Protect Your Dog From Adverse Drug Reactions to Ivermectin

Uses for Ivermectin

Ivermectin belongs to a class of drugs known as antihelmintics. It works by paralyzing and killing parasites. Ivermectin was introduced in the mid 1980’s as a broad-spectrum anti-parasite medication. It is effective against most common intestinal worms (except tapeworms), most mites, and some lice. It is also effective against larval heartworms (the “microfilariae” that circulate in the blood) but not against adult heartworms (that live in the heart and pulmonary arteries). For further information on heartworm and treatment options refer to our article Heartworm: How one mosquito bite can kill your dog.

It is available in tablets and chewables for heartworm prevention, topical solution for ear mite treatment and as an oral or injectible solution for other parasite problems, such as mange. Ivermectin products for heartworm treatment go by tradenames: Stromectol, Ivermec, Heartgard and Heartgard Plus by Merial, Iverhart Plus and Iverhart MAX by Virbac and Tri-Heart Plus by Schering-Plough as well as other generic versions. For prevention of ear mites Acarexx is prescribed.

The most common uses in dogs for ivermectin would 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

Doses of ivermectin used for prevention and treatment of heartworm disease are approximately 50 times lower than doses used for other parasites, which is why it has been FDA approved. Other anti-parasite uses require higher dosages of the drug and have not been FDA approved and therefore are “off-label”. Ivermectin is used in higher dosages to treat both demodectic and sarcoptic mange in dogs.

Problems may arise when higher doses of ivermectin are employed. Side effects usually occur in animals with genetic sensitivity to ivermectin. Highly sensitive dogs are at low risk for trouble when using the low dose heartworm preventive products. It is when using the off-label skin parasite protocols that adverse drug reactions can result.

Side Effects and Adverse Reactions

Side effects of urgent concern are: dilated pupils and drunken gait which can progress to respiratory paralysis and death if medication is not withdrawn and supportive care is not initiated.
More commonly seen side effects are:

  • depression
  • lethargy
  • vomiting
  • diarrhea
  • lack of appetite
  • incoordination
  • seizures
  • excessive salivation

Ivermectin should not be used in high dosages with valium or tranquilizers related to valium, amitraz dips, collars or topical treatments, or spinosad (Comfortis) due to the potential of adverse drug interactions.

Dogs with Genetic Sensitivity to Ivermectin

Dog breeds with genetic sensitivity to ivermectin include Collies, Shetland sheepdogs, Australian shepherds, Merle colored Pomeranians and Old English sheepdogs. Not every individual dog from these breeds is sensitive to ivermectin, and other individual dogs from other breeds may be prone to sensitivity. This is because it is a genetic mutation that causes the sensitivity.

Studies have shown that dogs ivermectin sensitivity have been found to have a mutant gene (within the MDR1 gene) for what is called the “P-glycoprotein.” Approximately 35% of Collies have a genetic mutation creating a non-functional 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. 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.

Very low test doses are often recommended at the start of a treatment to identify thes individuals regardless of their breed. Alternatively, a blood test is now available to test for genetic sensitivity. This genetic test for P-glycoprotein mutation will identify ivermectin sensitive dogs. It is a DNA test using an oral swab. Test kits can be ordered directly from the Washington State University Veterinary School and other DNA animal testing laboratories.

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