Drug Sensitive Dogs

None — Multidrug Sensitivity in Dogs

Multidrug sensitivity is a condition which can cause toxic levels of medications to build up in a dog's brain. It's caused by a mutation in the MDR1 gene, which is responsible for the production of a protein, called, P-glycoprotein.

In a healthy dog, P-glycoprotein plays a key role in transporting drugs out of the brain, keeping brain tissue healthy when the animal is getting medical treatment. In dogs with the MDR1 gene mutation, P-glycoprotein doesn't function properly. When the animal is given certain drugs, it is unable to remove them from the brain. The medication is absorbed by the brain tissue, causing drowsiness, coma and possibly death.

Katrina Mealey, D.V.M., Ph.D., Veterinary Pharmacologist with the College of Veterinary Medicine at Washington State University in Pullman, WA, says only certain drugs are affected by the MDR1 gene mutation. Some drugs that may cause problems in affected animals include: acepromazine (a tranquilizer), butorphanol (analgesic), doxorubicin (an anticancer drug), emodepside (Profender®, a worm treatment), erythromycin (antibiotic), ivermectin (for heartworms), loperamide (Imodium™, for diarrhea), rifampin (an antibiotic), vinblastine (an anticancer drug) and vincristine (also used to treat cancer).

Testing for Multidrug Sensitivity

Mealey says the MDR1 gene mutation is much more common in certain breeds. About 70 to 75 percent of collies have the mutation. It also affects 50 percent of Australian Shepherds (including the minis), 65 percent of long-haired whippets, 30 percent of McNabs, 30 percent of Silken Windhounds and 15 percent of Shetland Sheepdogs. Other breeds that are affected to a smaller degree (10 percent or less) include the German Shepherd, Old English Sheepdog and Border Collie. The gene mutation is found in 10 percent of herding cross-breeds and about 5 percent of mixed breeds.

Even among high-risk breeds, not all the dogs will have the MDR1 gene mutation. More importantly, mixed breeds of unknown origin can have the mutation, even though they may not have physical characteristics of any of the high-risk breeds.

Researchers at Washington State University have developed an at-home genetic test that can be used to screen a dog for the defective gene. Owners who are interested can contact the University for a test kit (www.vetmed.wsu.edu/depats-NCPL/test.aspx, or by calling (509) 335-3745). There is no charge for ordering the kit, however a processing fee is due when the samples are mailed back.

When the test kit arrives, the owner uses collection brushes to take two samples of tissue on inner surface of the dog's lip. The brushes are then mailed back to the lab for processing. Payment must be included with the samples. Currently, it takes about one week to process the tests. The results are e-mailed or faxed to the owner and veterinarian. If a dog is found to have the MDR1 gene mutation, the veterinarian can take steps to avoid giving the animal medications known to cause drug sensitivity. Mealey says that if a drug-sensitive dog is accidentally given the wrong drug, there is a reversal medication that can be given. However, the treatment needs to be given within a short period of time to save the animal.

For information about multidrug sensitivity in dogs, or the genetic test kit, click here.

WSOC