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