIALLY IF ONE IS ATTEMPTING
TO MEASURE A DOSE APPROPRIATE FOR HEARTWORM PREVENTION.
THERE IS TREMENDOUS POTENTIAL FOR SERIOUS SIDE EFFECTS
IF IVERMECTIN IS INAPPROPRIATELY DOSED.

Page last updated: 4/17/2015