Ivermectin

(for veterinary information only)

BRAND NAMES: IVOMEC, HEARTGARD, IVERHART PLUS,
                                 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.

Problems may arise when higher doses, such as those used against skin mites, are employed but even then, side effects generally do not occur with any anti-mange doses of ivermectin except in animals with genetic sensitivity. Such individuals are usually Collies, Shetland sheepdogs, Australian shepherds, and Old English sheepdogs, though some individual animals that are not members of these sensitive breeds may also be prone to side effects. Very low test doses are often recommended to identify thes individuals regardless of their breed. Alternatively, a blood test is available to test for genetic sensitivity (see below).

Collies with ivermectin sensitivity have been found to have a mutant gene for what is called the "P-glycoprotein." The P-glycoprotein has been studied largely because overexpression of this protein (i.e. having more of it than normal) results poor function of chemotherapy drugs in the treatment of cancer. The P-glycoprotein appears to be involved in keeping drugs out of certain body tissues. Having excess P-glycoprotein keeps chemotherapy drugs from reaching the tumor. When it comes to ivermectin sensitivity the problem is the opposite: mutant or non-functional P-glycoprotein leads to failure to keep certain drugs out of the central nervous system, allowing them access to sensitive tissue. Ivermectin side effects stem from ivermectin entering the central nervous system.

Approximately 35% of Collies have a genetic mutation creating a non-functional P-glycoprotein. This allows for ivermectin doses that would normally be blocked from the central nervous system to gain access to it. Other herding breeds as listed above also have a tendency to express this mutation. There is now a test for P-glycoprotein mutation so that ivermectin sensitive dogs can be identified. 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

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 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.

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: 3/17/2013