Canine immune-mediated polyarthritis (IMPA) is overrepresented in retrievers, spaniels, and German shepherds. Dogs with IMPA may have nonspecific (eg, weight loss, inappetence, lethargy, reluctance to move) or more specific (eg, fever, stiff/stilted gait, swelling of multiple joints) clinical signs. Dogs with IMPA commonly present with no obvious joint swelling or localizable pain. Underlying infection, inflammatory disease, or neoplasia should be thoroughly investigated.
Arthrocentesis and synovial fluid analysis, along with CBC, blood chemistry, urinalysis, and urine culture, should be considered. IMPA can cause mild to moderate elevation in serum alkaline phosphatase (SAP), and special culture techniques may be required to identify bacterial causes. Echocardiography may also be necessary to look for endocarditis. IMPA treatment should target the underlying cause if identified. If tick-borne diseases are geographically prevalent, initial therapy should be limited to analgesics and empiric treatment with doxycycline. Treatment with immunosuppressive drugs may be necessary, but because IMPA is rarely life-threatening, it is preferable to withhold these drugs until bacterial arthritis has been excluded. Patients should be monitored closely; some may require lifelong treatment.
Neutrophilic inflammation of a single large joint suggests septic arthritis, whereas IMPA commonly affects multiple small joints (carpi, tarsi). I do not recommend joint taps until a 3- to 5-day course of doxycycline has failed; if there is no response, further diagnostics should be performed (CBC, platelet count, serum biochemistry profile, urinalysis, protein:creatinine ratio). I routinely tap 4 joints (both tarsi and carpi). Joint taps from the stifles are less valuable because concurrent cruciate rupture may complicate interpretation. I routinely treat IMPA using prednisone with azathioprine, gradually tapering once complete clinical response has occurred but do not perform serial joint taps to monitor response. Many cases can be tapered off medication over 6 months without relapse. Prognosis is good in most cases.—Michael Stone, DVM, DACVIM
Canine immune-mediated polyarthritis: Part 2: Diagnosis and treatment. Johnson KC, Mackin K. JAAHA 48:72-81, 2012.