Feline Polyarthritis and Osteoarthritis

Polyarthritis
Polyarthritis - by definition inflammation affecting two or more joints - is an uncommon but important cause of lameness and PUO in cats. Affected cats are often systemically ill with pyrexia, lethargy, inappetence/ anorexia and the clinical signs can wax and wane over protracted periods. Some cats may not show overt lameness but may be simply reluctant to move or just appear ‘stiff’, while others will have genuine shifting lameness. Extreme pain/discomfort is seen in some cats with any movement apparently eliciting marked discomfort and palpation (anywhere) being strongly resented. Although clinical signs may be variable, and overt lameness may be absent, most cats with polyarthritis will display pain on manipulation of affected joints. This is often easiest to demonstrate (and perhaps more severe) in distal joints (elbows, stifles, carpi, tarsi and interpahalangeal joints). There may be palpable joints swelling/effusion in some and flexion/extension of the joints is resented.
Arthrocentesis is technically quite easy in cats, but generally only small volumes of synovial fluid can be aspirated, and the volume is usually too small for true cell counts to be performed. We normally submit a drop of fluid for culture, and make smears from the remainder. These can be stained with ‘Diff Quik’ for rapid interpretation - when examined under x40 objective (x400 total magnification) each cell in the field of view represents approximately 1000 cells per microlitre of synovial fluid. Normal synovial fluid typically contains <3000 cells per microlitre and thus an increased cell count is easily determined along with the cell morphology. Normal joints fluid has a predominance of mononuclear cells, and this is often maintained in degenerative joint disease 9although the cell count may be moderately raised). Inflammatory polyarthritis typically has high numbers of cells, with neutrophils typically (although not invariably) predominating. Septic arthritis will be characterised by the presence of degenerate neutrophils and possibly the presence of organisms.
A number of infectious agents have been associated with feline polyarthritis - some of these will establish infection in multiple joints and directly cause inflammation, while others will initiate inflammation secondary to an immune-mediated process (circulating complexes that lodge in the capillaries of the synovial membrane) triggered by the infection. Cytology shows typical findings of an infective process (neutrophlic inflammation with degenerate neutrophils), but special media may be required to culture the organisms. [...]