Dogs with a mutation in the MDR1 (multidrug resistance 1) gene may have severe and life-threatening reactions to some common drugs. Research has shown the mutation occurs more frequently in certain breeds.
Approximately three of every four collies in the United States have the mutant MDR1 gene. The frequency is about the same in France and Australia, so it is likely that most collies worldwide have the mutation. The MDR1 mutation has also been found in Shetland sheepdogs (Shelties), Australian shepherds, old English sheepdogs, English shepherds, German shepherds, long-haired whippets, silken windhounds, and a variety of mixed-breed dogs.
The only way to know if an individual dog has the mutant MDR1 gene is to have the dog tested. As more dogs are tested, more breeds will probably be added to the list of affected breeds.
Can collie crosses or other herding breed crosses carry the MDR1 gene mutation?
The MDR1 (multidrug resistance 1) gene mutation is common in collies and other herding breeds, and while it is less likely to occur in a mixed breed, it is still possible, and testing is recommended to determine the risk to your pet.
The mutant gene was found in a Saint Bernard mix that had an adverse drug reaction. The veterinarian noted the dog’s eyes were different colors, like some Australian shepherds.
| Breed | Approximate Frequency |
|
|---|---|---|
 | Australian Shepherd | 50% |
 | Australian Shepherd, Mini | 50% |
 | Black Mouth Cur | Not enough tested to calculate |
 | Border Collie | < 5% |
 | Carolina | Not enough tested to calculate |
 | Chinook | 25% |
 | Collie | 70% |
 | English Shepherd | 15% |
 | German Shepherd | 10% |
 | Herding Breed Cross | 10% |
 | Long-haired Whippet | 50% |
 | McNab | 30% |
 | Mixed Breed | 5% |
 | Old English Sheepdog | 5% |
 | Shetland Sheepdog | 15% |
 | Silken Windhound | 30% |