Health & Diseases

MDR1 Defect in Dogs: Medication Risk and Genetic Testing

The MDR1 defect (also ABCB1 mutation) is a genetic alteration that causes severe hypersensitivity to a number of medications in certain dog breeds. The MDR1 gene (Multi-Drug-Resistance gene 1), which codes for a transport protein (P-glycoprotein) in the blood-brain barrier, is affected. This protein pumps foreign substances—including many medications—back into the bloodstream from the brain tissue.

MDR1 deficiency in dogs: medication risk and genetic testing

What is MDR1 deficiency in dogs?

MDR1 deficiency (also called the ABCB1 mutation) is a genetic alteration that causes severe hypersensitivity to a range of medications in certain dog breeds. It affects the MDR1 gene (multi-drug resistance gene 1), which encodes a transport protein (P-glycoprotein) in the blood-brain barrier. This protein pumps foreign substances — including many medications — out of brain tissue and back into the blood.

Dogs with the MDR1 mutation (homozygous: both gene copies affected) lack this protective mechanism. Medications that normally barely cross the blood-brain barrier accumulate in the brain — with severe neurotoxic effects, sometimes life-threatening.

Background + scientific context

Mealey et al. (2001, Pharmacogenetics, PubMed 11668219) were the first to identify the causative deletion in the mdr1 gene (4-base-pair deletion) in Collies as the basis for the known ivermectin sensitivity: The mutation leads to a non-functional transport protein. Ivermectin at doses that are safe for normal dogs accumulates in the brain of dogs homozygous for the mutation and causes central nervous system toxicity up to coma. Collies, Shetland Sheepdogs, and related breeds are predisposed.

Mealey and Meurs (2008, Journal of the American Veterinary Medical Association, PubMed 18558882) examined the breed distribution of the ABCB1 mutation in more than 5,000 dogs: Affected breeds include not only Collies but also Australian Shepherds, Border Collies, McNabs, Old English Sheepdogs, Longhaired Whippets, and various mixed-breed dogs of Collie ancestry. Heterozygous dogs (one mutated copy) show intermediate sensitivity — relevant when considering dosage.

Gramer et al. (2011, Veterinary Journal, PubMed 20167517) determined mutation frequencies in European Collie populations: In German and Austrian Rough Collie populations, more than 50% of dogs carried at least one mutated gene copy; approx. 25% were homozygous. This shows that MDR1 deficiency in Collie-related breeds is not a rare exception, but an epidemiologically relevant issue.

Vitomalia Position

The MDR1 defect is a clear reason for genetic screening before drug treatment in Collie breeds. The standard principle “the dose makes the poison” applies only to a limited extent here: dosages that are harmless for unaffected dogs can be fatal for MDR1-homozygous animals. Every Collie, Australian Shepherd, or related dog should be tested — and the test result belongs in the patient record.

When does the MDR1 defect become relevant?

  • Before administering antiparasitics in affected breeds (ivermectin, high-dose milbemycin oxime, moxidectin)
  • Before administering MDR1 substrate medications: loperamide, vincristine, cyclosporine, digoxin, doxorubicin
  • Before surgical procedures (anesthetics)
  • In cases of unexplained neurological symptoms after medication administration
  • Breeding planning: MDR1 status in the breeding program

Practical application

Breeds with a high MDR1 mutation frequency:

breed Mutation frequency (homozygous) Recommendation
Rough/Smooth Collie ~20–30% Testing mandatory
Shetland Sheepdog ~15% Testing mandatory
Australian Shepherd ~10–15% Testing mandatory
Border Collie ~5% Testing recommended
Old English Sheepdog ~10–15% Testing recommended
Longhaired Whippet ~50% Testing mandatory
McNab ~30% Testing mandatory

MDR1-relevant medications (substrate list): - Antiparasitics: Ivermectin (caution: dosage!), milbemycin oxime (high dose), moxidectin (high dose) - GI medications: Loperamide (Imodium) — even therapeutic doses can be neurotoxic in MDR1-affected dogs - Cytostatics: Vincristine, doxorubicin, vinblastine — check MDR1 status before cancer treatment - Other: Cyclosporine, digoxin, acepromazine (use with caution)

Genetic test: - Buccal swab, sent to specialist laboratories (e.g. Laboklin, University of Veterinary Medicine Hannover) - Result: normal/normal (+/+), carrier (+/-), affected (-/-) - One-time test; no follow-up test needed

Common mistakes & myths

  • “My Collie has never had any problems — so he isn’t affected.” Previous tolerance does not prove that a dog is not affected. MDR1-related problems occur specifically with certain substances and dosages; without exposure, there are no symptoms.
  • “Normal dewormers contain too little ivermectin to be dangerous.” Antiparasitic sprays for horses and livestock dosages are often ten times higher than dog dosages — accidental ingestion, such as horse manure after ivermectin treatment, has killed MDR1-affected dogs.
  • “Only Collies are affected.” Mixed-breed dogs with Collie ancestry can also carry the mutation — if the dog’s lineage is unknown, testing is worthwhile.

Scientific status 2026

MDR1/ABCB1 is one of the best-characterized pharmacogenetic traits in dogs. Genetic tests are routinely available and affordable. Awareness in veterinary practice has increased; nevertheless, MDR1 dogs are still regularly treated with contraindicated substances. Washington State University (WSU) maintains an up-to-date database for the MDR1 substrate list; veterinarians can use it to provide information on safe dosages.

Frequently asked questions

Which breeds are affected by the MDR1 defect?

Mainly collie-related herding dog breeds: Rough/Smooth Collie, Shetland Sheepdog, Australian Shepherd, McNab, Border Collie, Longhaired Whippet, Old English Sheepdog. Mixed-breed dogs from these breeds may also be affected. For all these breeds, the following applies: genetic testing before the first administration of medication with MDR1 substrates.

How do I have my dog tested for MDR1?

The test is performed using a buccal swab (cheek mucosa), which is sent to a specialist laboratory. In Germany, laboratories such as Laboklin and the University of Veterinary Medicine Hannover offer the test. Your veterinarian can order the test; the cost is approx. €50–80. The result is valid for life.

Is loperamide (Imodium) dangerous for my collie?

Yes — loperamide is a classic MDR1 substrate. In MDR1-homozygous dogs, it accumulates in brain tissue and can cause severe neurotoxicity (sedation, ataxia, coma) — even at commercially available doses. Do not use loperamide in collie breeds without an MDR1 test.

Related terms

Sources & further reading

  1. Mealey, K. L., Bentjen, S. A., Gay, J. M., & Cantor, G. H. (2001). Ivermectin sensitivity in collies is associated with a deletion mutation of the mdr1 gene. Pharmacogenetics, 11(8), 727–733. https://pubmed.ncbi.nlm.nih.gov/11668219/

  2. Mealey, K. L., & Meurs, K. M. (2008). Breed distribution of the ABCB1-1Delta (multidrug sensitivity) polymorphism among dogs undergoing ACTH stimulation testing for hypoadrenocorticism. Journal of the American Veterinary Medical Association, 233(6), 921–924. https://pubmed.ncbi.nlm.nih.gov/18558882/

  3. Gramer, I., Leidolf, R., Döring, B., Klintzsch, S., Krämer, E.-M., Yalcin, E., … Geyer, J. (2011). Breed distribution of the nt230(del4) MDR1 mutation in dogs. Veterinary Journal, 189(1), 67–71. https://pubmed.ncbi.nlm.nih.gov/20167517/

Wissenschaftliche Einordnung

Mealey et al. (2001, Pharmacogenetics, PubMed 11668219) first identified the causative deletion in the mdr1 gene (4-base pair deletion) in Collies as the basis for the known ivermectin sensitivity: The mutation leads to a non-functional transport protein. Ivermectin at dosages that are safe for normal dogs accumulates in the brains of homozygous mutant dogs and causes central nervous system toxicity, leading to coma. Collies, Shetland Sheepdogs, and related breeds are predisposed.

Mealey and Meurs (2008, Journal of the American Veterinary Medical Association, PubMed 18558882) investigated the breed distribution of the ABCB1 mutation in over 5,000 dogs: Affected breeds include not only Collies but also Australian Shepherds, Border Collies, McNabs, Old English Sheepdogs, Longhaired Whippets, and various mixed-breed dogs of Collie ancestry. Heterozygous dogs (one mutated copy) show intermediate sensitivity — relevant for dosage considerations.

Gramer et al. (2011, Veterinary Journal, PubMed 20167517) determined mutation frequencies in European Collie populations: In German and Austrian Rough Collie populations, over 50% of dogs carried at least one mutated gene copy; approximately 25% were homozygous. This shows that the MDR1 defect in Collie-related breeds is not a rare exception but an epidemiologically relevant problem.