The assumption has long been held that the best way to avoid many inheritable diseases in dogs is to adopt a mixed breed dog, or a mutt. While many of the well known inheritable diseases have been linked to specific breeds of dogs, no study has ever set out to show the lack of these diseases in mixed breed dogs, until now.
A group of veterinary researchers from the University of California, Davis set out to investigate how frequently many of the commonly seen inheritable diseases would occur in mixed breed dogs as compared to pure bred dogs. In contrast to many veterinary studies with very small numbers of dogs, this study analyzed 27,254 dogs with 24 various diseases that are known to be inheritable. Some of the more common internal medicine diseases evaluated included Cushing’s disease, hypothyroidism, liver shunts, and Addison’s disease. Other well known conditions included many congenital heart defects, various types of cancer including lymphoma and mast cell tumor, and multiple orthopedic conditions including hip and elbow dysplasia, as well as cruciate ligament tear.
As was to be expected, 10 of the diseases were seen more commonly in pure bred dogs, including hypothyroidism, dilated cardiomyopathy, and cataracts. Somewhat surprisingly, however, 13 of the 24 diseases that were evaluated did not show any bias towards pure bred dogs compared to dog of mixed breeding. This means that there is no significant difference in risk of contracting one of these diseases for a mixed breed dog compared to a purebred. This group of diseases included Cushing’s disease, Addison’s disease, hip dysplasia, and multiple types of cancer. Cranial cruciate ligament rupture, a very common orthopedic condition, was actually seen significantly more often in mixed breed dogs.
Inherited diseases are, by definition, associated with some degree of genetic mutation. Since we know that all breeds of dogs originated from a common ancestral pool, and genetic diversification and breed selection / separation has only occurred in the more recent past, it stands to reason that diseases caused by genetic mutations long ago should not be found in any higher proportion in pure bred versus mixed breed dogs. Genetic mutations that have occurred much later in the diversification process, for example causing hypothyroidism, likely will affect a smaller subset of dogs making pure bred dogs more at risk. This may be better conceptualized using a family tree image. The common ancestors may be grouped together in the “trunk” of the tree, and any mutation occurring in this area is likely to affect all branches. A mutation that occurs on an isolated branch, or in the case of dogs after the selection process of specific breeds, is only likely to affect those specific breeds, and not the rest of the tree.
So how does this impact you when it is time to pick out a new companion? There are many positive attributes to bringing home both mixed breed dogs available at the local shelter and pure bred dogs from a reputable breeder. Deciding on what kind of dog to bring home should be a well thought out process, weighing all of the positives and negatives of the breed(s) involved. The conclusion of this study should in no way detract from adopting a mixed breed shelter dog; it should just be one more puzzle piece to help make the most informed decision.
Bellumori TP, Famula TR, Bannasch DL et al. Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995–2010). J Am Vet Med Assoc 2013; 242: 1549-1555.