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.