Risk Factors for Arthritis in Dogs

Last year, I participated in a research project evaluating risk factors for the diagnosis of osteoarthritis in dogs, using the large dataset in the Banfield medical records system. The research was published in November, 2023-

Graves JL, McKenzie BA, Koch Z, Naka A, Spofford N and Morrison J (2023) Body weight, gonadectomy, and other risk factors for diagnosis of osteoarthritis in companion dogs. Front Vet Sci 10:1275964.

This week, Dr. JoAnn Morrison from Banfield and I had the opportunity to present our research at the annual American Veterinary Medical Association convention. While this sort of research is not as drastic or “media-friendly” as reports of astonishing new discoveries, it is a critical foundation for understanding the health conditions our pets experience and how we can best identify and treat them or, ideally, prevent them.

RISK FACTORS FOR DEVELOPMENT OF OSTEOARTHRITIS IN COMPANION DOGS: RESULTS OF A LARGE RETROSPECTIVE STUDY 

BASICS OF OSTEOARTHRITIS
Osteoarthritis (OA) is the most common joint disorder in dogs, and it contributes significantly to pain, disability, and ultimately euthanasia in aged dogs.^1,2 The reported prevalence of OA varies between populations, and differences in methodology between studies make direct comparisons of these figures challenging. Prevalence of OA from 2.5% to over 80% have been reported, depending on factors such as the age of the population and how the condition is detected.^1,3,4

The prevalence will be higher in older populations because OA is an incurable condition and so will be present and available for diagnosis longer as dogs age. It is also progressive and more likely to become clinically apparent over time. The condition is typically detected earlier and more frequently with radiographs compared with diagnosis based on clinical examination or symptoms, such as lameness. However, while radiographs are more sensitive, they may not accurately reflect the clinical significance of OA.⁵ By any measure, however, OA is a common condition with significant impact on quality of life.

Previous studies have reported a large and varied set of potential risk factors for OA.³ Age is the strongest and most consistent of these, reflecting the nature of OA as chronic, progressive, and an aging-associated condition. Higher body weight is also frequently identified as predictive of a higher risk of OA diagnosis. However, the relative importance of body size, body condition, or some interaction between the two, is not always clear. Both larger dogs and dogs with overweight or obese body condition appear to be at higher risk.^3,6

Breed is another common risk factor, though again the contributions of body size, conformation, precipitating conditions (such as joint dysplasia or a predisposition to cranial cruciate ligament rupture), and genetics are often entangled. Neuter status, sex, activity, diet, and many other factors have been associated with OA risk in some studies. These associations are often of uncertain clinical relevance, and there are frequently conflicting findings from different studies.^3,6

Complex interactions between multiple factors likely influence the development and severity of OA in any one dog. However, a clearer understanding of the main risk factors, especially those which could potentially be modified in individuals or altered through breeding in populations, would help veterinarians reduce the harmful impact of this condition on the welfare of companion dogs. 

THE STUDY AND THE DATA
In this retrospective cohort study, Cox proportional hazard models were used to test for associations between osteoarthritis incidence and age at baseline, sex, maximum body weight, maximum body condition score, neuter status, and age at neutering. The same model was used to test these associations in 12 representative breeds, chosen based on breed weight and sample size.

THE RESULTS
Older age, higher adult body weight, gonadectomy, and younger age at gonadectomy were significantly associated with higher risks of osteoarthritis in the total cohort and in all 12 breeds evaluated. Higher body condition scores and sex were also significantly associated with osteoarthritis but with minimal effect sizes in the overall cohort, and these risk factors were not consistently significant in all breeds tested.

CLINICAL IMPLICATIONS
The results of this study confirm that OA is commonly diagnosed in companion dogs seen in primary care practice. The strongest risk factor by far was age, consistent with expectations based on previous research. Aging is currently not seen as a modifiable risk factor, but research in various species, including dogs, suggests that interventions may be developed which can alter the underlying aging process and delay or prevent some age-associated conditions, including OA. One study in Labrador retrievers found that alterations in metabolism and body composition in dogs exposed to lifelong caloric restriction were associated with delayed onset of radiographic and clinical joint disorders.^7,8 Novel approaches involving diet, exercise, and pharmaceuticals may be developed to capture these benefits in  more targeted and pragmatic interventions.

The current study also supported the importance of body weight as a risk factor for OA.  Higher body weight was significantly associated with increased risk, as was an increase in weight after full growth, which likely represented development of overweight or obesity. Larger dogs were at greater risk overall, and they were more significantly impacted by increased weight after maturity. 

Body condition score (BCS) was also associated with an increased risk of OA diagnosis, but the magnitude of this effect was small. This is likely not an indication that overweight and obesity are not important risk factors but rather that there are significant limitations of BCS as a measure of this variable. In this study, over 80% of dogs were categorized as BCS=5 (normal weight) or BCS=7 (overweight). This limited range of scores reduced the sensitivity of BCS as a measurement tool. 

BCS was a stronger predictor of OA diagnosis in smaller dogs, and there was also a wider range of scores assigned in these dogs, suggesting it may be easier for veterinarians to distinguish gradations of body condition in smaller patients. Developing more object and consistent alternatives to BCS as measures of overweight and obesity might make it easier to disentangle the relationships between body weight, body size, body condition, and OA risk.

Obviously, body condition is an important modifiable risk factor for clinicians to target. Overweight and obesity are risk factors for all-cause mortality, neoplasia, and other health conditions as well as for OA, so supporting clients in achieving a lean body condition in their dogs is a critical effort for reducing the prevalence of these conditions and improving companion dog welfare.^9,10

Neuter status has been consistently identified as a risk for OA, with intact individuals at lower risk than neutered dogs.³ There is also evidence that earlier neutering is associated with an increased risk of OA and conditions predisposing to OA, though this effect is sometimes only significant in larger dogs. Consistent with this existing literature, the current study found neutered individuals were at higher risk of OA in large and medium dogs and in some, but not most, small breeds. Younger age at neutering was also associated with OA risk, though this effect diminished rapidly after 2 years of age. This relationship was found in dogs of all sizes, so the increased risk associated with earlier neutering did not appear to impact larger dogs more than smaller breeds. 

Neutering is another potentially modifiable risk factor for OA. Owners can choose whether or not to neuter their dogs and at what age to perform the procedure. The existing evidence is consistent that delaying or forgoing neutering likely reduces the risk of OA later in life. However, the exact relationship between neuter status and OA is not clear. 

Neutering does promote overweight and obesity, and it has been associated with increased risk of conditions that predispose to OA, such as cranial cruciate ligament rupture. In this study, the effect of body weight gain after maturity on OA risk was not different between intact and neutered dogs, so it did not appear that obesity was the main factor raising OA risk in neutered dogs. One previous study also suggested that obesity mediated only a small proportion of the increased OA risk associated with neutering.¹¹ There remains, however, significant uncertainty about the exact nature of the relationship between neuter status, age at neutering, and OA. 

Based on the existing evidence, it is reasonable to consider delaying neutering until full skeletal maturity or perhaps up to two years of age, especially in larger dogs. However, discussions about this between vets and clients must include the larger context of all the known risks and benefits of neutering.^12,13 The net effect of neutering on the health and wellbeing of an individual dog is difficult to predict and will involve the influence of many other variables. Increased OA risk is only one consequence of neutering, and neuter status is only one of many factors associated with the development of OA. As always, clinical decisions should consider the complete and nuanced relationship between risks and benefits in the context of each individual patient.

Sex has been inconsistently reported as a risk factor for OA in previous research. In this study, males were at statistically lower risk than females, but the effect was small and likely not clinically meaningful. There were also breed differences in OA risk, and these were generally consistent with the pattern of larger breeds being at greater risk. 

Breed is a complex risk factor involving differences in genetic makeup, body size and conformation, and potentially also lifestyle variables influenced by owners, such as feeding practices and the type and intensity of activity. Because of the enormous phenotypic variability among dogs, evaluation of the role of body weight in OA risk can easily confound the effects of body size and breed with those of overweight and other breed differences. This study confirmed the previously reported influence of body size on OA risk. Beyond that, other relevant breed variables were not examined.

Sex is, of course, not a modifiable OA risk factor in dogs. Breed may be modifiable on a population level to the extent that genetic risk factors and conformation can be impacted by selective breeding. However, from a clinical perspective, these factors are fixed and can only be factored into the overall assessment of risk for individual patients, not targeted to reduce risk.

There are many other risk factors for OA that were not evaluated in this study. Month of birth, level and type of activity during development, diet, and factors associated with the individual patient’s medical history, such as trauma and infectious diseases, are all potentially relevant to overall OA risk.³ The findings of this study, however, reinforce some of the most significant factors that clinicians can use to assess and modify OA risk in specific patients. 

Maintaining a healthy body weight will certainly benefit all dogs. Delaying neutering may reduce the development of OA, particularly in larger dogs and those with other factors that put them at increased risk. In dogs that are at high risk due to size, breed, body condition, or personal history, more aggressive surveillance and treatment may be beneficial. As with all significant health conditions, OA risk must be evaluated and managed within the overall context of the specific case and with awareness of the nuances and uncertainties of the available evidence.

TAKE-HOME POINTS

• OA is a common and serious condition that negatively impacts the welfare of many older companion dogs
• Major risk factors for OA include
  • Older age
  • Larger body size
  • Overweight or obese body condition
  • Neutering
  • Earlier age at neutering
• The most important intervention for reducing OA risk is maintaining a healthy body condition
• Delaying neutering until full maturity or up to two years of age may reduce OA risk
  • This intervention should be considered in the context of the overall risk and benefit profile for each individual patient

REFERENCES

1. O’Neill DG, James H, Brodbelt DC, Church DB, Pegram C. Prevalence of commonly diagnosed disorders in UK dogs under primary veterinary care: results and applications. BMC Vet Res. 2021;17(1):1-14. doi:10.1186/S12917-021-02775-3/TABLES/2

2. Robinson NJ, Dean RS, Cobb M, Brennan ML. Investigating common clinical presentations in first opinion small animal consultations using direct observation. Vet Rec. 2015;176(18):463-463. doi:10.1136/VR.102751

3. Anderson KL, O’Neill DG, Brodbelt DC, et al. Prevalence, duration and risk factors for appendicular osteoarthritis in a UK dog population under primary veterinary care. Sci Rep 2018 81. 2018;8(1):1-12. doi:10.1038/s41598-018-23940-z

4. Johnston SA. Osteoarthritis. Joint anatomy, physiology, and pathobiology. Vet Clin North Am Small Anim Pract. 1997;27(4):699-723. doi:10.1016/S0195-5616(97)50076-3

5. Millis D, Tichenor MG, Hecht S, Hunt T. Prevalence of Osteoarthritis in Dogs Undergoing Routine Dental Prophylaxis. In: World Small Animal Veterinary Association World Congress Proceedings. ; 2014.

6. Graves JL, McKenzie BA, Koch Z, Naka A, Spofford N, Morrison J. Body weight, gonadectomy, and other risk factors for diagnosis of osteoarthritis in companion dogs. Published online August 3, 2023:2023.08.03.550998. doi:10.1101/2023.08.03.550998

7. Huck JL, Biery DN, Lawler DF, et al. A Longitudinal Study of the Influence of Lifetime Food Restriction on Development of Osteoarthritis in the Canine Elbow. Vet Surg. 2009;38(2):192-198. doi:10.1111/J.1532-950X.2008.00487.X

8. Kealy RD, Lawler DF, Ballam JM, et al. Five-year longitudinal study on limited food consumption and development of osteoarthritis in coxofemoral joints of dogs. J Am Vet Med Assoc. 1997;210(2):222-225.

9. Marchi PH, Vendramini THA, Perini MP, et al. Obesity, inflammation, and cancer in dogs: Review and perspectives. Front Vet Sci. 2022;9:1004122. doi:10.3389/fvets.2022.1004122

10. Salt C, Morris PJ, Wilson D, Lund EM, German AJ. Association between life span and body condition in neutered client-owned dogs. J Vet Intern Med. 2019;33(1):89-99. doi:10.1111/jvim.15367

11. Simpson M, Albright S, Wolfe B, et al. Age at gonadectomy and risk of overweight/obesity and orthopedic injury in a cohort of Golden Retrievers. PLOS ONE. 2019;14(7):e0209131. doi:10.1371/journal.pone.0209131

12. McKenzie B. Evaluating the benefits and risks of neutering dogs and cats. CABI Rev. 2010;2010:1-18. doi:10.1079/PAVSNNR20105045

13. Hart BL, Hart LA, Thigpen AP, Willits NH. Assisting Decision-Making on Age of Neutering for 35 Breeds of Dogs: Associated Joint Disorders, Cancers, and Urinary Incontinence. Front Vet Sci. 2020;7:388-388. doi:10.3389/FVETS.2020.00388/BIBTEX