MDR1 and Ivermectin Sensitivity in Dogs

The MDR1 mutation is a change in the ABCB1 gene that leaves some dogs without a working pump at the blood brain barrier, so certain drugs build up in the brain instead of being cleared. Dogs that carry it, including many Collies, Australian Shepherds and related breeds and their crosses, can react badly to ordinary doses of ivermectin and other macrocyclic lactones. The practical answer is to know your dog's status before the next deworming, anaesthetic or chemotherapy plan, because the risk is manageable once it is known.

What is the MDR1 mutation and which dogs carry it?

The gene involved codes for P glycoprotein, a transport protein that pushes drugs and toxins back out of the brain and other tissues. A mutation in that gene produces a shortened, non working protein. Dogs with two copies, often written as affected or homozygous, have little or no functional pump. Dogs with one copy, called carriers, have reduced function and usually cope with normal doses but can still react to high ones.

The trait is inherited in a simple autosomal recessive pattern. A carrier bred to a carrier can produce affected puppies, which is why a clear dog from a clear line is not the same as a tested clear dog. Prevalence is highest in Collies, where a large share of the breed carries at least one copy, and it appears in Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, Border Collies, German Shepherd Dogs and several other herding and mixed lines. Crosses matter: a dog that looks like a Labrador but has one Collie parent can still carry the mutation, and appearance tells you nothing reliable. Testing is the only way to know, and it is worth doing before a dog is bred or before any drug with a narrow margin is used.

If your dog is in one of the breeds or crosses above, the background on the mutation, its transmission and the testing options is set out in plain terms at MDR1 and drug sensitivity, alongside the drug lists that matter day to day.

What are the signs of ivermectin toxicity and how is an emergency handled?

Ivermectin and related macrocyclic lactones act on parasite nerve channels. In a dog without the pump, the drug crosses into the central nervous system and produces signs that usually appear within hours of the dose, though they can be delayed.

Early signs include dilated pupils, drooling, vomiting, wobbling, and a dull or unsteady walk. As it progresses you may see tremors, head pressing, loss of balance, blindness, seizures, and in severe cases coma or difficulty breathing. The dose and the dog's genetic status both matter: a sensitive dog can show signs at a dose that a normal dog would shrug off, and a very high dose can affect any dog.

Emergency care is time critical. Call your veterinarian or an emergency clinic immediately, state the product, the strength and the amount, and the dog's weight and breed background. Do not wait for signs to become severe, and do not try to induce vomiting unless a veterinarian tells you to. Treatment is supportive and aimed at limiting further absorption and controlling neurological signs: activated charcoal when it is safe and recent, intravenous fluids, seizure control, and monitoring of breathing and temperature. Lipid emulsion therapy is used in serious cases, where an intravenous fat emulsion is thought to pull the drug out of the brain and into the bloodstream; it is a hospital treatment, not a home remedy. Recovery is possible, but it can take days, and some dogs need intensive care.

Which medicines and daily situations need care in an MDR1 dog?

The list is longer than most owners expect. It includes ivermectin at doses used for mange or high dose heartworm prevention, milbemycin, moxidectin, selamectin and other macrocyclic lactones, plus some common drugs such as loperamide, certain anti nausea medicines, some chemotherapy agents, and a number of anaesthetic and sedative combinations. Not every product on the list is dangerous at every dose, which is exactly why the decision belongs to a veterinarian who knows the dog's status.

Heartworm prevention deserves its own note. Many monthly preventives contain a macrocyclic lactone at a low dose that most affected dogs tolerate, but the margin is not the same for every product, and the injectable form used for treatment is a different matter. If your dog is affected, ask specifically which preventive is safe rather than assuming the usual one is fine.

Anaesthesia and surgery need a pre procedure conversation. Tell the clinic the dog's MDR1 status in writing, and ask which sedatives, pain relief and anaesthetic agents are planned. Chemotherapy is the same conversation at a higher stakes level, because several common protocols include drugs that are problematic in affected dogs.

Food and household items also count. Some antiparasitic products sold for livestock, and residues in certain medicated feeds, have caused poisonings. Keep horse and cattle dewormers, pour on products and medicated premises away from dogs, and read labels on anything applied to the skin or given by mouth. Boarding kennels and travel add another layer: staff may use a routine dewormer or flea product, so leave written instructions naming the drugs to avoid, and carry your own supply when you can. A short card in the dog's file, listing the diagnosis and the drugs to avoid, prevents most of these problems.

How do the tests work, and what is different for puppies?

Testing is a DNA test, usually done from a cheek swab or a blood sample, and it reports one of three results: clear, carrier or affected. The result does not change with age or health, so a puppy's test is as valid as an adult's. What differs is timing. Puppies are often dewormed in their first weeks, before a test result is back, and they are also more vulnerable to neurological signs because their barrier and liver clearance are still developing. If the breed background suggests risk, ask the veterinarian to use a product that is safe for a potentially affected dog until the result arrives. Breeders can test before mating to avoid pairing two carriers, which is how the frequency of the mutation comes down over time.

What should you do next?

Get the test if your dog is in a breed or cross with known risk, and put the result in the dog's records where any clinic, kennel or sitter can find it. Then build a short list with your veterinarian: the dewormers and flea products that are safe, the heartworm preventive to use, and the drugs to avoid in an emergency. Keep the list on your phone and a paper copy in the dog's travel bag. If a dose is ever given by mistake, call for help straight away rather than watching and waiting. The mutation does not shorten a dog's life on its own, and with the right drug choices most affected dogs live ordinary, active lives.

The clinical detail behind the mutation is summarised in the NCBI Bookshelf entry on ivermectin toxicity, and the same rule holds at home, where the questions to bring to a veterinary visit belong in the same folder as the medication record. Where a gene sits behind a visible trait, the screening tests used in herding breeds show how that is handled in practice.