At least six dog breeds, including collies, are sensitive to multiple drugs because of a genotype or mutation of the MDR1 gene, and new research suggests that the genotype also may lead to adrenal gland disorders, said Katrina Mealey, DVM, PhD, ACVIM, director of the Veterinary Clinical Pharmacology Laboratory at Washington State University (WSU) College of Veterinary Medicine. “We hypothesize that adrenal gland problems may also occur in these dogs under certain situations, but we are still studying that,” Mealey said, adding that researchers are “trying to determine if dogs with the mutation have altered endocrine function … i.e., if the mutation contributes to disease rather than simply to drug sensitivity.”

Sensitivities, such as neurotoxicity, to at least three anticancer drugs, ivermectin as well as other prescriptions and some over-the-counter antidiarrheal medications have been proven in collies, Australian shepherds, long-haired whippets and Shelties by researchers from around the world. Studies have been published in several industry journals, including the August 2004 issue of the Journal of Veterinary Pharmacology and Therapeutics. Researchers also tested herding dogs in France and Australia and discovered the mutation.

In 2001, Mealey discovered the mutation in the MDR1 gene that codes for P-glycoprotein, an important part of the blood-brain barrier. Dogs with the mutation experience sensitivity to ivermectin, an antiparasitic agent, while dogs without it can tolerate the drug, Mealey explained. She helped to develop an assay to determine the genotype and has sold hundreds of tests to veterinarians and dog owners since it became available May 15, 2004. The test, which varies from $51 to $60 if ordered in bulk, requires veterinarians to take a swab from a dog’s cheek and send it into the WSU lab for evaluation.