1336 JAVMA| JUN 15, 2020| VOL 256| NO. 12 changes to joints. Larger-breed dogs are commonly thought to develop osteoarthritis at an earlier age, with more severe clinical signs than smaller breeds. 24 In a records-based study25 of 148,741 dogs from 93 clinics across England, researchers found that some purebred dogs had a higher prevalence of degenerative joint disease than others, suggesting inherited genetic defects in particular breeds. Other study26, 27 results indicate that Labrador Retrievers in particular have risks for hip dysplasia, elbow dysplasia, and cranial cruciate ligament rupture that are 3.4, 20.5, and 5.5 times those for mixed-breed dogs. Other factors such as the age and sex of an animal also play roles in the development of osteoarthritis. Although osteoarthritis can develop at any age, its higher prevalence in older dogs indicates that it is a disease associated with aging. It is often a result of long-term wear and impact on joints, with higher prevalence and frequency of diagnosis in older dogs when mobility is affected and obvious clinical signs are observed. Less information is available for cats, but studies have shown that 16.5% to 91% of cats (prevalence increasing with age) have radiographic evidence of osteoarthritis even when clinical signs are not observed. 7–9 Although the exact mechanisms by which age affects the development of osteoarthritis are unknown, it is likely related to a combination of changes in the capacity of joint tissues to adapt to biomechanical damage. 28

The sex of animals also has been associated with osteoarthritis risk, with male dogs having a greater risk of developing hip joint osteoarthritis than females. 29 In people, sex hormones (gonadal steroid hormones) modulate the joint tissues during tissue development and throughout the life cycle. 30 Although there is little evidence available on the means by which sex hormones influence the joint tissues of dogs, it has been shown that chondrocytes have unique responses to sex hormones owing to differing actions of the hormones and receptor numbers in people. 30 Although many factors may contribute, the increasing incidence of osteoarthritis may be linked with obesity in dogs. 31, 32 Obesity and overweight status in dogs have been defined as body weights that are 20% and 10% greater than the ideal weight, respectively. 33, 34 Obesity is a common problem, affecting an estimated 59.5% of cats and 55.8% of dogs in the United States. 35 It has also been defined as having sufficient excess body fat to cause or contribute to disease. 14 The long-term stress exerted on joints by excess body weight has been associated with loss of articular cartilage in people and dogs. 15, 36 In addition, obesity has been associated with increased oxidative stress and inflammation, which can cause cellular injury and tissue damage. 14 The reported high prevalence of obesity, particularly in mature patients, and the high frequency of osteoarthritis in dogs and cats emphasize the importance that veterinarians should place on educating owners about proper pet nutrition and healthy weight status.