Discovering Your Dog’s Hidden Health

08/02/2010

Genetic tests can uncover problems before they arise, or prevent them altogether.

Labrador Retriever with BallYou’re looking for a family dog, and you’ve found a registered breeder. Congratulations! Your next step may be asking questions about the line and even looking up the parents in one of several online registries such as the Canine Health Information Center.  These days, you can learn even more through genetic testing. Responsible breeders will conduct these tests before pairing the parents. These tests allow them to assure you your puppy is free from some inherited diseases known to that particular breed. It’s even possible to learn the background of your mixed breed dog through genetic testing.  If you are a breeder, genetic testing plays a central role. Even when the parent dogs show no signs of a disorder, they may carry a hidden gene for it. Genetic tests allow you to prevent diseases and disorders in your litters by making sure you chose the right parents. By knowing the genetic make-up of your pairings, you will contribute to the overall sturdiness of the line by reducing or eliminating inherited diseases like blindness, deafness and nerve disorders. Dozens of tests are available for every breed."It’s important to realize is that it’s not just one breed that needs testing," says Dr. Margret L. Casal, associate professor of medical genetics at the Veterinary Hospital of the University of Pennsylvania. "Anyone who is creating a good breeding program should test their dogs for the specific diseases that may be present in that breed. For instance, in Labs, congenital eye diseases are common, so you want to test for them. Especially because these are diseases that may not show up until later in life.

Genetic testing: the basics

Dogs have 78 chromosomes, each one a segment of its total DNA (deoxyribonucleic acid). This large molecule carries information that guides a dog’s development, including sections, known as genes, for identifiable traits such as eye color. If this molecule were composed of glass beads, Casal says, the DNA chain would stretch from New York to Los Angeles. When one of these genes has a change, or mutation, it may lead to a health problem. In a direct DNA test, a dog’s chromosomes are examined using a blood sample or cheek swab from inside your dog’s mouth.One of the most important applications of genetic testing has been for chronic, debilitating conditions such as the blindness caused by progressive retinal atrophy. PRA causes the retina to slowly deteriorate. PRA tests are now available for many breeds.Even dogs that appear normal may carry a hidden (or recessive) gene for a disorder. That dog, known as a carrier, passes on the mutation – and usually the disorder -- when bred with another carrier, although neither the sire nor the dam displays the condition.A few examples of direct DNA tests include screenings for:

Degenerative Myelopathy - a disease of the spinal cord that leads to paralysis. It typically appears sometime between 8 and 14 years of age.

Merle Gene -  responsible for a dappled coat color. When two copies are present, it may lead to a white coat color, but also deafness and blindness. 

Exercise Inducted Collapse - an inherited disorder causing Labrador retrievers to become severely weakened during strenuous activity. Black, yellow and chocolate Labs, Chesapeake Bay retrievers and curly-coated retrievers may also be affected.

von Willebrand’s disease - a bleeding disorder identified in a wide range of breeds.

Hyperuricosuria - a condition predisposing dogs to developing stones in their bladders or kidneys that may have to be surgically removed. It can occur in any breed but is often found in Dalmatians, bulldogs and black Russian terriers.

Musladin-Lueke Syndrome - a disease affecting connective tissue in beagles.

Karyotyping - to evaluate all chromosomes for defects. This test can determine why some dogs have reproductive or growth problems and allows breeders to identify animals that are poor performance or breeding risks.

Many other tests are available. View a complete list of available genetic tests.

Linkage tests

While direct DNA tests are available for many traits and conditions, for others there is less certainty. Researchers have found that some patterns of DNA indicate the presence of a mutation nearby. Casal likens these tests to looking for an ice cream factory – if you see ice cream trucks coming and going more frequently at one exit on the highway, you know the factory is nearby. These evaluations are known as linkage tests and are a good indication that a mutated gene is present.

Danika Bannasch, chief of veterinary genetics and professor of population health and reproduction at the University of California – Davis School of Veterinary Medicine, says direct DNA and linkage tests can serve an important role for owners and breeders.

"A person who buys a purebred puppy doesn’t necessarily have to concern themselves with genetic testing, although they can ask if the tests have been done and if the dogs are clear," she says. "But for people who are breeding dogs, it makes it possible to produce dogs that do not have diseases, and from the standpoint of making or producing healthy puppies it’s very important."

Linkage tests do have limitations, she adds. "There can be errors," she says. "But they can be helpful for breeders to make better decisions."

Predisposition and clinical exams

While genetic testing is one way to reduce or eliminate illnesses, owners must also rely on traditional examinations to evaluate their dog’s health. Clinical exams including X-rays and MRIs are still the gold standard for orthopedic problems such as hip and elbow dysplasia (although genetic tests may one day be available for these conditions, too). Enzyme tests can also determine the presence of disease and illnesses.

Some types of dogs or breeds are predisposed to disorders. Large dogs for instance may to have joint problems such as hip dysplasia. Small dogs with long torsos, such as Basset hounds, dachshunds and beagles, are predisposed to back problems.

"When you are aware these conditions are a problem, a savvy owner will ask a breeder what tests were done on the parents, and if the parents ever produced animals with the diseases they are interested in," says Rory Todhunter, director, medical genetic archives and professor of surgery at the College of Veterinary Medicine at Cornell University. "You may find that people have part of the story, don’t know the story at all, or don’t screen. But an intelligent person can start by asking these questions."

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