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Prevalence of inherited disorders among mixed breed and purebred dogs: 27,254 cases
Dec
Bellumori, T. P., Famula, T. R., Bannasch, D. L., Belanger, J. M., & Oberbauer, A. M. (2013). Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995-2010). Journal of the American Veterinary Medical Association, 242(11), 1549–1555. https://doi.org/10.2460/javma.242.11.1549
Summary by Charlie Clarke
Background
This study aimed to address the population wide assumption that mixed breed dogs are less affected by genetic disorders than purebred dogs. This assumption is based on the fact that when you create a breed and fix the type through inbreeding and rigorous selection, you increase the likelihood that recessive traits will be expressed and affect the dog. Recessive traits aren’t inherently bad for a breed as a whole, but they require accurate testing to assess the risk of a pairing, and breeders who are willing to take those recessive traits into account. Unfortunately the genetic tests that we do have are typically single factor traits, 1 gene for 1 disorder, but will miss multifactorial traits. This means that a pairing that may appear to be clear, can still produce affected offspring. Since dogs descended from a small population of wolves, there are most likely disorder traits that will be common to all dogs in addition to the stratification across breed.
Methods
The data used in this study was obtained from UCDavis Vet records from January 1st 1995 to January 1st 2010. The genetic disorders that were looked at in this study were those that are present in the population and debilitating. The researchers also made sure that they assessed disorders across all systems of the body to make sure that the results weren’t biased by body part affected. Below is a list of all disorders looked at, separated by respective categories.
Cancers: hemangiosarcoma, lymphoma, mast cell tumor, and osteosarcoma.
Cardiac: aortic stenosis, dilated cardiomyopathy, hypertrophic cardiomyopathy, mitral valve dysplasia, patent ductus arteriosus, and ventricle septal defect.
Endocrine: hyperadrenocorticism, hypoadrenocorticism, and hypothyroidism.
Orthopedic: elbow dysplasia, hip dysplasia, IVDD, patellar luxation, and ruptured cranial cruciate ligament.
Other: atopic dermatitis, bloat, cataracts, epilepsy, lens luxation, portosystemic shunt.
Each record was screened for a confirmed diagnosis, and for accuracy of diagnosis. Epilepsy was split into confirmed, probable, and suspect categories. They also selected a control population from the records that had none of these identified disorders. Once both of the populations were generated, the dogs were split by size, age, and sex. For every condition looked at, the control set was generated 50 times to give the researchers statistical accuracy. The number of affected dogs in each breed was then compared to the number of unaffected dogs of that breed.
Results
Of the 90,000 dogs assessed for this study, just over 27,000 had at least one condition, although they were permitted to have more than one condition. 13 disorders had no significant difference across purebreds and mixed breed dogs (all of the cancers looked at, hypertrophic cardiomyopathy, mitral valve dysplasia, patent ductus arteriosus, ventricular septal defect, hip dysplasia, patellar luxation, hypoadrenocorticism, and hyperadrenocorticism). 10 disorders were found to be more prevalent in purebred dogs (aortic stenosis, dilated cardiomyopathy, hypothyroidism, elbow dysplasia, IVDD, allergic dermatitis, bloat, cataracts, total epilepsy, and portosystemic shunt). The researchers then reported the top 5 breeds for each disorder and the percentage of dogs within the breed that have a confirmed diagnosis. The mixed breed dogs were more likely to have 2 disorders (cranial cruciate ligament rupture and being hit by a car).
| Aortic Stenosis | Dilated Cardiomyopathy | Hypothyroidism | Elbow Dysplasia | IVDD |
| Newfoundland 6.8% | Doberman 7.32% | Giant Schnauzer 11.45% | Bernese Mountain Dog 13.19% | Dachshund 34.92% |
| Boxer 4.49% | Great Dane 7.3% | Irish Setter 7.69% | Newfoundland 10.28% | French Bulldog 27.06% |
| Bull Terrier 4.15% | Neapolitan Mastiff 6.52% | Keeshond 6.63% | Mastiff 6.55% | Pekingese 20.59% |
| Irish Terrier 3.13% | Irish Wolfhound 6.08% | Bouvier 6.55% | Rottweiler 6.31% | Pembroke Welsh Corgi 15.11% |
| Bouvier 2.38% | Saluki 5.88% | Doberman 6.3% | Anatolian Shepherd 5.41% | Doberman 12.17% |
| Mixed Breed 0.15% | Mixed Breed 0.16% | Mixed Breed 1.54% | Mixed Breed 0.9% | Mixed Breed 4.43% |
| Allergic dermatitis | Bloat | Cataracts | Total Epilepsy | Portosystemic Shunt |
| West Highland Terrier 8.58% | St. Bernard 3.76% | Silky Terrier 22.76% | Catahoula 3.9% | Yorkshire Terrier 10.86% |
| Coonhound 8.33% | Irish Setter 3.42% | Miniature Poodle 21.49% | Beagle 3.57% | Norwich Terrier 7.41% |
| Wire Fox Terrier 8.16% | Bloodhound 3.39% | Brussels Griffon 20.51% | Schipperke 3.42% | Pug 5.88% |
| Cairn Terrier 6.91% | Great Dane 2.8% | Boston Terrier 19.61% | Papillon 3.4% | Maltese 5.87% |
| Tibetan Terrier 5.86% | Irish Wolfhound 2.7% | Tibetan Terrier. 18.92% | Standard poodle 3.19% | Havanese 4.35% |
| Mixed Breed 1.08% | Mixed Breed 0.2% | Mixed breed 4.04% | Mixed Breed 0.9% | Mixed Breed 0.35% |
Discussion
This paper found that purebred dogs are more likely than mixed breeds to suffer from genetic conditions. What’s important to remember is that no one breed dominated all of these conditions. Each breed has genetic conditions that are likely associated with the selection that was necessary to create breed type. Patellar luxation was found to have insignificant difference between purebreds and mixed breed dogs, but is very significant if you compare small dogs to large dogs. The second takeaway is that there are conditions common to all dogs. These conditions could have served an important role during the domestication process or they could be environmentally influenced. When looking at the results of this study, it’s critically important to remember that when bringing home a purebred dog there are ways to mitigate risk. Disorder testing the parents with tests that are breed specific, and tracking disorder prevalence within your lines can give an idea as to the risk of multifactorial disorders. The first potential limitation of this method design was that a specific disorder in a popular breed could artificially bias the results of purebreds as a whole when the specific disorder is mostly limited to that breed. The second limitation of this study is that these dogs were all evaluated at a teaching hospital and that referral out can bias purebred diagnosis if there is a history of a specific disorder in the breed. Additionally, there will be some dogs sorted into the healthy population that may simply have not yet achieved diagnosis due to age of onset or expense of diagnosis. Acute onset disorders could also be underrepresented in this data set because the owner may choose to work with their primary vet in an emergency situation. Despite these limitations, the experimental method did its best to control for these variables and to limit their ability to affect the results by utilizing a huge population of dogs and statistical controls.