Tales from the Pet Clinic

By Ann Hohenhaus, DVM

The refinement of purebred dogs over the past four or five centuries has created interesting versions of Canis familiaris, such as dogs with dreadlocks, wrinkles, extra toes, a double coat, or an innate ability to herd sheep.

Selective breeding of dogs to propagate characteristics related to coat, foot size, or herding prowess may also have created the predisposition of some purebred dogs to specific diseases. As humans bred dogs to meet their own specifications, the genes controlling disease tagged along into the next generation with the genes controlling desirable characteristics such as dreadlocks and double toes. But the cloning of the canine genome in 2005, coupled with the multi-generational pedigrees available in purebred dogs and the close genetic relatedness of dogs within a given breed gave scientists powerful tools to study genetic disease in purebred dogs.

Eye diseases no more

Genetic tools combined with the advanced clinical skills may also be used to eradicate some diseases. For example, veterinary ophthalmologists have nearly eradicated breed-related disease of the eye in several breeds, including progressive retinal atrophy in Irish Setters and Irish Red and White Setters. This inherited disease results in blindness due to a failure of the retina to transmit images to the brain. The diagnosis can be made by a veterinary ophthalmologist using CERF (Canine Eye Registration Foundation) testing of puppies as young as five or six weeks of age. Now a genetic test is available to identify dogs carrying the mutation causing progressive retinal atrophy in the two types of Irish Setters. The test requires only a small amount of blood and identifies dogs that are clear of the mutation, the ones optimal for breeding to avoid producing puppies with abnormal eyes. The Irish Setter Club actively supports research into the diseases of their breed and a list of open studies is on their website.

Inherited drug sensitivity

We mostly think of genetics as determining physical stature and predisposition to disease, but a genetic mutation, found predominantly in herding dogs, called MDR1 (multiple drug resistance) determines heightened sensitivity to a variety of medications.

Possibly most important on this list are two drugs commonly used to prevent heartworms in dogs, ivermectin and milbemycin. Collies, Australian Shepherds (both the standard and mini), and other breeds lack the ability to process not only the previously mentioned heartworm preventatives but also acepromazine, butorphanol, and chemotherapy drugs used on a daily basis by veterinarians. Standard doses of these drugs can prove fatal in a dog with the MDR1 mutation. Genetic testing is available to identify dogs with the mutation, allowing veterinarians to prescribe safer medications.

The current version of the AAHA-AVMA Canine Preventive Healthcare Guidelines recommends the use of genetic testing in dogs and if I see one of the breeds on the list for a MDR1 mutation, I would test that dog prior to administering chemotherapy, if the test had not be performed as part of a preventive healthcare program.

If you have a purebred dog, ask your veterinarian about genetic testing for any diseases that run in your breed. If you are thinking of adding a purebred dog to your family, do your homework and investigate possible health concerns. Also, ask the breeder if they are involved in helping to eradicate their breed’s diseases.