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.

A study involving 12 cats with rheumatoid-like disease was recently reported.3 Canine rheumatoid factor was detected in these cats, and while the author admitted that this finding is not specific, the author noted that cats with periosteal proliferative polyarthritis do not have elevated rheumatoid factor concentrations. However, the radiographic features of these two conditions are similar. Both new bone production (osteophytes) and joint surface damage (erosions) may be present.

Immune-mediated arthropathies that are nonerosive and exhibit little periarticular bone production and thickening are considered to be either systemic lupus erythematosus, if antinuclear antibody (ANA) test results are positive, or idiopathic, if ANA test results are negative. But because we know little about the sensitivity and specificity of the ANA test in cats, the definitive naming of these conditions must be done cautiously.

In our radiologic study, we frequently saw degenerative changes around the vertebral facets and spondylitic changes of the intervertebral region.1 While mild changes may be from primary degeneration, more extensive changes suggest immunologic disease similar to ankylosing spondylitis in people.4

Infectious agents may also initiate joint inflammation directly or through autoimmune-like reactions. Calicivirus and Mycoplasma species have been implicated.5 In the Infectious Diseases Laboratory at North Carolina State University's College of Veterinary Medicine, Bartonella species have been identified in the synovial fluid of inflamed joints in cats.6

In a recent retrospective study of osteoarthritis in cats,2 the cats were divided into four groups based on whether they had a known cause of the joint pathology and whether they had clinical signs possibly related to that joint disease. Cases in which the cause was not apparent were classified as primary arthritis, and cases in which the cause was determined were classified as secondary arthritis. I disagree with this approach; I think that the term primary should only be used when extensive testing of all possible secondary factors fails to reveal the cause. Even then, it is more likely that the testing is incomplete rather than that the joint has begun to deteriorate because of true primary degeneration. The problem with this study's choice of terms is that it could lead to complacency. One interpretation of the data might be that primary degeneration occurs commonly in cats, and future studies of the possible secondary causes might be discouraged.

What are the clinical signs of arthritis in cats?

If one joint is more painful than the others, the owner may report that the cat limps or you may notice lameness in the examination room. However, it is likely that only a small fraction of cats that are affected by arthritis exhibit this type of lameness, which may more commonly be caused by joint trauma such as from cat bites or cranial cruciate ligament rupture.

Most cats with degenerative joint disease appear to have a more diffuse and slow onset of joint disease, and, based on our radiologic study, it is frequently bilateral.1 So clinical signs of arthritis in cats are more likely to be reduced activity and an inability to perform physical tasks that the cats should be able to do or had done in the past. However, owners rarely recognize these signs because cats are not generally active indoors. And in some cases, owners may welcome their cats' choosing to spend more time lying on the couch with them rather than being outside. They attribute a lower activity level to advancing age and may not consider it a problem.

Activities involving jumping are likely to be altered as joint disease progresses, but many indoor cats may not need to jump to perform their daily functions, so a reduced ability to do so may not be observed. My own 15-year-old cat (who is not overweight) has to jump onto a table to get to her food. She used to do it in one easy leap, but for the last three years she will meow at the table until someone pulls a chair out so that she can do it in two smaller jumps. An overweight cat may have more obvious alterations in behavior and reduced activity and may even show lameness.7 Cats may groom incompletely if their mobility and flexibility are reduced. And a dislike of stroking may indicate back pain.

How is arthritis best diagnosed in cats?

The diagnostic approach to arthritis in cats is similar to that in dogs, though it generally seems incomplete and less exact and rarely leads to a specific diagnosis. When obtaining a history in an older cat, tailor your questions to focus on notable changes in behavior or activity, rather than asking if the cat has exhibited lameness.

Gait observation in cats is rarely helpful. An orthopedic examination focusing on detecting joint changes requires that we know what a normal joint feels like in cats.5 No data have been published regarding normal ranges of motion in cats. Because of a cat's small size and thick fur and skin, it is difficult to detect small changes indicative of joint distention or periarticular fibrosis.

Identifying the painful joint requires a somewhat relaxed patient until we manipulate that site. Many cats are not that cooperative. And then, once you've found one painful site, they become resistant to further manipulation. Of course, if you suspect a painful area, leave that area to last. Many cats are sensitive to spinal manipulation, but we don't know if we can attribute this to degenerative processes in the vertebral column. Similarly, cats may exhibit tail sensitivity, but we cannot say with certainty that it indicates lumbosacral disease. It is important to perform a complete examination for possible neurologic or systemic conditions that might mimic arthritic signs. Sedation is often necessary to complete the examination, particularly if joint instability is suspected.

Because physical examination findings can be difficult to interpret, radiography may help localize the problem.8 But keep in mind that radiographic findings can be misleading. In our analysis of arthritis in older cats, we specifically reviewed the radiographs of cats without a history of lameness.1 Since it was a retrospective radiologic study, we did not know whether the cats had behavioral, gait, or physical examination changes that coincided with the radiographic changes, but it does suggest that marked radiographic abnormalities do not necessarily correlate with clinically identifiable disease. Synovial osteochondromatosis is a good example of a pathologic process that has impressive radiologic findings,8 but the condition may or may not be associated with clinical joint disease. Conversely, cats with nonerosive immune-mediated diseases may exhibit pain because of acute joint inflammation but have few radiographic changes.

Hip dysplasia can cause severe hip degeneration in cats and is diagnosed similarly to how it is diagnosed in dogs. However, it is often an incidental finding. Young cats in the acute stage may present with lameness, but it usually resolves with conservative management (e.g. activity limitations, diet restrictions, supplements, possibly nonsteroidal anti-inflammatory drugs [NSAIDs]). However, a femoral head ostectomy may be indicated in cats with severe coxofemoral joint laxity and pain.

Lumbosacral degeneration can cause marked clinical signs in cats. Cats may no longer jump or seek affection. Sensitivity to stroking in the caudal lumbar region or tail manipulation may be noticed. Radiographic evidence of lumbosacral disease includes narrowing, sclerosis, spondylosis, or abnormal motion of the lumbosacral space. Myelography, epidurography, computed tomography, or magnetic resonance imaging may be needed to confirm the diagnosis.

Objective analysis of the gait in cats has been evaluated by using a force pad.9 This device records the weight placed on each paw as the animal walks across the pad. While a force pad is not likely to be available for routine clinical use, it will be a useful tool for evaluating treatment effects (e.g. assessing response to NSAID therapy). At North Carolina State University College of Veterinary Medicine, Dr. Duncan Lascelles has completed a similar study evaluating the gait of normal cats.10 He is also attempting to examine limb loading during landing from a jump and is comparing video-and harness-mounted motion detectors as other methods to objectively analyze activity.

What is the best medical management approach for cats with signs of arthritis?

If a cat is overweight, diet restriction (i.e. changing feeding from ad lib to controlled amounts, eliminating or reducing high-calorie treats or table scraps) helps reduce stress on the joints and, thus, makes the patient more comfortable. The environment of cats with arthritis can be restructured to reduce the need for stressful activities such as jumping. However, as in dogs and people, it may be beneficial in cats to encourage a little activity to keep the muscles and joints active. Creativity will be needed for these independent creatures.

Some cats respond to glucosamine and chondroitin sulfate products, though only anecdotal evidence regarding cats' response is available.11 One such product is specifically formulated for cats (Cosequin for Cats—Nutramax Laboratories).

Acupuncture can be considered, if available. Physical therapy techniques may help improve mobility and strength in an arthritic limb, though cats may not be cooperative.

NSAIDs can be used to treat chronic pain in cats, but it is imperative to balance a cat's need with the potential for drug-induced liver and kidney damage. Thus, it is prudent to try other approaches before resorting to pharmacologic intervention. An excellent article on managing pain in cats was recently published.12 If you think an NSAID is needed, perform a complete blood count and serum chemistry profile to obtain baseline values. Meloxicam (Metacam—Boehringer Ingelheim) has the advantage of ease of dosing in animals less than 11 lb (5 kg) (0.1 mg/kg orally on the first day, 0.05 mg/kg for the next two to five days, and then 0.025 mg/kg per day or every other day); the oral formulation is not labeled for use in cats.

Most of what we know about managing pain in cats relates to acute pain and short-term drug administration. Chronic arthritic pain is quite different. Studies in this area are difficult to conduct because of the nature of the subject. Basic research principles require that we create an effective model and measure the effects of the disease process so that we can then measure a response, for example, to a particular analgesic. Clinical research faces similar challenges. Dr. Lascelles and others at North Carolina State University are currently working to develop assessment methods, including behavior and activity questionnaires, activity monitors, and force pad assessment of limb loading, so that therapies can be assessed objectively.

Once problems have been identified and treated, have the owner complete a patient-specific activity questionnaire to assess response to treatment. For example, for my own cat, a good response to treatment would be a return to being able to get onto the table in one jump.

There are many unknowns regarding osteoarthritis in cats. We are getting better at detecting joint disease in cats but have little understanding of the primary causes. We are still working on ways to recognize and measure the effects of joint disease in cats. We have begun to explore nonpharmacologic methods for managing chronic pain but find it difficult to accurately judge response in this largely uncooperative patient group. We are learning more about the pharmacology of NSAIDs in cats, but cats' individual metabolisms may complicate NSAIDs' application clinically. Osteoarthritis probably has a larger effect on the cat population than we realize, so it is important for us to continue our efforts to recognize their plight and to find ways to make their lives more comfortable.

Simon Roe, BVSc, PhD, DACVS

Department of Clinical Sciences

College of Veterinary Medicine

North Carolina State University

Raleigh, NC 27606

REFERENCES

1. Hardie EM, Roe SC, Martin FR. Radiographic evidence of degenerative joint disease in geriatric cats: 100 cases (1994-1997). J Am Vet Med Assoc 2002;220:628-632.

2. Godfrey DR. Osteoarthritis in cats: a retrospective radiological study. J Small Anim Pract 2005;46:425-429.

3. Hanna FJ. Disease modifying treatment for feline rheumatoid arthritis. Vet Comp Orthop Traumatol 2005;18:94-99.

4. Kim TH, Uhm WS, Inman RD. Pathogenesis of ankylosing spondylitis and reactive arthritis. Curr Opin Rheumatol 2005;17:400-405.

5. Leonard CA, Tillson M. Feline lameness. Vet Clin North Am Small Anim Pract 2001;31:143-163.

6. Breitschwerdt E. Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC: Personal communication, 2004.

7. Scarlett JM, Donoghue S. Associations between body condition and disease in cats. J Am Vet Med Assoc 1998;212:1725-1731.

8. Allan GS. Radiographic features of feline joint diseases. Vet Clin North Am Small Anim Pract 2000;30:281–302.

9. Romans CW, Conzemius MG, Horstman CL, et al. Use of pressure platform gait analysis in cats with and without bilateral onychectomy. Am J Vet Res 2004;65:1276-1278.

10. Lascelles BDX, Findley K, Correa M, et al. Kinetic evaluation in normal cats using a pressure sensing walkway. Am J Vet Res: in press.

11. Beale BS. Use of nutraceuticals and chondroprotectants in osteoarthritic dogs and cats. Vet Clin North Am Small Anim Pract 2004;34:271-289.

12. Robertson SA. Managing pain in feline patients. Vet Clin North Am Small Anim Pract 2005;35:129–146.