Many of us talk about “hybrid vigor”- the notion that mixed-breed dogs avoid the inherited medical maladies passed along to their purebred counterparts. Does proof exist that mixed-breed dogs are indeed healthier? An article within the most recent edition of the Journal of the American Veterinary Medical Association helps answer this question. Titled “Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995-2010)”, this study determines the proportion of mixed-breed and purebred dogs with 24 common genetic disorders including four types of cancer, orthopedic issues, birth defects, bloat, hormonal imbalances, eye issues, allergic dermatitis, and epilepsy.
Compared to their purebred counterparts, mixed-breed dogs were more susceptible to only one inherited disorder- tearing of the cranial cruciate ligament (the main stabilizing ligament within the knee).
Purebred dogs were more likely to develop ten specific inherited disorders including:
Aortic stenosis (a birth defect within the heart)- Breeds most affected: Newfoundland, Boxer, Bull Terrier, Irish Terrier, Bouvier des Flandres
Dilated cardiomyopathy (disease of the heart muscle resulting in heart failure)- Breeds most affected: Doberman Pinscher, Great Dane, Neapolitan Mastiff, Irish Wolfhound, Saluki
Hypothyroidism (inadequate production of thyroid hormone)- Breeds most affected: Giant Schnauzer, Irish Setter, Keeshond, Bouvier des Flandres, Doberman Pinscher
Elbow dysplasia (malformation within the elbow joint)- Breeds most affected: Bernese Mountain Dog, Newfoundland, Mastiff, Rottweiler, Anatolian Shepherd
Intervertebral disk disease (slipped disk)- Breeds most affected: Dachshund, French Bulldog, Pekingese, Pembroke Welsh Corgi, Doberman Pinscher
Allergic dermatitis (skin allergies)- Breeds most affected: West Highland White Terrier, Coonhound, Wirehaired Fox Terrier, Cairn Terrier, Tibetan Terrier
Bloat (gastric torsion, twisting of the stomach)- Breeds most affected: Saint Bernard, Irish Setter, Bloodhound, Great Dane, Irish Wolfhound
Cataracts- Breeds most affected: Silky Terrier, Miniature Poodle, Brussels Griffon, Boston Terrier, Tibetan Terrier
Epilepsy- Breeds most affected: Catahoula Leopard Dog, Beagle, Schipperke, Papillon, Standard Poodle
Portosystemic shunt (a birth defect causing shunting of blood around rather than through the liver): Breeds most affected: Yorkshire Terrier, Norwich Terrier, Pug, Maltese, Havanese
No differences in disease incidence between the mixed-breed and purebred groups were found for 13 different disorders including:
The cancers evaluated (hemangiosarcoma, lymphoma, mast cell tumor, osteosarcoma)
Hypertrophic cardiomyopathy (a disease of the heart muscle)
Cardiac birth defects (mitral valve dysplasia, patent ductus arteriosus, ventricular septal defect)
Patellar luxation (displacement of the knee cap)
Addison’s disease (a hormonal imbalance)
Cushing’s disease (a hormonal imbalance)
Lens luxation (displacement of the lens within the eye)
The authors of this study surmised that genetic mutations resulting in inherited defects may have developed at different times during the evolution of dogs. For example, a mutation introduced very early on into the canine genome (closer in time to the wolf progenitor) would have been spread throughout the entire dog population, purebred and hybrid alike. For disorders affecting purebred dogs in higher proportions, the underlying genetic defect may have occurred more recently, after the gene pools for a particular breed (or related breeds) evolved.
It is possible that the same genetic traits that predisposed to domestication are chromosomally connected to the genetic traits that predisposed to the disorders shared equally by purebred and mixed-breed dogs.
It is possible that the hard wiring for some of the genetic disorders is chromosomally linked to the DNA that determines dog size rather than dog breed. While patellar luxations and lens luxations occur without breed specificity, they do occur primarily in small dogs. Conversely, cancers are more likely to occur in large breed dogs.
Lastly, the results of this research may have been influenced by the hospital population (study performed at UC Davis). For example, we know that Standard Poodles are genetically predisposed to Addison’s disease, yet the results of this study do not demonstrate this. If the study population contained an overabundance of Standard Poodles (evaluated for a wide array of diseases), it is possible that their specific predilection for Addison’s disease may have been “diluted out”.
So now what do we have to say about hybrid vigor? Clearly, for some inherited diseases, this theory fits. For others, it is inapplicable.
Do the prospects for future diseases influence which dogs you choose to adopt?
Nancy Kay, DVM
Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
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Please visit http://www.speakingforspot.com to read excerpts from Speaking for Spot and Your Dog’s Best Health. There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at www.speakingforspot.com, Amazon.com, local bookstores, and your favorite online book seller.