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 certain 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 an institution located in the East Coast or elsewhere 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 (note that pit bull types were evaluated separately) seen at UC Davis, which is 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, we have simply paraphrased what they stated and referenced as their supporting evidence.
While this was a good foundation survey, it was not an in-depth analysis. Sometimes, when the sample population (in this case over 27,000 dogs) becomes too large, things can be overlooked. Additionally, rapid advancements in veterinary diagnostics and major sociological trends have occurred between and since 1995-2010.
Hypothyroidism
For example, we 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 whether they 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 it 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 has included both TGAA and TSH as a part of its complete thyroid testing panel since 2016; but were these tests included and relied upon in their published 2013 paper?
By the way, we suggest testing dogs for hypothyroidism with Hemopet’s Thyroid Profile 5™, which measures T4, freeT4, T3, freeT3 and TGAA.
Sociological Shifts
For decades, the predominant theory has been that mixed breed dogs are healthier than purebreds, presumably because of their outbred “hybrid vigor”. Based on observational experiences, most veterinarians would agree with that sentiment. In thinking about the dogs we’ve examined and treated for the past 60 years, we 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, however, we have witnessed major 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 and Goldendoodle. Do not misunderstand us, they were bred very carefully over several generations to be a lower allergy guide and companion dog. Unfortunately, some breeders have embraced this marketing niche by 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 you will love and that fits best in your home and lifestyle. We 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.
References
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, https://avmajournals.avma.org/view/journals/javma/242/11/javma.242.11.1549.xml.
Dodds, W. Jean., DVM, and Diana Laverdure, MS. The Canine Thyroid Epidemic: Answers You Need for Your Dog. Wenatchee, WA: Dogwise Pub., 2011, 192 pp, https://www.amazon.com/Canine-Thyroid-Epidemic-Answers-Need/dp/1617810169.