Australian Shepherd Drug Hypersensitivity (MDR1/ABCB1): What the Research Actually Shows

The short answer: The Australian Shepherd is one of America’s most popular breeds (AKC’s #12 in 2024), and it is also a herding breed that carries the MDR1 (ABCB1) drug-sensitivity trait. Research reports the mutation in Aussies at roughly 16–29% (allele frequency) — not the extreme of the Rough Collie (about 55–75%), but far more common than in the similarly herding Border Collie (about 0–1%) (Neff et al. 2004; Gramer et al. 2011; OMIA). Dogs with the mutation are more prone to serious neurological reactions to certain dewormers and anti-diarrheals — a form of drug (multidrug) hypersensitivity. This is constitutional information, not a disease diagnosis. A single DNA test reveals a lifelong trait and helps you and your veterinarian double-check medications in advance. Dosing decisions always remain the veterinarian’s call.

What the MDR1 (ABCB1) gene actually does — the brain’s “drug gatekeeper”

SamA friend from my running club has an Australian Shepherd and heard it could collapse from a common medication. Scary.

Elena MarshUnderstandable worry. The cause is a deletion in the ABCB1 gene that disables the pump clearing drugs from the brain — the mechanism Mealey’s team pinned down in 2001. Aussies are one of the breeds where it shows up, so knowing the mechanism matters.

MDR1 is properly named ABCB1. It is the blueprint for P-glycoprotein (P-gp), a “drug efflux pump” stationed at protective barriers such as the blood–brain barrier. P-gp pumps specific drugs back out of cells so that they do not accumulate in the brain and other sensitive organs (Mealey et al. 2023, J Vet Pharmacol Ther).

Some herding dogs, including many Australian Shepherds, carry a four-base deletion in this blueprint called ABCB1-1Δ (nt230[del4]), so the pump is not built correctly. With the “gate” out of order, drugs that should stay out of the brain get in, which can cause ataxia, drooling, tremors, seizures, and coma. Mealey and colleagues first identified this deletion in 2001 as the cause of ivermectin sensitivity in herding breeds (Mealey et al. 2001, Pharmacogenetics).

How common is it in the Australian Shepherd — the breed-frequency research

SamSo it’s fairly common in Aussies, not a rare fluke?

Elena MarshRight. In Neff 2004 and Gramer 2011 the Australian Shepherd runs around 16–29% for the mutant allele — well above most breeds. Not as extreme as the Rough Collie, but common enough that an individual test is worth it.

The Australian Shepherd sits in the middle of the MDR1 risk spectrum: much more affected than most breeds, but below the extreme Collie lineage. Pooling several large surveys gives roughly the following (frequency of the mutant allele).

An allele frequency around 16–29% means a substantial share of Australian Shepherds carry at least one copy of the mutation. Neff et al. (2004, PNAS) traced ABCB1-1Δ to a single ancestral collie-type lineage, and the Australian Shepherd — despite the “Australian” name, a breed developed largely in the western United States — belongs to that broader herding group. Because carriers are common in the breed, “my Aussie looks fine” is not a substitute for knowing its genotype.

Why “ivermectin accidents” happen — the drugs involved

SamShould the Aussie stop its monthly heartworm pill, then?

Elena MarshPlease don’t stop it on your own. Research shows preventive-dose ivermectin is usually within the safe margin even with the mutation. The danger is high-dose mange treatment. Let the vet set the line.

The effect was first recognized with ivermectin, a deworming drug used in heartworm prevention and mange treatment (Mealey et al. 2001). But the problem is not one drug. The related macrocyclic lactone milbemycin oxime also caused significantly more neurological adverse reactions in ABCB1-1Δ dogs during treatment of generalized demodicosis (Barbet et al. 2009, Vet Dermatol).

Because P-glycoprotein transports many drugs beyond antiparasitics, reduced ABCB1 function can affect a broader range of medications, as reviewed by Mealey et al. (2023). Testing labs such as Washington State University’s program also flag loperamide (anti-diarrheal) and some chemotherapy and sedative agents as “use with caution.” The crucial nuance is dose: heartworm-prevention doses of ivermectin are generally within a safe margin even for MDR1 dogs, whereas the high doses used to treat mange call for caution. That line is the veterinarian’s to draw.

Heterozygous vs homozygous — copy number and phenotype

SamSo one copy means it’s fine?

Elena MarshNot that simple. Research shows even one copy gives intermediate sensitivity — it’s incomplete dominance. Homozygotes are affected most. That’s why the copy number, not just presence, matters.

This is not simply “has it or not.” Because the gene is inherited from each parent, there are three states: normal, heterozygous (one copy), and homozygous (two copies). The trait was first assumed to be autosomal recessive, but later work showed that heterozygous dogs also show intermediate sensitivity, so it is now treated as incomplete (co-)dominance (OMIA; Mealey et al. 2003/2008/2023).

In Australian Shepherds, where carriers are common, this gradient matters: a homozygous dog (two mutant copies) is affected most strongly, while a heterozygous dog (one copy) is more drug-sensitive than a normal dog but less so than a homozygote. That is exactly why a test result showing not only “carries the mutation” but “how many copies (zygosity)” is meaningful when medication is being considered.

What the test can and cannot tell you — a trait, not a diagnosis

SamSo a test means we can choose medications with confidence?

Elena MarshKnowing the trait in advance is real peace of mind. But the result is a reference, not a diagnosis. Since DNA never changes, test once and share it with your vet — that’s the smart way to use it.

An MDR1 DNA test tells you about an inborn tendency to be sensitive to certain drugs. Because DNA does not change over a lifetime, the test is a one-time affair, and sharing the result with your regular veterinarian lets you confirm safety when medication is eventually needed.

At the same time, the result is neither a disease diagnosis nor a prediction of illness. An Australian Shepherd with the mutation can safely receive many drugs at appropriate doses, and one without it is not automatically safe with every drug. The test also only covers known variants, so it cannot capture variants outside the panel or other factors (a false-negative is possible). If you notice concerning symptoms or are actually planning treatment, use the result as one reference point and always get a veterinary diagnosis.

FAQ — Frequently Asked Questions

Q. I have an Australian Shepherd. Do I really need the MDR1 test?
Research puts the Aussie mutant-allele frequency at roughly 16–29%, so a meaningful share of the breed carries it — much more than most breeds. Because carriers look completely normal, testing is the only way to know before medication is needed. Discuss timing with your veterinarian.

Q. If my Aussie has the mutation, can it never take heartworm prevention?
No. Research indicates preventive-dose ivermectin and similar drugs are generally within a safe margin even with the MDR1 mutation. The higher doses used to treat mange are the ones that tend to cause problems. The dose line is drawn by the veterinarian. Do not stop prevention on your own judgment.

Q. Is the test result valid for life after one test?
Yes. Because the DNA sequence does not change over a lifetime, a gene test like MDR1 is generally a one-time test. Sharing the result with your regular veterinarian helps with future medication.

Q. If the result is “clear,” is any drug safe?
No. An MDR1 test only tells you about the specific drug sensitivity tied to ABCB1. Adverse reactions from other causes are a separate matter, so always follow your veterinarian’s instructions for medication.

References

• Mealey KL, et al. (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/11692082/
• Neff MW, et al. (2004) Breed distribution and history of canine mdr1-1Δ. PNAS 101(32):11725-11730. https://pmc.ncbi.nlm.nih.gov/articles/PMC511012/
• Mealey KL, Meurs KM. (2008) Breed distribution of the ABCB1-1Delta polymorphism among dogs undergoing ABCB1 genotyping. JAVMA 233(6):921-924. https://pubmed.ncbi.nlm.nih.gov/18795852/
• Gramer I, et al. (2011) Breed distribution of the nt230(del4) MDR1 mutation in dogs. The Veterinary Journal 189(1):67-71. https://pubmed.ncbi.nlm.nih.gov/19171022/
• Barbet JL, et al. (2009) ABCB1-1Δ genotype and adverse reactions to milbemycin oxime in dogs with generalized demodicosis. Veterinary Dermatology 20(2):111-114. https://pubmed.ncbi.nlm.nih.gov/19392768/
• Mealey KL, Owens JG, Freeman E. (2023) Canine and feline P-glycoprotein deficiency: What we know and where we need to go. J Vet Pharmacol Ther. https://pubmed.ncbi.nlm.nih.gov/36326478/
• OMIA:001402-9615 Multidrug resistance 1, ABCB1-related in Canis lupus familiaris. University of Sydney. https://omia.org/OMIA001402/9615/
• American Kennel Club — Most Popular Dog Breeds of 2024. https://www.akc.org/expert-advice/news/most-popular-dog-breeds-2024/
• Washington State University, Program in Individualized Medicine (PrIMe) / VCPL. https://prime.vetmed.wsu.edu/

About the author

Elena Marsh

Editor & writer (not a veterinarian)

A writer with a molecular-biology background and a lifelong dog and cat owner. Not a veterinarian — she translates peer-reviewed genetics research and primary data into plain language, always as information rather than diagnosis.