Multi-Drug Sensitivity (MDR1) – Collie

In certain breeds a mutation on the MDR1 gene (which stands for Multi Drug Resistance 1) makes affected animals sensitive to certain drugs. The first drug that this defect was found to be present for was Ivermectin, used to treat mange and prevent heartworm. Affected dogs suffer seizures when given this drug. It has since been found that the mutation on the MDR1 gene means that the brain is not able to efficiently pump some drugs out of its protected environment the way normal brain vessels do – hence these drugs can enter and build up in the brain tissue, and cause toxic effects such as seizures.

A range of drugs are usually pumped out of the brain by the protein pump that the MDR1 gene is responsible for, and so dogs carrying the defective (“mutant”) gene are sensitive to a whole range of drugs. Dogs carrying two copies of the mutant gene are more sensitive to these drugs than those with one copy of the gene. For more details on the drugs involved in this disease, information can be found at VCPL/drugs.aspx

Your vet should be aware if your dog is carrying an affected MDR1 gene, or 2 copies of the gene, as the amount of these drugs given needs to be reduced to avoid toxic effects, or alternative drugs used if available. This genetic defect is known to occur very commonly in the Collie, with between 55 – 75% of the breed carrying at least one copy of the mutant MDR1 gene. A DNA test is available to determine if your dog is carrying abnormal MDR1 gene/s or not.