MDR1 Gene Mutation in Dogs 2025: Vet Reviewed Guide to Drug Sensitivity, Testing & Safe Care 🧬🐶

By Dr. Duncan Houston BVSc

The MDR1 mutation (also known as ABCB1 mutation) affects drug transport in dogs, leading to heightened sensitivity—sometimes fatal—to common medications. This guide explains what owners need to know: from genetics and breed risks to testing, altered drug protocols, and proactive care strategies using Ask A Vet.

1. 🧬 What Is the MDR1 (ABCB1) Gene?

It encodes a protein called P-glycoprotein (P-gp), which pumps drugs and toxins out of the brain and other tissues at barrier sites. A 4-base-pair deletion (nt230(del4)) leads to truncated P‑gp and drug buildup in sensitive tissues.

2. 🐕 Who’s at Risk?

• Collies: ≈ 70 % affected – the origin breed.
• Australian Shepherds: ≈ 50 %.
• Shetland Sheepdogs: ~10 %.
• Other herding breeds: Border Collie (~<5 %), Old English Sheepdog, German Shepherd, other collie-line breeds.

Breeds with white feet or herding background often carry the gene—remember the adage: “white feet don’t treat”.

3. ⚠️ Why It Matters: Drug Toxicity Explained

With impaired P‑gp, drugs accumulate in the brain, triggering neurotoxicity even at low doses. Clinical signs include:

• Vomiting, drooling
• Ataxia (incoordination), tremors
• Seizures, pupil dilation
• Lethargy, coma, respiratory arrest, death.

🔍 Responsible drug use:

• Macrocyclic lactones (ivermectin, moxidectin): fatal in high doses for affected dogs; low-dose heartworm preventive is safe.
• Antidiarrheal: loperamide (Imodium) – avoid.
• Sedatives/opioids: acepromazine, butorphanol—dose reduction advised.
• Antibiotics & chemotherapy: erythromycin, doxorubicin, vincristine, vinblastine – adjust or avoid.
• Other medications: cyclosporine, digoxin, fentanyl, maropitant, ondansetron, etc. – caution advised.

4. 👨👩👧👦 Genetics & Inheritance

MDR1 is autosomal, incomplete dominant:

Homozygous affected dogs have the highest sensitivity; heterozygotes can also exhibit mild toxicity.

5. 🧪 Genetic Testing

• Simple cheek swab or blood test—widely available (e.g., Washington State University pharmacogenetics lab).
• Highly accurate results: N/N, M/N, or M/M reveal status.
• Recommended for all herding breeds—essential before sedation, chemo, or parasite control.

6. 🛡️ Safe Care Guidelines for MDR1-Positive Dogs

• Veterinary precautions:

• Flag genetic status in records; use alternative medications or reduced dosing.
• Monitor closely for toxicity signs when administering sedatives or anthelmintics.
• Prepare emergency treatment—IV hydration, anticonvulsants—for accidental overdose.

• Home awareness:

• Inform all caregivers (kennels, groomers, sitters) of MDR1 status.
• Avoid OTC medications without vet approval.
• Carry a note/ID tag: “MDR1 Sensitive – Avoid ivermectin/loperamide.”

• Vet check routines:

• Confirm MDR1 before anesthesia, parasite treatment, or chemotherapy.
• Use alternative safe drugs (e.g., milbemycin at low dose OK, maropitant instead of ondansetron, dexmedetomidine instead of acepromazine).
• Healthy lifestyle: nutrition, enrichment,

7. 📚 Prognosis & Quality of Life

MDR1-positive dogs live full, healthy lives with care. Avoiding harmful drugs is key. Once status is known, use tailored medication protocols and watchful monitoring. **Ask A Vet App** is vital for guidance on safe medication choices.

8. 📤 Summary for Dog Parents

• Test before treating—cheek swab reveals MDR1 status.
• MDR1 dogs need special drug protocols: avoid or reduce dosing of risky medications.
• Always alert vets, caregivers, and pharmacies about MDR1 sensitivity.
  
• Keep Ask A Vet on hand for fast decisions and peace of mind.