Immune-mediated polyarthritis (IMPA) is a noninfectious, inflammatory joint disorder in which the immune system targets synovial membranes, resulting in synovitis and effusion. IMPA usually affects multiple joints symmetrically and is classified as an autoimmune disease. The inflammation is due predominantly to the deposition of immune complexes within the synovium, leading to neutrophilic infiltration and joint fluid accumulation.
In both dogs and cats, IMPA can be primary (idiopathic) or secondary to systemic disease. The most widely accepted classification includes four types:
Type I (idiopathic) IMPA is the most common form and has no identifiable underlying cause.
Type II (reactive) IMPA is associated with remote infections (eg, urinary tract infection, respiratory infection, infective endocarditis).
Type III (gastrointestinal) IMPA is usually linked to chronic GI disease such as inflammatory bowel disease (IBD).
Type IV (neoplastic) IMPA is associated with underlying malignancy.
Immune-Mediated Polyarthritis in Dogs
Immune-mediated polyarthritis is more common in dogs than in cats.
Clinical Signs of Immune-Mediated Polyarthritis in Dogs
Clinical signs in dogs with IMPA typically include fever, lethargy, anorexia, shifting leg lameness, and, in some severe cases, recumbency. Multiple joints (especially carpi and tarsi) are painful on manipulation and can be swollen or effusive. Some dogs might also exhibit systemic signs such as lymphadenopathy or weight loss.
Diagnosis of Immune-Mediated Polyarthritis in Dogs
Diagnosis of IMPA in dogs relies on synovial fluid analysis. Arthrocentesis of the effused joints (at least three joints) reveals increased nucleated cell counts, predominantly nondegenerate neutrophils, and decreased viscosity. Cytological evaluation and culture should be performed to exclude septic arthritis and other infectious causes.
Radiography might show joint effusion and soft tissue swelling in nonerosive IMPA. In erosive IMPA, erosive changes and joint collapse are visible in radiographs.
Nonerosive IMPA includes idiopathic IMPA, systemic lupus erythematosus (SLE), reactive polyarthritis (drug reactions like those with sulfonamides), infection, and neoplasia. It occurs as part of breed-specific syndromes such as in Akitas, Chinese Shar-Pei, or spaniel breeds.
Erosive IMPA includes rheumatoid arthritis and erosive polyarthritis of Greyhounds. Cartilage damage and joint destruction could also occur in chronic cases of septic arthritis or polyarthritis.
CBC and biochemical panel, urinalysis, thoracic and abdominal imaging, and infectious disease screening (eg, tickborne disease panel) help to identify secondary causes of IMPA.
Treatment and Prognosis of Immune-Mediated Polyarthritis in Dogs
Initial treatment of IMPA in dogs typically involves immunosuppression with corticosteroids (eg, prednisolone or prednisone), with adjunct immunosuppressive agents such as azathioprine or cyclosporine in refractory cases. Leflunomide can also be used instead of steroids, with good results.
If there is a history of tick exposure or travel to endemic regions, doxycycline can be administered initially, until the results of serological testing are available.
Dogs with identified secondary causes of IMPA require treatment of the underlying disease as well.
The prognosis for dogs with IMPA is generally good in idiopathic cases, and many patients respond well to medical therapy. However, relapses can occur, and long-term management might be needed.
Immune-Mediated Polyarthritis in Cats
Immune-mediated polyarthritis is less common in cats than in dogs; however, cases in cats are increasingly recognized. IMPA can be primary or secondary to other infectious diseases, such as feline infectious peritonitis (FIP), feline leukemia virus (FeLV) disease, feline immunodeficiency virus (FIV) infection, toxoplasmosis, or chronic infections.
Clinical Findings of Immune-Mediated Polyarthritis in Cats
Cats with IMPA are typically young to middle-aged and can present with lameness, stiff gait, joint swelling, fever of unknown origin, lethargy, or decreased activity. Some cats show systemic signs such as weight loss, poor coat, or behavioral changes due to discomfort. Joint effusion and pain can occur in affected joints. In some proliferative forms, enlargement of the joint and decrease in range of motion can also occur.
Diagnosis of Immune-Mediated Polyarthritis in Cats
Diagnosis of IMPA in cats is based on synovial fluid analysis, which shows high nucleated cell counts, mainly neutrophils in nonseptic cases. A sample should be sent for culture to rule out septic arthritis.
Additional diagnostic testing for IMPA in cats should include diagnostic imaging of the joints affected and abdominal and thoracic radiography or CT, FeLV/FIV testing, CBC, serum biochemical analysis, urinalysis, and testing for infectious agents such as coronavirus or Toxoplasma. Radiography of joints can reveal joint effusion and soft tissue swelling, and in cases of erosive polyarthritis, erosive changes or proliferative changes can be evident in the joint.
IMPA in cats can be classified into nonerosive or erosive types. Nonerosive types include systemic lupus erythematosus (SLE), idiopathic polyarthritis, and reactive polyarthritis. Erosive types can be feline rheumatoid-like arthritis and feline chronic progressive polyarthritis (FCPP).
There are two forms of FCPP: proliferative periosteal arthritis in young cats (1–5 years old), characterized by fever, joint pain, lymphadenopathy, edema, osteophyte formation, and joint collapse; and deforming erosive arthritis, which affects older males and causes chronic joint deformity and erosion.
Treatment and Prognosis of Immune-Mediated Polyarthritis in Cats
Treatment of IMPA in cats typically includes prednisolone and cyclophosphamide.Immunosuppressive therapy, usually starting with prednisolone, is the mainstay of IMPA management. If there is no response or if adverse effects occur, other drugs, such as chlorambucil, cyclosporine, leflunomide, or mycophenolate, may be considered.
The prognosis for cats with IMPA varies. It is generally poor in cases of erosive IMPA, especially with concurrent FeLV or myeloproliferative disease. It is better for idiopathic polyarthritis, particularly type I.
Key Points
IMPA is a noninfectious, inflammatory joint disease, caused by immune system dysregulation, that often leads to joint pain, swelling, fever, and shifting lameness.
Diagnosis is based on synovial fluid analysis, imaging, and exclusion of infectious causes.
Treatment consists of immunosuppression, primarily with corticosteroids; additional agents (eg, cyclosporine, chlorambucil, or leflunomide) are administered in refractory cases.
In cats, polyarthritis is more likely to be infectious than immune mediated.
For More Information
Bennett D, Nash AS. Feline immune-based polyarthritis: a study of thirty-one cases. J Sm Anim Pract. 1988;29(8):501-523.
Lemetayer J, Taylor S. Inflammatory joint disease in cats: diagnostic approach and treatment. J Feline Med Surg. 2014;16(7):547-562.
Noonan M. Immune-mediated polyarthritis (IMPA) in dogs and cats. Vet Specialists; 2020.
Also see pet owner content regarding immune-mediated arthritis in dogs and cats.
