MDR1 Gene Mutation: The MDR1 gene encodes P-glycoprotein, a drug transport pump that plays an important role in limiting drug absorption and distribution (particularly to the brain), as well as enhancing the excretion of many drugs used in dogs. Some dogs, particularly herding breeds, have a mutation in the MDR1 gene, leaving them defective in their ability to limit drug absorption and distribution. These dogs also have delayed excretion of drugs that are normally transported by P-glycoprotein, making them susceptible to severe drug toxicity.

Drugs Affected by the MDR1 Gene Mutation: Acepromazine, Apomorphine, Butorphanol, Doramectin, Doxorubicin, Erythromycin, Ivermectin, Loperamide, Milbemycin, Moxidectin, Paclitaxel, Selamectin, Vinblastine, Vincristine, Vinorelbine

Breeds Affected by the MDR1 Mutation

Breed Approximate Frequency
Australian Shepherd 50%
Australian Shepherd, Mini 50%
Border Collie < 5%
Collie 70%
English Shepherd 15%
German Shepherd 10%
Herding Breed Cross 10%
Long-haired Whippet 65%
McNab 30%
Mixed Breed 5%
Old English Sheepdog 5%
Shetland Sheepdog 15%
Silken Windhound 30%