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.