09/03/2026
GREAT info from Carol Price of Collieology.
Read and share - for the health of your dog.
The more you know...
BORDER COLLIE HEALTH: Today our breed expert, Carol Price, looks at THE MDR1 GENE DEFECT in Border collies, and why all owners should know about it
Today I wanted to write about the MDR1 (Multi Drug Resistance) gene defect in Border collies, as it can make both dogs who are affected by it, or carriers of it, far more vulnerable to different toxins entering their brains. And that includes some more regularly prescribed veterinary medications. In the worst case scenario the effect can be fatal. A fuller list of these medications can be found at the end of this feature.
Although Rough collies and Australian shepherds are most susceptible to this defect, as well as some other herding breeds, it can occur in Border collies too. So it is definitely worth knowing about.
WHAT THE MDR1 GENE DOES
You may have sometimes heard of people getting their dogs tested for the "MDR1 gene", when what they really mean is the MDR1 gene DEFECT. For, when acting normally, the MDR1 gene has the effect of keeping any kind of toxin pumped out of cells in the body and, most crucially, at the blood-brain barrier, to stop them getting to the brain. However, when the gene is defective it cannot perform this function so well, and thus toxins can more readily access a dog's brain, leading to some more severe neurological symptoms and even potentially death.
If dogs only have one copy of this genetic defect, the effects may be milder, but if they have inherited the defect from both parents they can be far more dangerous. Again, as listed at the end of this feature, drugs known to trigger these more dangerous toxic reactions in affected Border collies include parasite treatments, like Ivermectin, plus a range of different sedatives, heart drugs, anti-diarrhoeal agents, pain killers and chemo drugs for cancer treatment.
SCREENING
One of the main problems in the past with this genetic defect in collies, is that neither owners nor vets could know if a dog had it, before giving them a drug that then had such a devastating reaction in them. However today there is a simple DNA screening test available that highlights if a dog has inherited either one copy of the mutation from a parent dog, or the same mutation from both parents; the latter making them more severely affected. Most good Border collie breeders now routinely do this test on their dogs, along with a bundle of other breed related inherited conditions you can now DNA screen for.
Other owners whose collies’ parents were not screened for it, or whose status for this defect is less known, may want to do this test just to put their mind at rest before giving their dog any more potentially risky treatments. Although the chance of your collie having this genetic defect may be relatively low, compared to other breeds, sometimes you just cannot put a price on peace of mind.
Meanwhile, should it further help other owners, breeders or vets, here is a fuller list of medications potentially more harmful to dogs with the MDR1 defect:
1. DRUGS THAT SHOULD NOT BE USED IN DOGS WITH THE MDR1 DEFECT: Ivermectin substances ("Anti parasites") - (Diapec®, Ecomectin®, Equimax®, Eqvalan®, Ivomec®, Noromectin®, Paramectin®, Qualimec®, Sumex®, Virbamec®
Doramectine substances "Anti parasites": (Dectomax® )
Loperamide substances "Anti-diarrhoeal ": (Imodium®)
Moxidectine substances "Anti Parasites" (Cydectin®, Equest®)
2. DRUGS THAT SHOULD ONLY BE USED UNDER STRICT VETERINARY SUPERVISION
Cytostatics "Chemotherapy drugs": (Vinblastine, Vincristine, Doxorubicine, Paclitaxel, Docetaxel, Methotrexat, Vincristine)
Immunosuppressive drug: (Cyclosporine A)
Heart glycosides: (Digoxine, Methyldigoxine)
Opioids: (Morphium)
Antiarrhythmics (Heart meds) : (Verapamil, Diltiazem, Chinidine)
Antiemetics (Anti-sickness drugs) (Ondansetron, Domperidon, Metoclopramide )
Antibiotics (Sparfloxacin, Grepafloxacin, Erythromycin)
Antihistamin (Ebastin)
Glucocorticoid (Dexamethason)
Acepromazine (tranquiliser and pre-anaesthetic agent) *
Butorphanol "analgesic and pre-anaesthetic agent" *
OTHER DRUGS: Etoposide, Mitoxantrone, Ondansetron, Paclitaxel, Rifampicin
3. DRUGS THAT CAN ONLY BE USED IN THE PERMITTED APPLICATION FORM AND DOSE
(Anti-parasite treatments): Selamectin (Stronghold®), Milbemax® and Advocate®.
* In dogs with the MDR1 mutation, Acepromazine and Butorphanol tend to cause more profound and prolonged sedation in dogs. It is recommended to reduce the dose by 25% in dogs carrying one copy of the faulty gene and by 30-50% in dogs carrying two faulty copies – i.e. one from each parent. Also be aware there may be other problem drugs or treatments for dogs with this gene defect yet to be identified.
©Carol Price/Collieology 2026
