What does the phrase “white feet do not treat” mean?
Certain breeds, including dogs with white feet are at risk for being unable to process a number of drugs. Why does this happen? The answer is that it is all about the DNA. Within the DNA of dogs there is a gene called MDR1. This gene like all genes has a function. The MDR1 gene codes or makes a protein called P-glycoprotein.
P-glycoprotein is important because it is a drug transport pump. In other words, it plays a major role in distribution, metabolism, and excretion of many drugs. Dogs that have a defective or mutant MDR1 gene have defective P-glycoprotein, and therefore have delayed excretion of certain drugs. This means that drugs can build up in their body's causing toxicity.
What breeds are affected, that is who has the mutant MDR1 gene? Classically the herding breeds are affected, with 70 percent of Collies being affected! The most common US breeds include the Collie, Australian Shepherd, and Shetland Sheepdog.
Other breeds include the German Shepherd, herding breed crosses, and the long haired Whippet. Interestingly, less than 5 percent of Border Collies are affected.
There are several commonly used drugs in veterinary medicine that are affected by the mutant MDR1 gene. These drugs include ivermectin, acepromazine, and butorphanol. When these drugs are used in affected patients, the drugs are not excreted and hence build up in the system leading to toxicity.
Taking this one step further, these same affected patients can have trouble eliminating chemotherapy drugs. Cancer fighting drugs often are quite potent and would cause serious toxicity in this group of patients.
Fortunately there is testing to determine if a patient has the mutant MDR1 gene. In fact this testing should be a prerequisite in any dog with white feet prior to chemotherapy.
Conversely certain cancer patients have a surplus of the normal MDR1 gene resulting in a faster clearance of medications. These patients will have drug resistance because the drugs are eliminated too quickly.
As we can see genetics is playing a bigger part of understanding appropriate patient care. Any dog can be tested for the mutant gene, and breeds at risk likely should be tested. The test is a simple blood sample or cheek swab.
The future will bring more genetic testing and allow veterinarians to be more specific for each individual patient.