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Multidrug Sensitivity in Herding Breeds: MDR1 Gene Mutation
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
Butorphanol
Doramectin
Doxorubicin
Erythromycin
Ivermectin
Loperamide
Milbemycin
Moxidectin
Paclitaxel
Selamectin
Vinblastine
Vincristine
 
Breeds affected by the MDR1 mutation (frequency %)
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 %
 
Testing a Dog for the MDR1 Mutation is Easy:
Order a testing kit by contacting the Veterinary Clinical Pharmacology Laboratory at Washington State University: www.vcpl.vetmed.wsu.edu or by phone 509-335-3745