MDR1 Multi-Drug Resistance Testing for Dogs Explained

MDR1, or Multi-Drug Resistance 1, is a genetic condition that causes certain dogs to be extremely sensitive to a wide range of medications, including many commonly prescribed antibiotics and pain relievers. Dogs with this mutation have a defect in proteins that normally pump drugs out of the brain, leading to dangerous drug accumulation that can cause seizures, neurological problems, blindness, or even be fatal.

This inherited trait cannot be caught or developed outside genetic inheritance. For breeders and owners of at-risk breeds in the U.S., pre-breeding MDR1 testing is a vital step to make informed breeding decisions and keep sensitive dogs safe from harmful medications.

In this article, we’ll dive into MDR1: covering the breeds most at risk in the U.S., the genetics behind the mutation, how to get your dog tested, and why these tests are important for your dog’s health and safety.

What Is Multi-Drug Resistance (MDR1) in Dogs?

MDR1 commonly occurs in herding and working breeds. The mutation causes increased sensitivity to various drugs including antibiotics, painkillers, sedatives, and chemotherapy medications.

When a dog with MDR1 is given certain medications, their brain cannot effectively eliminate the drugs. This leads to toxic levels building up, resulting in severe symptoms such as seizures, nerve damage, blindness, and potentially death if untreated.

Which Dog Breeds Are Most At Risk for MDR1?

In the U.S., MDR1 is primarily found in herding dog breeds such as the Border Collie puppies, Old English Sheepdog puppies, Collie puppies (both rough and smooth), and German Shepherd puppies. Other breeds including Australian Shepherds, Shetland Sheepdogs, English Shepherds, Miniature American Shepherds, McNab Shepherds, and White Swiss Shepherds also carry the MDR1 mutation in the U.S.

Mixed-breed dogs with herding ancestry may have this mutation and should be considered for testing, especially before administering any drugs known to cause reactions. Your veterinarian can help determine if testing is needed based on your dog’s breed or mixed heritage.

The Genetics of MDR1: Understanding Heredity

MDR1 is inherited in an autosomal recessive manner. Dogs must inherit two copies of the faulty gene (one from each parent) to display drug sensitivity and be at risk for serious adverse effects.

If both parents carry the mutation, their puppies might inherit two mutated copies and be affected. Puppies with only one mutated copy are carriers and typically do not show symptoms but can pass the mutation on. When both parents test clear, the puppies will not carry the mutation.

Knowledge of this inheritance pattern helps breeders avoid pairing two carriers and reduces the likelihood of producing affected puppies, promoting healthier breeding practices across the U.S.

How to Test Your Dog for MDR1

MDR1 testing is recommended if you own or plan to breed dogs from at-risk breeds. The test helps breeders make safe pairing decisions and guides veterinarians to prescribe medications that won’t harm your dog.

Testing involves a simple cheek swab or blood sample, which your veterinarian can collect and send to specialized labs. In the U.S., laboratories like Washington State University's Veterinary Clinical Pharmacology Lab and companies such as ImpriMed offer MDR1 genetic testing.

Results typically return within a week and reveal if your dog is clear, a carrier, or affected. Many vets will add this information to breed health records to aid future breeding decisions and health research.

Testing costs in the U.S. generally range from $60 to $90 depending on the lab and testing service.

Why MDR1 Testing Is Crucial for Your Dog’s Health

Knowing your dog’s MDR1 status is essential to avoid harmful medications that could cause severe or fatal reactions. Many commonly prescribed veterinary drugs, including ivermectin (used for heartworm prevention) and certain chemotherapy agents, can be dangerous for MDR1-affected dogs.

By understanding your dog’s genetic makeup, you and your veterinarian can use safer medicines and dosage plans tailored to protect your pet, preventing avoidable tragedies.

Ultimately, MDR1 testing empowers owners and breeders in the U.S. to safeguard their dogs and improve overall breed health by reducing the impact of this severe hereditary condition.