Part of the official UK Kennel Club testing scheme in Australian Shepherd, Border Collie, Collie (Rough) Collie (Smooth), Old English Sheepdog and Shetland Sheepdog
In certain breeds a mutation on the ABCB1 gene, which encodes the MDR1 protein (which stands for Multi Drug Resistance 1) can cause animals that carry the mutation to be particularly sensitive to certain drugs. The variant was first detected in a subpopulation of dogs that were highly sensitive to Ivermectin-induced neurotoxicity. The variant on the ABCB1 gene results in the brain being unable to efficiently pump some drugs out, causing a toxic build-up of these drugs in the brain. Dogs subsequently experience and range of symptoms from vomiting and diarrhea to lethargy, seizures, or coma.
A whole range of drugs are pumped out of the brain by the MDR1 protein pump, including antiparasitics, immunosuppressants, antidiarrheals, chemotherapy, antibiotics, antihistamines and more. More details on the drugs that can be dangerous for dogs can be found here. Your vet should be aware if your dog has one or two copies of the defective ABCB1 gene, as it may affect future treatment options.
This genetic defect is known to occur in up to 75% of some dog breed populations, and is particularly common in Rough Collies, Smooth Collies, Shetland Sheepdogs, Australian Shepherds, Miniature Australian Shepherds, Silken Windhounds and White Swiss Shepherds.