Some Herding Dogs Sensitive to Horse Dewormers

Most horses' immediate reaction to being dewormed is to spit as much of the offending paste out as soon as possible after dosing. As well as being aggravating for owners, this habit can be dangerous if there's a herding breed dog (collie, Australian Shepherd, Shetland Sheepdog, or related dog) in the family--for some dogs, licking that blob of paste could be fatal.

For years, veterinarians and dog owners have known that some collies and related breeds can die when given a certain class of antiparasitic drug. The class of antiparasitic drugs, the avermectins, includes ivermectin, moxidectin, milbemycin, selamectin, and others. The avermectins are commonly used against parasites in animals and humans.

In 2001, researchers at Washington State University's College of Veterinary Medicine, discovered the reason herding breed dogs are extremely sensitive to the toxic effects of avermectins. These dogs have a defect in a gene that encodes a protein pump called P-glycoprotein. P-glycoprotein is thought to have developed to protect the body from environmental toxins.

To date, over 10,000 dogs have been tested for this defect, called the MDR1 mutation. A gene known as the multi-drug resistance gene, MDR1, normally codes for the production of P-glycoprotein. Katrina Mealey, DVM, PhD, Dipl. ACVIM, the primary investigator, found that in susceptible dogs MDR1 contains a "deletion mutation"--parts of the genetic code are missing. As a result, synthesis of P-glycoprotein is unsuccessful.

At improper doses, avermectins are powerful neurotoxins. In normal dogs, P-glycoprotein pumps this toxin from the brain. In dogs with a deletion mutation in the MDR1 gene, P-glycoprotein is no longer available to protect the brain. Ingestion of the wrong dose of any of these avemectin compounds can cause neurological toxicity in a dog with the MDR1 mutation. Signs of avermectin toxicity include unsteadiness, dilated pupils, excessive salivation, and can include coma and death.

Another potential route of exposure for dogs is by ingesting feces of horses or other livestock that have recently been treated with one of the avermectins. Because these drugs are designed to act in the intestinal tract, they achieve high concentrations there, and subsequently relatively high concentrations in feces.

Mealey found that about 75% of collies have the genetic anomaly that stops production of P- glycoprotein. Other breeds that might be affected include Australian Shepherds, Border Collies, English and German Shepherds, and more. See a complete list of affected breeds.  

The MDR1 test is available to the public. Testing kits, instructions, and information about what breeds of dogs are affected and what drugs to worry about are available on the Washington State University Veterinary Clinical Pharmacology Laboratory Web site.  

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