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MDR1 stands for Multi Drug resistance 1. And it is important that Owners know the status of their dogs and not just breeders. Details of UK bases labs that carry out the testing are on the DNA Page.

It is a genetic mutation which affects the way the drugs are transported through the dog. In dogs which are effected by MDR1 the drugs travel to the brain, but then fail to be transported out again. This leads to a toxic build-up of the drug in the brain, and ultimately neurological problems including seizers, and in severe cases of toxicity possibly even death.  MDR1 doesn’t just affect Australian Shepherds, it mainly affects breeds in the pastoral groups, but it has been known to affect other breeds as well.

Dogs who have one or two copies of the gene will show sensitivities to drugs. Two copies and the dog is classed as Affected. One copy is is often written as a 'Carrier'. However these dogs show a level of sensitivity although not as severe as an Affected dog.


MDR1 is easily identifiable through a simple DNA test. This is usually done by taking some mouth swabs from the dog.

MDR1 is a simple recessive trait so the status of the dog can often be worked out if the parents status is known.

A dog will inherit one copy of the gene from each parent and the overall status of the dog is known by combining the 2 copies of the gene. A dog affected by MDR1 will have 2 bad copies of the gene. A carrier has one bad copy and one good copy. A clear dog has no bad copies of the gene.

MDR1 is a dominant gene which means that dogs which are carriers of MDR1 still show sensitivity to the same drugs an affected dog will, but they may not be as severe.


MDR1 is not a genetic problem that will not affect the day to day living of your dog. What is does mean is that if the dog ever receives veterinary attention you need to be careful what drugs your dog is being given.

If the status of your dog is unknown, or there is a possibility they could have inherited copies of the gene from their parents it is best to play safe and treat them as an affected.

For many years it has often been referred to that ‘collie type’ dogs are not to be given Ivermectin. This is effectively MDR1, however there are more drugs than Ivermectin that cause the problems.

It is advisable if your dog is affected/carrier/or status unknown for MDR1 that you give a list of problem drugs to your vet to keep on file. This means if there was ever an emergency you don’t have to rush around trying to find a list of the drugs.

There are a number of lists, available that list the problem drugs. One such list is the Washington State University, who carry out research into MDR1

Please note that even if you have a dog that is clear for MDR1 it is still worth, taking caution when giving your Aussie medications. There is a possibility that there may be another gene involved as well.


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