Breed-Associated Disease: A Very Short Overview
The first step in my argument will be traversed the most quickly: my claim is that pedigree-breeding produces comparatively more disease than outbreeding. As a historical claim this seems almost indisputably true, and I strongly suspect that it is true of necessity.
Pedigree-breeding causes disease in two ways. First, the various physical attributes for which we have selectively bred dogs and cats—large heads; excessive skin folds; short, stubby legs; corkscrew tails; short, “smushed” faces; distinctive coats; and so on—can directly cause disease or predispose the animal to disease. One recent review classifies these as “conformation-related” diseases and identifies a total of eighty-eight distinct conformational attributes in the top fifty UK breeds of dog causing or predisposing animals to disease (Asher et al. 2009). An example of a conformation-related disease in dogs is Brachycephalic Airway Obstruction Syndrome (BAOS), seen in brachycephalic (“short-faced”) breeds such as English Bulldogs and Pugs. In BAOS, a constellation of common anatomic features in the animal’s upper airway makes it difficult for them to breathe comfortably; these features are a direct result of selection for these breeds’ characteristic facial appearance. At the milder end, BAOS can result in uncomfortable breathing and exercise intolerance. At the more severe end, BAOS can result in respiratory distress, loss of consciousness, and occasionally even death, either spontaneously or because of euthanasia (Gough and Thomas 2004; Lodato and Hedlund 2012).
The second mechanism underlying breed-associated disease arises from the fact that pedigree-breeds are created and maintained through inbreeding in a closed population. It is well known that inbreeding increases the chance of an individual’s developing recessive genetic diseases, and this has been seen with pedigree-breeds. A recent review identified over three hundred breed-associated inherited diseases in dogs that were not conformation-linked, that is, that were due to the tendency of pedigree-breeding to cluster disease-causing genes. These diseases include heart defects, skin problems, cancers, neurologic diseases, and many others. By definition, these are diseases for which pedigree-breeds are at an increased risk as compared to mixed-breeds (Summers et al. 2010; see also Gough and Thomas 2004).
This is not the place to discuss in detail the welfare impacts of breed-associated diseases or their risk assessment, but two important points will be made. First, though the severity of breed-associated diseases is variable, they often impose a substantial welfare burden on affected animals, in terms of discomfort, pain, impaired functioning, and early death (Rooney and Sargan 2009; Gough and Thomas 2004). Second, many breed-associated diseases occur commonly. In some cases we can base this judgment on prevalent data, but since these data are often unavailable, in other cases the judgment is based on the clinical experience of many veterinarians, including my own when I was in practice. We also know that relative risks for breed-associated diseases are often significantly elevated in pure-breeds compared to mixed-breeds. Further, when mating two purebred animals, knowledge of the parents’ ancestry and genetics, as well as the anticipated anatomy of the offspring, will sometimes give us more specific reason to think that breed-associated disease will occur. These points are worth emphasizing because some stakeholders have called for more research on the incidence and prevalence of these diseases before action is taken. According to the argument presented in the next section, additional research would be helpful but not necessary.
Two further notes are in order. First, the availability of medical and surgical treatments for breed-associated diseases does not compensate for or nullify the added harm associated with pedigree-breeding. Treatments are not always available, and are often of variable efficacy when available. Treatments (e.g., surgery) often add morbidity to an animal’s condition, and even if a condition is corrected, it does not make up for the welfare cost imposed by the disease before it was treated. And, perhaps most importantly, treatment is often not pursued.
Second, some commentators (e.g., Jeppsson 2014) have argued that pedigreebreeding is desirable on health grounds as compared to outbreeding because pedigree-breeds sometimes receive the added benefits of additional health screens and/or selection against health problems. But this argument is misleading, because it conflates the question of whether pedigree-breeding increases disease with the question of what quality medical care we choose to provide to pedigree- versus mixed-breed dogs. There is no reason why we cannot or should not provide outbred dogs with the same quality of medical care that is sometimes provided to pedigree-breed animals.