In June 2013, the Journal of American Veterinary Medical Association published a study called “Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995–2010)” that was compiled by The University of California-Davis School of Veterinary Medicine (UC Davis).
The researchers searched all of UC Davis’ medical records for confirmed diagnoses of inheritable diseases in dogs. Overall, 22 of the 24 genetic, inherited disorders were more present in purebred dogs, but only 10 of these were believed to be statistically significant. In other words, 12 of the 24 occurred at the same rate in both mixed breed and purebred dogs with a little more in purebred dogs.
As with all research studies, there are biases and limitations. For instance, the researchers admitted a potential West Coast bias as all of the dogs were patients at UC Davis. It would be interesting if another institution like the University of Tennessee conducted a similar analysis as a counterweight the UC Davis study.
Additionally, the sample number of purebreds was 20,937, whereas 5,990 dogs represented mixed breeds (pit bull types were evaluated separately) that were seen at UC Davis, typically a referral hospital. In this scenario, the researchers stated that a referral hospital may examine more purebred dogs because their caregivers are more apt to pursue extensive treatment and spend more money than mixed breed caregivers. Inevitably, this difference would cause an overrepresentation of some disorders in purebred dogs. Remember, I have simply paraphrased what they stated and referenced as supporting evidence.
I think that while this is a good foundation survey, it is not an in-depth analysis. Sometimes, when the sample population (in this case over 27,000 dogs) becomes too big, things can fall through the cracks. Additionally, rapid advancements in veterinary diagnostics and major sociological trends have occurred between 1995-2010.
I question the number of confirmed cases of hypothyroidism. The researchers did admit that hypothyroidism demanded a more intense diagnosis. What they did not state was if this diagnosis was differentiated between heritable autoimmune thyroiditis and other familial or nutrition-based hypothyroidism.
Autoimmune thyroiditis is the heritable form of hypothyroidism and occurs in up to 90% of the canine cases. However, accurate diagnosis relies on testing the TGAA (thyroglobulin autoantibody). In some cases, circulating autoantibodies to T4 and /or T3 are present (T4AA and/or T3AA).
For years, we believed that including canine TSH (thyroid stimulating hormone) was an important indicator of thyroid function, like it is in humans. However, in dogs, TSH only has a 70% specificity rate, which means the other 30% of the time TSH provides false-positive or false-negative results. [In humans, this discordancy only occurs in 5% of cases.] The reason for this species difference is because the dog has a more active thyroid regulatory pathway through growth hormone. UC Davis does include both TGAA and TSH as a part of its thyroid testing panel in 2016; but were these tests included and relied upon in their published 2013 paper?
For decades, the predominant theory has been that mixed breed dogs are healthier than purebred, presumably because of their outbred “hybrid vigor”. Based on observational experiences, most veterinarians would agree with that sentiment. If I think about the dogs I have examined and treated for the past 50 years, I generally agree.
When this theory gained popularity, it was over 40 years ago when neighborhood dogs were literally “Heinz 57”. Back then, the spay/neuter movement was just gaining momentum. So several dogs were “the offspring of the neighborhood stud dog.”
Over the past 20 years we have witnessed massive shifts in the mixed breed dog category. Nowadays, we have an abundance of purpose bred hybrid breeds and other types of mixes called “designer breeds”. This trend is exemplified by the popularity of the Labradoodle. Do not misunderstand me, the Labradoodle was bred very carefully over several generations to be a low allergy guide dog. Unfortunately, some opportunistic breeders are mixing together an assortment of purebred breeds without considering the genetic risk factors that the individual dogs may have.
On the positive side, we have seen an increase in responsible breeding to wean out these genetic risk factors. Nevertheless, this positive influence does not negate the reality of unaware, backyard or puppy mill breeders, even though public awareness of these issues is now at an all-time high.
Ultimately, lumping all mixed breeds together in one pool can lead to disproportionate study results.
Most importantly, this or other studies should not guide you to choose a purebred or a mixed breed dog, but should help you select the dog that fits best in your home and that you will love. I strongly encourage adopting dogs from rescues or shelters. If you want an adult dog of a specific breed, look to breed-specific rescues first. If you want a puppy, then seek out responsible breeders in your area.
Beuchat, Carol, PhD. “Health of Purebred vs Mixed Breed Dogs: The Actual Data.” The Institute of Canine Biology, 29 Mar. 2015. Web. 04 Dec. 2016. http://www.instituteofcaninebiology.org/blog/health-of-purebred-vs-mixed-breed-dogs-the-data.
Bellumori, Thomas P., Thomas R. Famula, Danika L. Bannasch, Janelle M. Belanger, and Anita M. Oberbauer. “Prevalence of Inherited Disorders among Mixed-breed and Purebred Dogs: 27,254 Cases (1995-2010).” Journal of the American Veterinary Medical Association 242.11 (2013): 1549-555. Print.
Dodds, W. Jean., DVM, and Diana Laverdure, MS. The Canine Thyroid Epidemic: Answers You Need for Your Dog. Wenatchee, WA: Dogwise Pub., 2011. Print.