Immune-Mediated Polyarthritis

Immune-mediated polyarthritis (IMPA) is a systemic inflammatory disorder characterized by sterile, non-infectious inflammation of multiple joints, resulting from an aberrant immune response directed against synovial tissues. It is one of the most common immune-mediated diseases in dogs and is occasionally reported in cats. The condition is classified into two main categories: erosive (e.g., rheumatoid arthritis) and non-erosive (e.g., idiopathic, reactive, or drug-induced). Non-erosive IMPA is further subdivided into type I (idiopathic) and type II (reactive to an underlying infectious, neoplastic, or drug trigger). The disease is typically immune-complex-mediated, with type III hypersensitivity reactions leading to neutrophil-rich synovial inflammation. Clinical manifestations include acute or chronic lameness, joint swelling, pain, stiffness, and systemic signs such as fever, lethargy, and anorexia. Early diagnosis and appropriate immunosuppressive therapy are critical to prevent irreversible joint damage and systemic complications.

The exact etiology of IMPA is often unknown (idiopathic), but several triggers have been identified. In dogs, type II IMPA can be secondary to infectious diseases such as bacterial endocarditis, ehrlichiosis, borreliosis (Lyme disease), leishmaniasis, and dirofilariasis. Viral infections (e.g., canine distemper virus) and fungal infections (e.g., blastomycosis) have also been implicated. Drug-induced IMPA has been associated with sulfonamides, penicillins, cephalosporins, and some vaccines. Neoplastic conditions, particularly lymphoma and multiple myeloma, can trigger paraneoplastic IMPA. In cats, IMPA is less common but may be associated with feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and Mycoplasma spp. infections. The underlying mechanism involves the formation of immune complexes that deposit in synovial membranes, activating complement and attracting neutrophils, leading to synovitis. Genetic predisposition may play a role in certain breeds, but specific genetic markers are not well-defined.

IMPA is primarily a disease of dogs, with a reported incidence of approximately 0.1% in the general canine population, but it is more common in certain breeds. Breeds with a higher predisposition include the Akita, Bernese Mountain Dog, Boxer, Cocker Spaniel, German Shepherd, Golden Retriever, Great Dane, Newfoundland, and Weimaraner. The disease typically affects young to middle-aged dogs, with a median age of 4-6 years, but can occur at any age. There is no strong sex predilection, though some studies suggest a slight female predominance. In cats, IMPA is rare, with no clear breed or age predisposition. Geographic variation exists, with higher incidence in regions where tick-borne diseases (e.g., ehrlichiosis, Lyme disease) are endemic. Seasonal patterns may be observed in areas with high tick activity. The disease is not contagious, but underlying infectious triggers may have zoonotic potential (e.g., leptospirosis).

The pathophysiology of IMPA involves a type III hypersensitivity reaction. An inciting antigen (e.g., infectious agent, drug, or self-antigen) triggers the production of antibodies, forming circulating immune complexes. These complexes deposit in the synovial membrane and synovial fluid, activating the complement cascade. Complement activation leads to the release of chemotactic factors (e.g., C5a) that attract neutrophils into the joint space. Neutrophils release proteolytic enzymes, reactive oxygen species, and pro-inflammatory cytokines (e.g., TNF-α, IL-1, IL-6), causing synovial inflammation, cartilage degradation, and joint effusion. In erosive forms, such as rheumatoid arthritis, there is a more prominent T-cell-mediated response, leading to pannus formation and progressive cartilage and bone erosion. The systemic signs (fever, lethargy) are due to the release of pyrogenic cytokines. Chronic inflammation can lead to fibrosis and joint ankylosis in severe cases. The disease is not typically associated with a single organ system, but secondary effects on the bone marrow (anemia of chronic disease) and liver (reactive hepatopathy) can occur.

Predisposing factors for IMPA include genetic susceptibility, as certain breeds are overrepresented. Concurrent infectious diseases, particularly tick-borne infections (e.g., Ehrlichia, Borrelia, Anaplasma), are significant risk factors. Recent vaccination, especially with modified-live vaccines, has been implicated as a trigger in some cases. Drug administration, particularly sulfonamide antibiotics, can induce a drug-induced lupus-like syndrome. Underlying neoplasia, especially lymphoma, can predispose to paraneoplastic IMPA. Age is a factor, with young to middle-aged dogs more commonly affected. Environmental factors, such as living in endemic areas for infectious diseases, increase the risk. Stress and immunosuppression may also play a role in disease onset. In cats, retroviral infections (FeLV, FIV) are important predisposing factors.

Clinical signs of IMPA can be acute or chronic. The hallmark is lameness, which may be shifting or affect multiple limbs. Joint swelling, pain on palpation, and decreased range of motion are common. Stiffness, especially after rest, is often reported. Systemic signs include fever (often >103°F), lethargy, anorexia, and weight loss. In severe cases, the animal may be reluctant to move or exhibit a stilted gait. Physical examination may reveal joint effusion, periarticular soft tissue swelling, and pain on manipulation. In erosive forms, crepitus and joint deformity may be present. Some dogs may have concurrent signs of the underlying disease (e.g., lymphadenopathy in ehrlichiosis, cardiac murmur in endocarditis). Cats may show more subtle signs, such as hiding, decreased activity, and poor grooming. The disease can be classified as peracute (sudden severe lameness), acute (within days), subacute (weeks), or chronic (months).

Differential diagnoses for IMPA include: 1) Septic arthritis: typically monoarticular, with purulent joint fluid, positive culture, and systemic signs; differentiate by joint fluid cytology (degenerate neutrophils, bacteria) and culture. 2) Osteoarthritis: usually chronic, non-inflammatory joint fluid with low cell count; radiographic evidence of osteophytes. 3) Trauma: history of injury, monoarticular, joint fluid may be hemorrhagic. 4) Polyarthritis due to systemic lupus erythematosus (SLE): positive ANA test, multiple organ involvement. 5) Tick-borne diseases (e.g., ehrlichiosis, Lyme disease): serology/PCR positive, thrombocytopenia, other systemic signs. 6) Rheumatoid arthritis: erosive changes on radiographs, positive rheumatoid factor. 7) Mycoplasma arthritis: especially in cats, PCR positive. 8) Neoplasia (e.g., synovial cell sarcoma): monoarticular, radiographic bone lysis, cytology/histopathology. 9) Drug-induced lupus: history of drug exposure, resolution after withdrawal. 10) Polyarthritis associated with inflammatory bowel disease or other immune-mediated diseases.

The diagnostic algorithm for IMPA begins with a thorough history and physical examination, focusing on joint palpation and gait assessment. If polyarthritis is suspected, the next step is arthrocentesis of multiple joints (at least 4-6 joints) for synovial fluid analysis. Synovial fluid should be evaluated for color, turbidity, viscosity, mucin clot, total nucleated cell count (TNCC), and cytology. In IMPA, the fluid is typically turbid, with TNCC > 2000 cells/μL (often > 10,000), and > 90% neutrophils, which are non-degenerate. A search for an underlying cause is essential: complete blood count (CBC), serum biochemistry, urinalysis, and infectious disease testing (e.g., Ehrlichia, Anaplasma, Borrelia, Leptospira, fungal titers, FeLV/FIV in cats). Thoracic radiographs and abdominal ultrasound may be indicated to rule out neoplasia or endocarditis. If erosive disease is suspected, radiographs of affected joints are taken. Antinuclear antibody (ANA) testing is performed to rule out SLE. If no underlying cause is found, a diagnosis of idiopathic IMPA (type I) is made. In cases of suspected drug-induced IMPA, withdrawal of the offending drug and monitoring for improvement is recommended.

Laboratory findings in IMPA are non-specific but may support the diagnosis and identify underlying causes. Complete blood count may reveal mild to moderate anemia (anemia of chronic disease), leukocytosis with neutrophilia, and occasionally thrombocytopenia (especially with tick-borne diseases). Serum biochemistry may show mild hyperglobulinemia, hypoalbuminemia, and elevated liver enzymes (reactive hepatopathy). Urinalysis may reveal proteinuria or hematuria if glomerulonephritis is present. Synovial fluid analysis is the most important laboratory test: it is typically turbid, with decreased viscosity, poor mucin clot, and elevated TNCC (often > 5,000 cells/μL, sometimes > 50,000). Cytology shows a predominance of non-degenerate neutrophils (> 90%), with variable numbers of macrophages and lymphocytes. In erosive IMPA, the cell count may be lower with more mononuclear cells. Serology for infectious agents (e.g., Ehrlichia, Anaplasma, Borrelia, Leptospira, fungal antigens) and PCR for infectious agents (e.g., Mycoplasma, Bartonella) should be performed. ANA testing is positive in a subset of dogs with SLE-associated IMPA. Rheumatoid factor (RF) may be positive in erosive rheumatoid arthritis. Blood cultures are indicated if bacterial endocarditis is suspected.

Imaging findings in IMPA are often unremarkable in the early non-erosive form. Radiographs of affected joints may show soft tissue swelling, joint effusion (widening of the joint space), and periarticular osteophyte formation in chronic cases. In erosive forms, subchondral bone erosion, joint space narrowing, and periarticular new bone formation may be seen. Thoracic radiographs are important to rule out underlying neoplasia or fungal disease. Abdominal ultrasound may reveal splenomegaly, lymphadenopathy, or other organ involvement. Advanced imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) is rarely needed but may be useful to assess joint damage in erosive disease. Ultrasonography of joints can detect effusion and synovial thickening, but arthrocentesis remains the gold standard. Echocardiography is indicated if bacterial endocarditis is suspected.

Cytological evaluation of synovial fluid is the cornerstone of diagnosis. In IMPA, the fluid is inflammatory, with a predominance of non-degenerate neutrophils. The total nucleated cell count is typically > 2,000 cells/μL, often > 10,000. The mucin clot is poor. No bacteria are seen on cytology, and culture is negative. Histopathology of synovial membrane is rarely performed but may be useful in chronic or erosive cases. Findings include synovial hyperplasia, villous hypertrophy, infiltration of neutrophils, lymphocytes, and plasma cells, and fibrin deposition. In erosive disease, pannus formation and cartilage erosion are seen. Special stains (e.g., Gram stain) are negative for infectious agents. Histopathology is more commonly used to rule out neoplasia or other infiltrative diseases.

The treatment of IMPA involves immunosuppressive therapy and management of any underlying cause. The mainstay is glucocorticoids, such as prednisone or prednisolone, at an initial dose of 1-2 mg/kg/day PO divided q12h. In severe cases, pulse therapy with methylprednisolone (10-20 mg/kg IV once daily for 1-3 days) may be used. If response is inadequate or glucocorticoid-sparing is needed, additional immunosuppressive agents are added: azathioprine (2 mg/kg PO q24h or 2.2 mg/kg q48h), cyclosporine (5-10 mg/kg PO q24h), or leflunomide (2-4 mg/kg PO q24h). Mycophenolate mofetil (10-20 mg/kg PO q12h) is also used. In refractory cases, human intravenous immunoglobulin (IVIG) at 0.5-1 g/kg IV over 6-12 hours may be considered. Non-steroidal anti-inflammatory drugs (NSAIDs) are generally avoided due to the risk of gastrointestinal and renal side effects, but may be used short-term for pain relief if no contraindications. Analgesics such as gabapentin (10-20 mg/kg PO q8-12h) may be used for pain. If an underlying infectious cause is identified, appropriate antimicrobial therapy is initiated (e.g., doxycycline 5-10 mg/kg PO q12h for tick-borne diseases). Supportive care includes fluid therapy, nutritional support, and joint rest. Physical therapy may be beneficial after acute inflammation resolves.

The prognosis for non-erosive IMPA is generally good to excellent with appropriate therapy. Most dogs respond to glucocorticoid therapy within 1-2 weeks, and remission is often achieved within 2-4 months. However, relapses are common, and long-term immunosuppression may be required. The prognosis is worse for erosive forms, which are more difficult to manage and may lead to chronic pain and disability. Negative prognostic indicators include lack of response to initial therapy, development of erosive changes, and presence of concurrent systemic disease (e.g., SLE, neoplasia). The mortality rate is low (<10%) but can be higher if the underlying cause is severe. With aggressive treatment, many dogs can achieve a good quality of life. In cats, the prognosis is also good, but they may be more sensitive to corticosteroid side effects.

Follow-up is essential to monitor response to therapy and adjust drug dosages. Initially, re-evaluation should occur every 2-4 weeks. At each visit, a physical examination, including joint palpation, and synovial fluid analysis (if possible) should be performed. The goal is to taper glucocorticoids to the lowest effective dose, typically by 25-50% every 2-4 weeks once remission is achieved. If additional immunosuppressive agents are used, their doses may be adjusted based on clinical response and blood work. Complete blood count and serum biochemistry should be monitored every 2-4 weeks during the induction phase, then every 3-6 months during maintenance. If azathioprine is used, CBC and liver enzymes should be checked every 2 weeks for the first 2 months, then monthly. Long-term follow-up is recommended every 6-12 months to monitor for relapse and drug side effects. Owners should be educated to watch for signs of relapse (lameness, fever) and to avoid sudden discontinuation of medications.

Pearls: 1) Always perform arthrocentesis on multiple joints, even if only one appears affected, as IMPA is often polyarticular. 2) Synovial fluid with >90% non-degenerate neutrophils is highly suggestive of IMPA. 3) Rule out infectious causes before starting immunosuppressive therapy, as immunosuppression can exacerbate infections. 4) Use a combination of glucocorticoids and a steroid-sparing agent early in severe cases to reduce side effects. 5) Consider drug-induced IMPA if the patient has recently received sulfonamides or other drugs. Pitfalls: 1) Failing to perform arthrocentesis due to perceived difficulty, leading to misdiagnosis. 2) Starting immunosuppressive therapy without ruling out sepsis, which can be fatal. 3) Using NSAIDs as sole therapy, which is ineffective and may cause side effects. 4) Tapering steroids too quickly, leading to relapse. 5) Ignoring the possibility of an underlying neoplastic process, especially in older animals.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: Prednisone/Prednisolone: Dogs: 1-2 mg/kg PO q12h for 7-14 days, then taper by 25-50% every 2-4 weeks. Cats: 1-2 mg/kg PO q12h, with caution. Methylprednisolone: 10-20 mg/kg IV once daily for 1-3 days for pulse therapy. Azathioprine: Dogs: 2 mg/kg PO q24h, or 2.2 mg/kg q48h; cats: 0.3-0.6 mg/kg PO q48h. Cyclosporine: Dogs: 5-10 mg/kg PO q24h; cats: 5-7 mg/kg PO q24h. Leflunomide: Dogs: 2-4 mg/kg PO q24h. Mycophenolate mofetil: Dogs: 10-20 mg/kg PO q12h; cats: 10 mg/kg PO q12h. Human IVIG: 0.5-1 g/kg IV over 6-12 hours, repeated if needed. Doxycycline: 5-10 mg/kg PO q12h for 14-28 days for tick-borne diseases. Gabapentin: 10-20 mg/kg PO q8-12h for pain. All immunosuppressive drugs require monitoring for bone marrow suppression, hepatotoxicity, and gastrointestinal signs. Dosages should be adjusted in renal or hepatic impairment. Contraindications: Glucocorticoids should be used with caution in patients with diabetes mellitus, heart disease, or infections. Azathioprine is contraindicated in cats with severe bone marrow suppression. Drug interactions: Glucocorticoids may interact with NSAIDs, increasing GI ulceration risk; azathioprine may interact with allopurinol, increasing toxicity.

Evidence-based literature supports the use of glucocorticoids as the first-line therapy for IMPA, with response rates of 70-90% in dogs. A retrospective study by Clements et al. (2004) found that dogs with non-erosive IMPA treated with prednisone alone had a median survival time of 4.5 years, and those receiving additional immunosuppressive agents had similar outcomes. A study by Colopy et al. (2010) reported that combination therapy with prednisone and azathioprine was effective in reducing relapse rates. The use of cyclosporine has been evaluated in a small case series, showing efficacy as a steroid-sparing agent. Leflunomide has been shown to be effective in refractory cases. Human IVIG has been used in severe, refractory cases with anecdotal success. Consensus guidelines from the American College of Veterinary Internal Medicine (ACVIM) recommend a stepwise approach: start with glucocorticoids, add a steroid-sparing agent if response is inadequate, and consider IVIG for refractory cases. There is limited evidence for the use of NSAIDs in IMPA, and they are generally not recommended due to potential side effects. The importance of ruling out infectious causes is emphasized in the literature, as immunosuppression can worsen underlying infections.