Martha Love had a lot to be thankful for Thanksgiving 2009. Her 1-year-old Miniature Australian Shepherd, “Cash,” walked out of the WSU Veterinary Teaching Hospital on November 17 after being hospitalized for 26 days. Eight of those days Cash was in a coma on a mechanical breathing ventilator.

Martha first noticed Cash wasn’t feeling well after they visited several horse stables.  On the evening of October 21 Cash began staggering and quickly lapsed into a coma. He was first rushed to Pets ER for supportive care and the next morning presented to Dr. Colin Bonnett, a WSU alumnus in Montana, who placed a breathing tube because Cash was having difficulty breathing. Dr. Bonnett continued supportive care, but immediately referred him to the WSU Veterinary Teaching Hospital. Cash was flown by chartered plane to WSU. Kimber Feher, a veterinary technician, volunteered her time to accompany Cash and breathed for him using a manual breathing ventilator until he arrived safely in Pullman.

Because of his symptoms, the veterinary team suspected that Cash had eaten ivermectin, a commonly used drug likely given to the horses at one of the stables. Ivermectin, which is used to remove worms or prevent heartworm disease, is safe at prescribed doses for many animals, including dogs. Large doses of ivermectin can be toxic or even deadly. But some dog breeds have a genetic mutation that makes them more susceptible to ivermectin toxicity, even at prescribed doses.

Dr. Katrina Mealey at Washington State University discovered the MDR-1 gene mutation that causes increased sensitivity to certain drugs, including ivermectin. The MDR-1 gene helps create a protein called P-glycoprotein that pumps drugs and toxins out of the brain. Without this protein, high levels of certain drugs can accumulate in the brain impairing function, leading to coma and an inability to breathe that can result in death. Dogs with the MDR-1 gene mutation do not produce P-glycoprotein or do not produce enough of the protein to effectively pump particular drugs, which leads to toxicity. Ivermectin toxicity causes the nervous system to shut down.

Tests confirmed that Cash had high levels of ivermectin in his system and had the MDR-1 mutation, which made him more susceptible to poisoning. The MDR-1 mutation is more common in some breeds of dogs including the Collie, Australian shepherd, Miniature Australian shepherd, Shetland sheepdog, Old English sheepdog, and Long-haired whippet. Approximately 50 percent of Australian Shepherds and 70 percent of Collies have this mutation.

Cash received around the clock intensive care from fourth year WSU veterinary students, ICU veterinary technicians, and Dr. Heather Wright, a neurology resident. While recovering Cash remained on a ventilator to breathe, he was given nursing care to prevent bedsores and infection, and was fed intravenously. His team of doctors included Dr. Annie Chen-Allen, a board certified neurologist; Dr. Katrina Mealey, a board certified internist and pharmacologist; and Dr. Patricia Talcott, a board certified toxicologist.

Cash gradually emerged from his coma, began eating on his own, playing with his toys, and walking without assistance. Now fully recovered, Cash is home with his family and he is not expected to have any lasting effects from the ivermectin.

More information about the MDR 1 mutation including how to test your dog.