Osteoarthritis in cats: Still a mass of unknowns
When addressing arthritis in cats, we presume similarities to arthritis in dogs, interpreting radiographs and clinical signs with canine differential diagnoses in mind. And we develop therapies based on how dogs are managed. But these presumptions have little scientific basis. In fact, we know little about how many cats have arthritis, what effect their arthritis has on their lifestyles, or to what degree therapy improves their comfort level.
In this article, I answer questions about feline arthritis with the information we have at the moment. Much more research is needed in cats to approach the level of understanding we have about arthritis in dogs.
Do we have an accurate estimate of the prevalence of feline degenerative joint disease in clinical practice and in the cat population?No, we do not know the extent to which the cat population is affected with arthritis. Two other clinicians and I described the prevalence of degenerative joint disease in cats older than 12 years of age that were radiographed for reasons unrelated to lameness.1 We identified degenerative changes in axial and appendicular joints that were visible on the films of 100 cats. Ninety percent had evidence of at least one area of radiographically detectable degeneration. The elbow had the highest frequency of obvious disease overall, with 12 of 71 joints (17%) having moderate or severe osteophytosis. Spinal changes were also common, with severe changes at the lumbosacral space related to signs described in the history such as pain and reluctance to jump. No other relationships between recorded health status and these radiographic changes were present.
Another recent study, which included cats that had been presented for evaluation of lameness, suggested similar findings across a wider age range (older than 1 year).2 In this study, 63 of 292 cats (22%) had arthritic changes detected radiographically. Clinical signs were evident in 21 of these 63 cats (33%), with owners reporting lameness, a stiff gait, and difficulty jumping.
Additional epidemiologic information is needed to better determine the prevalence of arthritis in cats, particularly in two areas. First, a prospective study is needed in cats that are presented for evaluation of lameness. The inclusion criteria could be expanded to include cats with reduced activity or less ability to jump. Second, the general population should be surveyed, including all age groups of cats. Patient history and physical examination data are needed to examine correlations with whole-body radiograph findings. The survey should be performed in multiple locations and include enough numbers of specific breeds to identify predilections. Other factors that should be studied include environment (indoor vs. outdoor), body condition, and vaccination status.
What do we know about the etiology and pathogenesis of arthritis in cats?
We know little about how factors such as obesity, breed, age of onset, or activity level and environment (indoor vs. outdoor) relate to the causes of arthropathy in cats. Most of what has been described about the causes and pathogenesis of feline arthritis is based on the assumption that cats have conditions similar to those in dogs. But this assumption is likely incorrect because the clinical presentation of cats with arthritis is different, and radiographic changes in arthritic cats differ slightly from those in arthritic dogs. In cats, joints with advanced degeneration may not exhibit a marked proliferative bony response.
Joint injury and subsequent degeneration are one suspected cause of arthritis in cats. However, joint injury in cats seems uncommon. Cats' ligaments may be relatively stronger than their bones compared with these tissues in dogs. In the population of cats that I see, it is my impression that when a limb is overloaded, fractures seem to be the mode of failure rather than joint disruption. Fractures and dislocations of the hip and hock are other possible causes of arthritis in cats. Hip dysplasia has also been identified in cats. As in dogs, the cause of the laxity that leads to articular degeneration and joint remodeling in the hip is unknown. Cranial cruciate ligament rupture and patellar luxation can also result in arthritis and clinical signs in cats.
From our study, we know that the elbow frequently has marked radiographic changes,1 but we do not know the cause of these changes. An elbow-dysplasia-like process would be one possibility in cats exhibiting bilaterally symmetric changes.
Immune-mediated diseases may be expressed as suppurative arthropathy in cats. It is difficult to definitively diagnose these conditions because we lack specific tests. However, the clinical and radiographic features of these polyarthropathies are consistent with a general diagnosis of immune-mediated arthritis. Synovial fluid analysis may provide additional evidence to support this diagnosis.