Global animal welfare concerns having orthopaedic origin constitute a vast topic, not compressible within a 50-minute presentation. To condense the information we will apply an algorithm based on prevalence, severity, and proportion of life afflicted, to determine the orthopaedic conditions on which to focus. This presentation will be confined to animal welfare issues of companion animals in the developed world.
Orthopaedic genetic diseases as consequence of errant breeding practices are typically singled out as the genesis of all animal welfare concerns in companion animals, particularly dogs. However, there are other lesser issues that impact animal welfare. For example trauma can result from irresponsible pet ownership, causing conditions such as fractures secondary to automobile trauma or gunshot injuries. Musculoskeletal injuries can result from excessive exertion in the many sporting events popularized recently. Service dogs (with their handlers) at times must work in harsh environmental conditions. While these non-genetic sources of welfare concern may be occurring at some level, they are rare when compared to the many genetic diseases and purposeful genetic defects that exist in pure and mixed-breed dogs today.
Establishing a Welfare Impact Score
There are two types of genetic orthopaedic conditions, those that are undesirable causing pain or disability, and those that are desirable as defined by the breed standard - but also capable of causing pain and disability. For our purposes, the former class will be termed 'diseases,' the latter 'conditions.' One study reported that each of the top 50 breeds of dogs was predisposed to at least one disorder linked to their conformation and many breeds have multiple disorders.1 Breed standards change with time and the tendency has been to breed for extreme morphological characteristics that are often detrimental to the health of the animal. A common sense "welfare impact score" (WIS) has been developed to quantify and prioritize inherited disorders creating welfare concerns. Without going into details, the WIS is the product of the prevalence of the disease times the severity of the disease times the proportion of the animal's life with the affliction. For many diseases the data are incomplete, particularly prevalence data. However, in the case of orthopaedic conditions/diseases such as hip dysplasia, elbow dysplasia, patella luxation, and osteochondrodysplasia and many others, the diseases are highly prevalent (particularly hip dysplasia), they manifest early and they cause variable degrees of pain and disability for the life of the dog. Such conditions from an animal welfare perspective then should receive high priority to lessen their severity and reduce their prevalence.
Genetics of Orthopaedic Conditions/Diseases
Most orthopaedic conditions/diseases of the dog involve multiple genes. Such phenotypes are termed quantitative traits. A quantitative trait or disease, such as hip dysplasia, has both genetic components and non-genetic components, sometimes called environmental factors. The relative role (%) the genes play in coding for a phenotype is represented by the metric known as heritability (h2). Heritability runs from 0 to 1 with 1 meaning that the variation in the phenotype of interest within a given population is totally explained by the genes. Conversely, a h2 of 0 means the genes play no role in the ultimate phenotypic variation. Heritability is an extremely important concept because the rate of making genetic change, say to improve the quality of hips, is largely determined by the magnitude of heritability. The other determinant of genetic change is 'selection pressure.' Selection pressure is determined by subtracting the mean parental phenotype from the mean population phenotype. So the more distant the parents are from the population average, the more rapid the genetic change. These two metrics are important to know because the rate of genetic change expected in a single generation of breeding a sire with a dam is simply the heritability times the selection pressure applied. In many of the diseases/conditions to be mentioned below, various forms or early detection are used, often called screening tests. For screening to be effective, the test phenotype, say hip score at 2 years of age, must have close association with the ultimate disease of interest, which in the case of hip dysplasia is the development of osteoarthritis (OA). The screening phenotype should also have maximum h2 and there must be the ability to apply selection pressure. In other words, if all members of a given population have the same trait, selection pressure cannot be applied (i.e., the parental average is the same as the population average). Irrespective of heritability then, no further genetic improvement can be made. In such case, the disease or condition is said to be 'fixed' in the population. We will see that some conditions/diseases are fixed qualitatively, if not quantitatively. A few of examples follow.
Diseases and Conditions
For the purpose of this presentation a few of the most prevalent and welfare-impactful orthopaedic diseases/conditions will be discussed. By welfare-impactful is meant that the diseases/conditions are extremely prevalent and cause pain, largely from osteoarthritis, and loss of mobility.
We probably know more about canine hip dysplasia (CHD) and its genetic origins than the other orthopaedic diseases so it will be treated first. Canine hip dysplasia (CHD) is the most prevalent genetic orthopaedic disease in the dog. Prevalence figures vary by breed but can be as high as 90% of the breed. Hip dysplasia is associated with hip joint laxity early in life, ultimately leading to radiographic osteoarthritis and the pain and disability that accompany it. It can occur in any breed but has greatest morbidity in large-breed dogs. Accordingly, it is very much a concern from an animal welfare perspective.
Can CHD be reduced in prevalence? The answer is yes; if there is 1) a screening phenotype closely related to the ultimate development of OA, 2) a moderate or high h2, and 3) the ability to apply selection pressure. The most common hip-screening test used globally is scoring of hip phenotype from a hip-extended radiograph of dogs at 1 or 2 years of age. While the scoring systems vary somewhat, they have in common the assessment of hip laxity and the presence of OA.
In a recent study from The Seeing Eye, Inc. in Morristown, NJ, USA, it was found in 3 breeds of dog guides, Labrador retrievers, golden retrievers, and German shepherd dogs, that hip improvement had been made after 8 generations of selection based on hip-extended scoring incorporated into estimated breeding values (EBV).3 In fact, in generation 8, greater than 90% of offspring at 14–16 months of age had hip scores in the best category, 'Excellent.' The remaining 5–10% were scored as 'good.' The heritability of hip score was 0.16 in the Lab, 0.21 in the Golden, and 0.32 in the GSD. Despite the excellent hips, a portion of these dogs still goes on to develop hip dysplasia. Unfortunately, no further hip improvement can be made since no selection pressure remains. Results from a newer test that more accurately measures hip laxity, showed that well over 50% of these dogs with excellent hips at a young age were at risk to express hip OA later in life.
Currently, there is no gene-based test to screen for CHD; however, several regions within specific genes have been identified. An accurate gene test for CHD will not be available for years.
Like CHD, elbow dysplasia (ED) is a disease, not a condition. Unlike hip dysplasia, however, much less is known about elbow dysplasia. Elbow dysplasia is thought to be caused by a mismatch in growth of the radius and ulna. Like hip dysplasia, ED results in progressive OA of the elbow joint causing pain and impaired mobility. Diagnosis and screening is based on one or more radiographic views of the elbow. Estimates of prevalence (compiled by the Orthopedic Foundation for Animals [OFA]) vary in the seven most popular breeds ranging from 11% in Labs and Goldens to 40% in Rottweilers. The disease is known to be heritable in many breeds (h2 = 0.25 to 0.28).4 Despite the low heritability, however, some reduction in prevalence in response to selection has been reported in some of the more commonly afflicted breeds, such as Rottweilers and Bernese Mountain Dogs.
Patellar instability is a genetic disease that is most commonly observed in small-breed dogs (Pomeranian, Yorkshire Terrier, Cocker Spaniel, and Chow Chow, to name a few), but the disease is observed in large dogs too. Based on OFA data, the highest reported prevalence is 41% in the Pomeranian breed. Diagnosis and screening is based upon palpation and is divided into 4 grades of severity. The breed disposition of patellar instability suggests a heritable trait. Like CHD, it is thought to be polygenic and multifactorial; however, estimates of heritability are presently not available. Application of selection pressure has shown some moderate decrease in incidence within the few breeds studied. There is no DNA test and molecular genetic research has been disappointing.
Osteochondrodysplasia (OCD) is a growth and developmental abnormality of the bone and cartilage, that results in a lack of normal bone growth and subsequent bone deformities. In some breeds like the great Pyrenees, Alaskan malamutes, Labrador retrievers, it is a disease definitively departing from the breed standard, while in other breeds such as dachshund, Welsh corgi, Basset hound, Pekingese, and bulldog, it is a condition, a desirable genetic characteristic of the breed standard and therefore selected for. Osteochondrodysplasia is an autosomal dominant genetic disorder, meaning that it can be passed along by either gender and only one parent need carry the gene for an offspring to be affected. The clinical appearance of leg and/or skull deformities makes for easy diagnosis. To avoid the disease, refrain from breeding affected dogs. Dogs with severe angular limb deformities may require surgical correction.
Other orthopaedic diseases/conditions or trauma with animal welfare impact will be included if time permits.
1. Collins LM, Asher L, Summers J, et al. Getting priorities straight, risk assessment and decision-making in the improvement of inherited disorders in pedigree dogs. Vet J. 182;2009:402–411.
2. Smith GK, Karbe GT, Agnello KA, McDonald-Lynch MB. Pathogenesis, diagnosis, and control of canine hip dysplasia. In: Tobias KM, Johnston SE. Veterinary Surgery: Small Animal. Volume 1, Chapter 59. St. Louis, MO: Elsevier/Saunders; 2012: 824–848.
3. Smith GK, Leighton EA. Can the hip-extended radiographic score genetically eliminate hip osteoarthritis? Keynote Presentation. In: Proceedings from the 8th International Working Dog Conference. San Antonio, TX, 2013: 2.
4. Beuling R, Mues CH, Tellhelm B, et al. Prevalence and inheritance of canine elbow disease in German Rottweilers. J Anim Breed Genet. 2000;117:375–383.