MDR1 gene & Collies

MDR1 gene & Collies

MDR1 (or Multi-Drug Resistant protein gene)

The MDR1 gene protects the brain by ensuring that any harmful chemical are transported AWAY from the brain.  In some dogs who are members of the herding varieties such as Collies and Australian Shepherds, a mutation in this gene causes extreme reactions to various drugs including Ivermectin (found in some heart worm medication), Loperamide (found in things like Imodium for diarrhoea) and several others including pesticidal treatments.  The mutation causes the gene to be defective and makes it difficult for the dog to remove these drugs from the brain, leading to a build up of toxins.  As a result, the dog may experience problems of a neurological nature such as seizures, incoordination, or even death.

All our characteristics are derived by our genes – a copy of which we get from our mother and one from our father.  Dogs that have two copies of the mutation will display a sensitivity to these types of drugs.  It is also worth noting that they will pass on one copy of the mutation to any potential offspring so it is important that dogs are tested before entering a breeding programme.

Dogs that only have one copy of the genetic mutation may still react to these drugs if they are administered at high doses and they also have a 50% chance of passing on the mutation to their puppies too.

Breeds affected by the MDR1 mutation (frequency %)

Breed Approximate Frequency
Australian Shepherd 50%
Australian Shepherd, Mini 50%
Border Collie < 5%
Rough 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 %

The drugs fall into 3 categories – class A, B and C:


DO NOT USE in dogs with MDR1 defect.

An affected dog (-/-) carries two MDR1 gene mutations, having received one from each of its parents. It will also  pass on a mutant MDR1 gene to its offspring.

MDR1-affected dogs are likely to experience drug toxicity following normal doses of the drugs listed here:

Anti-Parasitic drugs:

Ivermectine substances: Diapec®, Ecomectin®, Equimax®,Eqvalan®, Ivomec®, Noromectin®, Paramectin®, Qualimec®, Sumex® & Virbamec®

Doramectine substances: Dectomax®

Moxidectine substances: Cydectin® & Equest®

Loperamide substances (anti-diarrhoea): Immodium®

Metronidazole (Flagyl ® – general antibiotic


Toxic reactions have been known to occur so only use under the close supervision of your vet

Cancer treatments (Cytostatics): Vinblastine, Doxorubicine, Paclitaxel, Docetaxel, Methotrexat & Vincristine

Glucocortisoids (steroids commonly used to treat auto-immune diseases): Dexamethason

Immuno-suppressants: Cyclosporine A

Heart glycosides: Digoxine & Methyldigoxine

Antiarrhythmics (heart problems): Verapamil, Diltiazem & Chinidine

Pain control: Morphine & Butorphenol

Anti-emetics (sickness/vomiting): Ondansetron,  Domperidon andMetoclopramide

Antibiotics: Sparfloxacin, Grepafloxicin & Erythromycin

Antihistamines: Ebastin, (although safe for most dogs, Piriton should be queried when dealing with MDR1 positive dogs)

Tranquillisers & pre-anaesthetic agents: Aceptomazine (ACP) & Butorphenol

Other drugs: Etoposide, Mitoxantrone, Ondanestron, Paclitaxel, Rifampicin


Can be used safely providing the correct dosage is given.

Stronghold®, Advocate® & Milbemax® can be used only in the recommended application and dosage.

The importance of knowing your Collie’s MDR1 status cannot be over-emphasised, as you never know when he or she may require surgery and/or drug treatment. If your dog is known to be affected, you will at least be in a position to inform your vet of the dangers of certain drugs, by printing off the information.

There are several laboratories offering DNA tests for MDR1 – Laboklin Laboratories of Manchester (UK branch of the Laboklin Company of Bad Kissingen, Germany).

UK lab:
Germany lab:

Animal Genetics is another lab that does this test. There are clinics with reduced rates throughout the year. You can find out more by contacting Pastoral Breeds Health Foundation

The test is carried out using simple buccal (cheek) swabs, which you can easily do yourself.

Testing is also available in the US where you can simply send for a kit to take a cheek swab and sent it off to the lab: – I understand that the cost is in the region of $55