Mixed Breed Dogs Are Not Protected From Breed Disease Heritage

A canine research group, led by Thomas Bellumori, from the University of California Davis has mapped the prevalence of 22 different hereditary disorders using health information from 27 254 dogs from a major animal hospital in the United States. With this huge amount of data they were able to compare the prevalence of some of the known hereditary diseases between different breeds.

It has been publicly discussed for years that hereditary disorders would be a direct consequence of the strict selective breeding of pedigree dogs and that for this reason the purebreds would have a much greater risk of developing hereditary disorders than the mixed breed dogs. According to the latest research by Bellumori and his group, this assumption does not seem to hold. Indeed many diseases seem to be as common in mixed breed as in pedigree dogs. The result might be explained with the fact that many mutations are widely spread in the entire dog population. In this case dogs of different breeds can carry the same mutation causing affected puppies.

Table 1: Comparison of disease prevalence between purebred and mixed breed dogs
 

More common in
purebred dogs
Equally common in purebred
and 
mixed breed dogs
More common in
mixed breed dogs
Aortic stenosis Hyperthropic cardiomyopathy Ruptured cranial cruciate ligament
Dilated cardiomyopathy Mitral valve dysplasia  
Elbow dysplasia Patent ductus arteriosus  
IVDD Ventricular septal defect  
Hypothyroidism Hemangiosarcoma  
Allergic dermatitis Lymphoma  
Bloat Mast cell tumor  
Cataracts Osteosarcoma  
Epilepsy Hip dysplasia  
Portosystemic shunt Patellar luxation  
  Hyperadrenocorticism  
  Hypoadrenocorticism  
  Lens luxation  

Although the number gives a hint on how widespread the diseases are, it has to be remembered that the genetic causes have not been confirmed in this study. The same disease can be caused by a number of mutations that may also vary between breeds. Some of the diseases are not solely genetic and especially the different cancers are usually caused by the joint effect of heritage and environment. In the article it is evaluated that pedigree dogs are more often diagnosed than mixed breed dogs, which could bias the results to the mixed breed favour.

In addition to the hereditary disease information, the group also collected statistics about the accidental deaths of dogs. In this data mixed breed dogs were clearly overrepresented. Quite curiously, it is mentioned in the article that a mixed breed dog more often dies of getting under a car. Looking at the onset ages of the diseases, which is in most of the presented cases higher than the mean age of accidental death (4.9 years), we see that mixed breed dogs might not live long enough to express its diseases as often as the pedigree dogs.

Based on the research, it still seems that ther would however be certain diseases that would be more enriched in purebreds, because the pourcentage is so much higher than in mixed breed dogs. Aortic stenosis, elbow dysplasia, IVDD, hypothyroidism and portosystemic shunt show a clear prevalence to a certain breed, with a clearly bigger part affected than in the other top five breeds on the list. This raises the question: Why? Could it simply be because of poor luck or is the disease inherited together with a desirable trait? In the other diseases, more commonly found in purebred dogs, the breed with the highest prevalence does not stick out and the second question is: What do these breeds have in common? Unfortunately, these questions will go unanswered for now.

For further canine genetic research these results may help idetify in which breeds to search the genetic causes for the diseases. Some of the diseases presented in the article already have a genetic test developed and some of these have also made their way into the MyDogDNA-analysis. However, many times a disorder can be caused by a number of mutations and a single gene test can only capture one of the genetic causes. Unfortunately, except for MyDogDNA, the results of commercial gene tests are not collected into one database for researchers to check whether an expected amount of gene tests turn out positive compared to existing clinical data, as the one collected by Bellumori et al.

Table 2: Disease prevalence within different breeds
 

Disease Position Breed Part of breed affected (%)
Aortic stenosis 1. Newfoundland 6.8
  2. Boxer 4.5
  3. Bull Terrier 4.1
  4. Irish Terrier 3.1
  5. Bouvier des Flandres 2.4
Dilated cardiomyopathy 1. Doberman Pinscher 7.3
  2. Great Dane 7.3
  3. Neapolitan Mastiff 6.6
  4. Irish Wolfhound 6.1
  5. Saluki 5.9
Elbow dysplasia 1. Berneses Mountain Dog 13.9
  2. Newfoundland 10.3
  3. Mastiff 6.6
  4. Rottweiler 6.3
  5. Anatolian Shepherd 5.4
IVDD 1. Dachshund 34.9
  2. French Bulldog 27.1
  3. Pekingese 20.6
  4. Pembroke Welsh Corgi 15.1
  5. Doberman Pinscher 12.7
Hypothyroidism 1. Giant Schnauzer 11.5
  2. Irish Setter 7.7
  3. Keeshond 6.6
  4. Bouvier des Flandres 6.6
  5. Doberman Pinscher 6.3
Allergic dermatitis 1. West Highland White Terrier 8.6
  2. Coonhound 8.3
  3. Wirehaired Fox Terrier 8.2
  4. Cairn Terrier 6.9
  5. Tibetan Terrier 5.9
Bloat 1. Saint Bernard 3.8
  2. Irish Setter 3.4
  3. Blood Hound 3.4
  4. Great Dane 2.8
  5. Irish Wolfhound 2.7
Cataracts 1. Silky Terrier 22.8
  2. Miniature Poodle 21.5
  3. Brussels Griffon 20.5
  4. Boston Terrier 19.6
  5. Tibetan Terrier 18.9
Epilepsy 1. Catahoula Leopard Dog 3.9
  2. Beagle 3.6
  3. Schipperke 3.4
  4. Papillon 3.4
  5. Standard Poodle 3.2
Portosystemic shunt (PSS) 1. Yorkshire Terrier 10.9
  2. Norwich Terrier 7.4
  3. Pug 5.9
  4. Maltese 5.9
  5. Havanese 4.4

 

Reference:

Thomas P. Bellumori, MD; Thomas R. Famula, PhD; Danika L. Bannasch, PhD; Janelle M. Belanger, MS; Anita M. Oberhauer, PhD

"Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995-2010)"

Journal of the American Veterinary Medical Association, June 1, 2013, Vol. 242, No. 11, Pages 1549-1555
doi: 10.2460/javma.242.11.1549