Meet the Researcher Who Discovered the MDR1 Gene

Herding dog owners everywhere should know their dogs may be sensitive to ivermectin, the powerful antiparasitic drug common in most worming medication. Now you can hear directly from the researcher who discovered the MDR1 gene mutation that causes this reaction. Washington State University’s Dr. Katrina Mealey, Ph.D., DVM discovered the gene literally by serendipity. While studying for her advanced degree, she encountered a journal article from the Netherlands. The article discussed treatment of a routine case of mites in laboratory mice, some of which had been engineered to remove the MDR1 gene. All of the mice who were missing the gene died from the treatment. This caused Mealey to begin research to locate the gene in dogs.

From the WSU Foundation: “Mealey had barely graduated from high school when the antiparastic drug ivermectin came on the market, which quickly became known as a super-weapon for animals and humans against parasites, such as mites, heartworms, and lice. In a small percentage of certain dog breeds, however, veterinarians found an ivermectin treatment could prove fatal. While the antiparastic could cure a poodle, it might kill a collie. Based on those results, veterinarians followed the guideline, ‘White feet; don’t treat.’ But, no one really understood the why behind the differing responses.”

Mealey also invented the cheek swab that tests whether dogs carry the MDR1 gene mutation or not.

“Seventy five percent of Collies have the MDR1 gene mutation that makes them susceptible to fatal reaction to antiparasitic drugs like ivermectin,” Mealey said. “Whereas, Shelties only have 10 percent of the population affected.”

Dominant mutation

The MDR1 mutation is a dominant trait, Mealey added. If a dog has one copy of the gene, it will have drug sensitivity. If it has two copies, it will have more severe sensitivity. Mealey’s ongoing research has indicated that dosage is the critical component to sensitivity. The low-level dosage of ivermectin contained in heartworm treatment is generally safe, she said, but the super high doses required to treat mange, for example, can be deadly. Mealey has found the mutation in Silken Windhounds and even a very small number of Boxers.

Early effect of popular sire syndrome

According to extensive research at UC-Davis, Mealey said they have concluded that the MDR1  mutation originated in a herding dog before specific breeds were established. “They theorize this was a dog particularly good at herding sheep, that became a popular sire. By the time breeds were established in the British Isles, they all carried this mutation,” Mealey said. Researchers have not identified the mutation in herding breeds that originated in other parts of the world. It is seen primarily in Collies, Shetland Sheepdogs, Old English Sheepdogs, Australian Shepherds, etc. For more information on Mealey’s work, check out: https://foundation.wsu.edu/2018/04/03/dogs-best-friend/