Horse Owners Take Note: Ivermectin Can Be Toxic to Dogs

Ivermectin, a powerful deworming medication used widely in veterinary medicine, treats a number of parasites in dogs, including ear and hair mites and internal parasites. This medication also prevents heartworm infections caused by Dirofilaria immitis. Specifically, ivermectin kills heartworm larvae (microfilaria) in the bloodstream of infected dogs.

While widely deemed safe, ivermectin can be toxic to dogs, causing neurological abnormalities such as depression; drooling, anorexia, and vomiting; dilation of the pupils and blindness; inability to stand or control voluntary movement; disorientation, tremors, and seizures; and coma.

Dogs most commonly present with ivermectin toxicosis either after accidental ingestion of large doses (e.g., ingesting the feces of large animals such as horses recently dewormed with ivermectin) or if they have a genetic mutation making them highly susceptible to the effects of this drug.

A genetic mutation in the MDR1 gene, also referred to as the ABCB1 gene, results in the production of a dysfunctional protein called p-glycoprotein. Located widely throughout the body, p-glycoprotein plays a key role in pumping drugs out of cells. The p-glycoprotein can also be found in the blood-brain barrier, functioning to keep certain medications such as ivermectin out of the central nervous system. If a dog has a mutated form of p-glycoprotein due to the genetic mutation, ivermectin can pass readily from the bloodstream into the central nervous system, causing neurological abnormalities.

First recognized in collies in the 1980s, researchers soon found that a variety of other herding-breed dogs also possessed this genetic mutation. Such breeds include the long-haired whippet; Australian, English, and German shepherds; Shetland and Old English sheepdogs, and border collies.

It is important to appreciate that only higher doses of ivermectin, such as those used to treat mange or ear mites, are typically toxic to dogs with a mutation in the MDR1/ABCB1 gene. All heartworm preventatives containing ivermectin produced in the United States and approved by the Food and Drug Administration are safe for all dogs, including those with the mutation. In other words, having a genetic mutation in the MDR1/ABCB1 gene does not mean a dog should be denied heartworm preventatives.

According to a recent study conducted by the American Heartworm Society, heartworm affects tens of thousands of dogs annually, and the number of dogs that test positive to a standard heartworm antigen test is on the rise. Specifically, of the almost seven million heartworm tests performed in 2013, over 77,500 were positive. By 2016, nearly 119,000 of 9.2 million heartworm tests were positive, reflecting a 15.28% increase in the number of positive tests. Thus, heartworm preventatives are clearly indicated for all dogs.

Horse owners should use care to store and dispose of equine deworming products to avoid accidental ingestion by dogs.

If a dog begins to show any neurologic clinical signs, seek veterinary care immediately. When diagnosed and treated promptly, dogs suffering from ivermectin toxicosis can make a full recovery.

Lastly, do not rely on the physical appearance of a dog, especially mixed-breed dogs, to rule in or out the MDR1/ABCB1 mutation. The only way of knowing whether or not a dog has the mutation is by genetic testing. Ask your veterinarian for information on genetic testing and interpreting the results.