To the resident veterinarians:

White Collies, like other Collies, are commonly affected by a genetic mutation in the MDR1 (multidrug resistance 1) gene, also known as the ABCB1 gene. This mutation impairs the function of P-glycoprotein, a critical molecule that acts as a protective barrier by preventing certain drugs from entering the brain. When this mechanism fails, drugs can accumulate in the central nervous system, leading to neurotoxicity.

Approximately 70%–75% of Collies carry at least one copy of the mutated MDR1 gene, making them highly sensitive to specific medications. Dogs with two copies (homozygous) are at the greatest risk, but even carriers (heterozygous) may experience adverse reactions under certain conditions.





Common Medications to Use with Caution
Dogs with the MDR1 mutation are particularly sensitive to drugs that are P-glycoprotein substrates. These include:

Ivermectin: While low, FDA-approved doses used in monthly heartworm preventatives are generally safe, higher doses (such as those used to treat mites or mange) can cause severe neurotoxicosis, including seizures, blindness, tremors, and even death.
Loperamide (Imodium): This over-the-counter anti-diarrheal is extremely toxic to MDR1-affected dogs and should be avoided entirely.
Milbemycin oxime and moxidectin: These macrocyclic lactones are used in parasite prevention. While safe at preventative doses, higher therapeutic doses may pose risks.
Selamectin (topical): Anecdotal reports link it to adverse reactions in sensitive breeds.
Certain chemotherapy drugs (e.g., doxorubicin, vincristine), immunosuppressants (e.g., cyclosporine), and sedatives (e.g., acepromazine, butorphanol) may require dose adjustments or avoidance.[/quote]



Safe Parasite Prevention Options
Despite sensitivities, safe and effective parasite control is available:

Interceptor Flavor Tabs (milbemycin oxime): Widely regarded as a top choice for MDR1-affected dogs. It prevents heartworm and treats intestinal parasites like hookworms, roundworms, and whipworms. It has been specifically tested and approved for use in Collies with the mutation.
Heartgard Plus (ivermectin/pyrantel): Safe at monthly preventative doses, though some owners prefer alternatives due to historical concerns. Adjustments have been made to improve safety.
Interceptor Plus: Contains praziquantel for tapeworm control, but safety in pregnant or lactating dogs has not been evaluated.
Always administer medications under veterinary guidance and monitor for side effects such as vomiting, diarrhea, or neurological signs.[/quote]





Testing for MDR1 Mutation
The only definitive way to determine MDR1 status is through genetic testing, typically performed using a cheek swab or blood sample. Testing is highly recommended, especially for purebred Collies or herding-breed mixes.

Knowing your dog’s status allows for personalized, safer treatment plans and helps avoid potentially life-threatening drug reactions. Many veterinary clinics and online services offer this test, and results should be shared with your veterinarian[/quote]