Now that I am fresh off a pedantic lecture on using medicines in the appropriate quantities instead of cutting back based on your “gut instinct”, I must confess that there is a small, very specific population of pets that may need to be …under-medicated. Please read on, especially if you have a herding type of dog at home.
Everyone’s cells have a molecule in the outer membrane called P-glycoprotein. It functions as a pump to drive away certain substances, forming a barrier that corrals those substances on one side only and excludes them from sensitive areas like the brain. Only certain drugs interact with P-glycoprotein, and the recommended dosing for those medications depends on the supposition that all of us have the same amount of pump molecules. However, that turns out not to be true. Not all of us.
A tiny sliver of the dog population has only broken, dysfunctional P-glycoproteins due to a gene mutation named MDR1 (also known as ABCB1). Without a competent barrier, ingested (or injected) substances are allowed to flood unchecked into areas where the body was not expecting them. It’s not always safe, particularly if the patient is asked to take more than one of those drugs concurrently. What medicines are on the list? A couple allergy medicines, a couple stomach medicines, a couple pain medicines, and a handful of chemotherapy drugs. The important thing here is to find out whether your dog is a carrier of the broken gene (i.e., a mutant), or not. Genes come in pairs, and if your dog has two mutant genes instead of just one, it’s all the more risky for him to take multiple medicines. This is worth investigating in advance.
The number of times I’ve recommended an independent test company for any of my patients is… vanishingly small. This particular test, however, is backed by a lot of legitimate research. It’s not a great investment for every pet, though. There is a very, very small number of affected dogs in the general population, but the risk of mutation skyrockets when the patient is a purebred collie (around 70% affected), Australian shepherd (50% affected), or Shetland sheepdog (still significant at 15%). Some studies show approximately 10% incidence in purebred German shepherds as well. Border collies are very rarely affected, despite their passing resemblance to the Australian shepherd. If you own an exceptionally unusual purebred, it might be worth checking the list of affected breeds. Felines can carry this mutation also, but it is very rare, with purebred cats at a slightly higher risk.
Please don’t panic when you’ve read this and realized that you may be harboring a mutant in your home. Thousands of dogs with the faulty gene go their whole lives without knowing their MDR1 status and never experience a mishap with their healthcare. You can’t predict their future, though; one day you could possibly find yourself in a perfect storm of pharmaceuticals (whether prescribed or through accidental drug exposure) where MDR1 status might influence the treatment plan. If you have a herding breed, bring it up with your vet next time, and see if genetic testing might be a worthwhile investment.
Dr. M.S. Regan