When the MDR1 mutation was first characterized in the 1990s, researchers initially associated it almost exclusively with Collies. Early reports called it Collie eye anomaly or simply described affected dogs as having Collie-type sensitivity. This narrow focus led to a dangerous misconception that persists today: many owners and even some veterinarians believe only Collies carry the mutation.
The reality is far more complex. The MDR1 mutation exists in dozens of breeds and countless mixed-breed dogs. After two decades of population studies and widespread genetic testing, we now have detailed prevalence data that every dog owner should understand. These numbers tell us which breeds face elevated risk and, crucially, which dogs should be tested before receiving certain medications.
The Highest-Risk Breeds
The mutation originated in a common ancestor of modern herding breeds, which explains why herding dogs are disproportionately affected. Research from Washington State University, which has maintained the largest database of MDR1 testing results, provides the most reliable prevalence estimates.
The Collie remains the most affected breed, with approximately 70 to 75 percent of individuals carrying at least one copy of the mutation. Within that population, roughly 35 percent are homozygous (M/M), carrying two mutant copies. These dogs have no functional P-glycoprotein and face the highest risk from affected medications. Another 35 to 40 percent are heterozygous (N/M), carriers who have intermediate sensitivity. Only 25 to 30 percent of Collies are genetically normal (N/N) for MDR1.
| Breed | Affected (M/M or N/M) | Homozygous (M/M) | Carrier (N/M) |
|---|---|---|---|
| Collie (Rough and Smooth) | 70-75% | 35% | 35-40% |
| Australian Shepherd | 45-50% | 15% | 30-35% |
| Miniature Australian Shepherd | 45-50% | 15% | 30-35% |
| Shetland Sheepdog | 15-20% | 5% | 10-15% |
| English Shepherd | 15% | 5% | 10% |
| Longhaired Whippet | 40-65% | 15-25% | 25-40% |
| Silken Windhound | 30% | 10% | 20% |
| Old English Sheepdog | 5-10% | 1-2% | 4-8% |
| Border Collie | 1-5% | 0.5% | 1-4% |
| German Shepherd | 5-10% | 1% | 4-9% |
| McNab | 30% | 10% | 20% |
Australian Shepherds: A Growing Concern
Australian Shepherds deserve special attention because they are one of the most popular breeds in North America, and their MDR1 prevalence is alarmingly high. Approximately half of all Australian Shepherds carry at least one copy of the mutation. With the breed's popularity soaring over the past two decades, this translates to hundreds of thousands of affected dogs.
The Miniature Australian Shepherd, increasingly popular as a more apartment-friendly version of the standard Aussie, carries similar mutation frequencies. Toy Australian Shepherds, sometimes marketed as Toy Aussies, also show high prevalence though population studies are limited by smaller sample sizes.
What makes Australian Shepherds particularly concerning is their frequent presence on farms and ranches where livestock medications are used. An Aussie on a cattle operation faces constant exposure risk to products that could be lethal. I have treated multiple Australian Shepherds for ivermectin toxicity after they accessed livestock dewormers, and the owners almost never knew their dog carried a genetic vulnerability.
Shetland Sheepdogs: Moderate but Significant Risk
Shelties have a lower prevalence than Collies or Aussies, but 15 to 20 percent still carry the mutation. Given how popular Shelties remain as companion dogs, this represents a substantial population of affected animals.
Shelties present a particular challenge because their small size makes dosing errors more dangerous. A medication dose appropriate for a 30-pound dog could overwhelm a 15-pound Sheltie with MDR1 sensitivity. The margin for error shrinks with body size.
The German Shepherd Question
German Shepherds present an interesting case. They are not typically grouped with herding breeds in the public imagination, yet 5 to 10 percent carry the MDR1 mutation. Given that German Shepherds are among the most common breeds worldwide, even this moderate percentage represents millions of affected dogs globally.
The mutation likely entered the German Shepherd population through historical crosses with Collie-type dogs during the breed's development in the late 19th and early 20th centuries. Some breed historians suggest specific crosses with Scottish herding dogs contributed to the German Shepherd's working abilities and simultaneously introduced the MDR1 mutation.
I routinely recommend MDR1 testing for German Shepherds, particularly those who will have access to livestock medications or who may need certain drugs during their lifetime. The cost of testing is trivial compared to the potential consequences of discovering the mutation during an emergency.
Sighthound Breeds: An Unexpected Finding
The presence of MDR1 in Longhaired Whippets and Silken Windhounds surprised many researchers. These sighthound breeds are not closely related to traditional herding dogs, and the mutation's pathway into these populations is still debated.
One theory suggests the Longhaired Whippet, developed from crosses that may have included Sheltie or other herding breed genetics to introduce the longhair gene, simultaneously acquired the MDR1 mutation. The Silken Windhound, developed more recently, has Borzoi and Whippet ancestry but also includes Sheltie crosses, providing another potential mutation source.
Regardless of origin, the prevalence in these breeds is substantial. Longhaired Whippet populations show 40 to 65 percent affected, depending on the study and geographic population tested. Silken Windhounds run around 30 percent. Any owner of these breeds should test their dog.
Border Collies: Lower Than Expected
Many people assume Border Collies, as quintessential herding dogs, would have high MDR1 prevalence. In fact, they show relatively low rates of 1 to 5 percent. This may reflect different founding populations or genetic bottlenecks that did not include as many mutation carriers.
However, 1 to 5 percent is not zero. Border Collies remain working dogs often found on farms and ranches. Any Border Collie with access to livestock medications should be tested. A 3 percent prevalence means 3 out of every 100 Border Collies carry vulnerability that their owners likely do not know about.
Breeds That Should Always Be Tested
Testing is strongly recommended for any dog with ancestry from these breeds:
- Collie (Rough or Smooth)
- Australian Shepherd (any size)
- Shetland Sheepdog
- English Shepherd
- Longhaired Whippet
- Silken Windhound
- Old English Sheepdog
- Border Collie
- German Shepherd
- McNab
- Australian Cattle Dog (rare reports)
Mixed Breed Dogs: The Hidden Population
Perhaps the most underappreciated at-risk population is mixed breed dogs. Any dog with herding breed ancestry can carry the MDR1 mutation. A dog that looks nothing like a Collie or Australian Shepherd may have an affected ancestor three or four generations back and still carry the mutation.
DNA breed testing has become common among mixed breed owners curious about their dog's heritage. These tests often include MDR1 status, providing valuable safety information. If your mixed breed has any herding ancestry in their DNA results, pay close attention to the MDR1 finding. Even a small percentage of Australian Shepherd or Collie in a dog's background can mean they inherited the mutation.
Shelter dogs present particular challenges. Many are of unknown or uncertain ancestry. If a shelter dog has a herding-type appearance or behavioral characteristics, testing before starting any medication is prudent. The cost is minimal and the information is potentially life-saving.
Geographic and Population Variations
MDR1 prevalence can vary by geographic region and specific breeding populations. European Collies, for example, may show different prevalence than American Collies due to different founding stock and breeding practices. Working-line dogs may differ from show-line dogs within the same breed.
Some breeders have worked to reduce mutation frequency in their lines through selective breeding. Responsible breeders test all breeding stock and make informed decisions about pairings. A breeder who can document multiple generations of MDR1 testing may have lower prevalence in their dogs than breed-wide statistics suggest.
Conversely, certain isolated populations or heavily inbred lines may have higher prevalence than average. Small breeding populations can see dramatic shifts in allele frequency over just a few generations.
Understanding the Numbers
These prevalence statistics have practical implications. Consider an Australian Shepherd with unknown MDR1 status presenting for treatment that requires a P-glycoprotein substrate drug. There is roughly a 50 percent chance this dog carries at least one mutant allele. A 15 percent chance they are homozygous affected with maximum sensitivity. These are not trivial odds.
When I see a Collie in my clinic, I assume they are affected until proven otherwise. The 70 percent affected rate means assuming normalcy would be wrong more often than right. I adjust my approach accordingly, using safer alternatives when possible and testing before proceeding with higher-risk medications. Our complete drug list for MDR1 dogs details which medications require caution.
For breeds with lower prevalence, the calculation differs but testing remains valuable. A Border Collie has only 3 percent chance of being affected, but if that dog is the one in thirty who carries the mutation, the consequences of not knowing can be severe.
The Case for Universal Testing in At-Risk Breeds
Given the availability of affordable MDR1 testing, I advocate for universal testing in all breeds with documented mutation prevalence. The test is simple, typically requiring only a cheek swab. Results are available within one to three weeks depending on the laboratory. Cost ranges from 40 to 75 dollars as a standalone test and is often included in comprehensive genetic panels costing 100 to 200 dollars.
Compare this to the cost of treating ivermectin toxicity, which can run thousands to tens of thousands of dollars for intensive care cases. Compare it to the emotional cost of losing a dog to a preventable adverse drug reaction. Testing makes overwhelming economic and emotional sense. Understanding how ivermectin causes toxicity reinforces why prevention through testing matters so much.
Washington State University's Veterinary Clinical Pharmacology Laboratory pioneered MDR1 testing and remains a gold-standard option. Multiple commercial laboratories now offer the test as well, including those that provide comprehensive canine genetic panels.
Implications for Breeders
Responsible breeders have an obligation to test breeding stock and provide MDR1 status to puppy buyers. The information should be shared along with other health certifications. Puppy contracts should include this genetic health information.
The goal is not necessarily to eliminate the mutation from breeding populations, which could unnecessarily narrow genetic diversity. The goal is informed breeding decisions. Pairing two carriers (N/M x N/M) produces approximately 25 percent affected puppies. Pairing a carrier with a normal dog produces carrier puppies but no affected individuals. Breeders can make these decisions consciously rather than blindly.
Puppy buyers should ask about MDR1 testing. A breeder who does not test, or who cannot provide documentation, may not be following best practices for genetic health management. For more information on inheritance patterns and breeding decisions, the genetics guide at The Herding Gene provides detailed explanations.
Beyond the Statistics
Numbers tell part of the story, but every affected dog is an individual. A Collie with M/M status is not a statistic but a beloved family member who needs protection from certain medications. An Australian Shepherd puppy deserves to start life with its owners knowing whether drug sensitivities will need management.
These prevalence statistics exist to inform decisions, not to frighten owners. Dogs with MDR1 mutations live full, healthy lives when their genetic status is known and respected. The mutation is not a disease. It is a vulnerability that can be completely managed with appropriate awareness.
Know your dog's breed. Understand the risks. Test before medicating. These simple steps protect dogs from preventable harm and give owners peace of mind. The statistics are clear: millions of dogs carry this mutation. Make sure you know whether yours is among them. For immediate steps to take if exposure occurs, review our first aid guide.