3.8.1 Summary
We see genetic disease in patients every day. The hallmark of genetic disease is its predictability. Therefore, we have the opportunity to intervene and lessen its impact for our patients. This includes prepurchase discussions with clients, recommendations on initial examination and throughout the life of the patient, and prebreeding evaluation. The hospital team should be knowledgeable about the genetic tests available, their proper use and interpretation, breed‐specific hereditary predispositions, and proper genetic counseling for both owners and breeders of dogs and cats.
3.8.2 Terms Defined
Purposefully Bred: A description indicating the intentional breeding of two animals, compared to the random matings that tend to occur in nature.
The predisposition to genetic diseases is lifelong. Consequently, these must be treated as chronic diseases and not just requiring intermittent treatment during clinical episodes. The opportunity to genetically counsel our clients occurs throughout the pet's life (see 1.2 Providing a Lifetime of Care). The most common examples are allergies in dogs and feline lower urinary tract disease in cats.
3.8.3 On a Client Acquiring a New Pet
If a client asks about adopting or purchasing a new dog or cat, the hospital team can discuss breed or mixed‐breed anatomical and behavioral expectations and whether they fit with the client's lifestyle and home environment (see 3.10 Advising Clients on Selecting an Appropriate Pet). Terrier and terrier‐type dogs have very different behavioral characteristics from retrievers or shepherds. Long‐haired versus short‐haired cats and dogs have different grooming requirements.
If the client is purchasing a purebred dog or pedigreed cat, there are breed‐specific prebreeding health screening recommendations for the parents. For dogs, these can be found at the OFA Canine Health Information Center (www.ofa.org/breedtests.html). Common genetic disorders and tests for cat breeds can be found on the International Cat Care website (https://icatcare.org/advice/?per_page=12&categories=cat‐breeds).
Online resources contain information on DNA tests available in dogs and cats from multiple laboratories. One can search by breed, disease, or laboratory. Some resources also link to the original published article on each disease test, as well as which laboratories run the tests: (http://research.vet.upenn.edu/WSAVA‐LabSearch). Such resources may list identified mutations, including rare mutations that may not be present in breeding populations. Many commercial genetic testing companies test and report unvalidated mutations that have no relevance to the dog or cat being tested. Caution and diligence must be exercised when providing counseling recommendations based on genetic test results (see 3.6 Genetic Testing).
Health‐conscious breeders are happy to provide official documentation of the results of health screening. The results of a puppy's parents' health screening can be looked up on the OFA website (www.ofa.org). Many other countries have searchable health testing databases, although there are more for dogs than for cats.
Some pure breeds and purposefully bred mixed‐breeds (designer or “bred‐for‐rescue” dogs) do not have specific prebreeding health screening requirements. All parents of purposefully bred litters should have prebreeding health examinations (musculoskeletal, heart, eye, etc.) and history taken for episodic hereditary disease (allergies, seizures, chronic gastrointestinal disease, etc.) to determine their suitability for breeding (see 3.4 Predicting and Eliminating Disease Traits). Prebreeding health evaluations should become as routine and commonplace as equine prepurchase examinations.
In most instances, the veterinary team is not asked for advice by a client prior to a purchase or adoption. When an appointment is being made for examination of a new purposefully bred puppy or kitten, the owner should be told to bring all paperwork provided by the breeder, pet store, broker, or agent. If evidence of health screening on the parents has not been provided, health test results that may have been performed on the parents may be looked up in online health testing databases such as the OFA.
If health screening results are not available, one can access the prebreeding health screening requirements or testable diseases and have the owner ask the breeder for health testing information on the parents. This may be the only way to educate the owner and breeder on the ethical obligation of health screening and health‐conscious breeding. Selective pressure is the only way to reduce the frequency of genetic disorders in purposefully bred dogs and cats. Health‐screened parents produce healthier kittens and puppies.
3.8.4 Genetic Counseling in Owned Animals
Veterinarians and the veterinary team should be knowledgeable about which genetic disorders have appropriate testing available, and in what patients they should be run (see 3.4 Predicting and Eliminating Disease Traits). If parental documentation of genetic testing is not available, certain breeds should undergo genetic testing early in life (see 3.6 Genetic Testing). For example, patients from breeds with an incidence of von Willebrand disease should be tested so that measures can be taken to prevent excessive hemorrhage during surgery or injury. Patients at risk of carrying the mdr1 mutation (mostly herding breeds) should be tested before drug treatment. Owners of large‐breed puppies can be counseled to feed lower calorie “large‐breed growth or puppy” foods to provide for a more uniform growth rate and better joint development. Boxers should be tested while young for the dominant arrhythmogenic right ventricular cardiomyopathy (ARVC) gene. Carriers can be auscultated for arrhythmias through life, and if they occur can be put on antiarrhythmic drugs to prevent heart failure.
As more research is being done, more disease liability genes are being identified. Some of these genes occur in only a small “familial” population of a breed or only in a research setting while some cause breed‐wide disease.
The focus of molecular genetic testing is now shifting to panel (multiplex) testing that includes many mutations found across all breeds. It is much less expensive to use a gene chip that runs hundreds of genetic tests for mutations, traits, and ancestry on a single sample. This is what is found with several commercial genetic testing companies for dogs and cats. With the results of such testing, genetic counseling becomes more important. Many genetic mutations that cause disease in some breeds have no health consequences in other breeds. An example is the sod1 mutation test for liability to develop degenerative myelopathy (DM). This is a significant clinical disease in eight breeds, although the sod1 mutation is found across all breeds and mixed‐breed dogs. The presence of the gene mutation is not predictive for clinical disease if the breed is not actually predisposed to the disease. Genetic testing companies should provide some genetic counseling based on the breed or mixed‐breed status of the specific patient. Decisions
