Polyarthritis

Polyarthritis is a systemic inflammatory condition characterized by simultaneous inflammation of multiple joints. In veterinary medicine, it is often immune-mediated, either erosive or non-erosive, and can be primary (idiopathic) or secondary to underlying infections, neoplasia, or drug reactions. The condition involves synovial inflammation, leading to joint effusion, pain, and lameness. It can be classified based on the predominant inflammatory cell type (neutrophilic, lymphoplasmacytic, or mixed) and the presence or absence of erosion. Systemic signs such as fever, lethargy, and anorexia are common. Polyarthritis can be a component of systemic immune-mediated diseases, including systemic lupus erythematosus (SLE), or occur as a distinct entity like idiopathic immune-mediated polyarthritis (IMPA).

The etiology of polyarthritis is diverse. Immune-mediated causes include type II, III, and IV hypersensitivity reactions. Type III hypersensitivity involves immune complex deposition in synovial membranes, triggering complement activation and neutrophil infiltration. This can be idiopathic (IMPA) or secondary to infections (e.g., bacterial endocarditis, borreliosis, leishmaniasis), neoplasia (paraneoplastic), or drug reactions (e.g., sulfonamides, penicillins). Erosive forms, such as rheumatoid arthritis (RA), are less common in dogs and cats and involve autoimmune responses against collagen and rheumatoid factor. Infectious causes include bacterial (e.g., Mycoplasma, Borrelia, Streptococcus), fungal (e.g., Blastomyces, Histoplasma), viral (e.g., feline calicivirus, feline leukemia virus), and rickettsial (e.g., Ehrlichia, Anaplasma) agents. In cats, chronic progressive polyarthritis is associated with feline syncytium-forming virus and feline leukemia virus. Non-infectious, non-immune causes include crystal-induced arthropathies (gout, pseudogout) and degenerative joint disease, but these are less common.

Polyarthritis is most commonly diagnosed in dogs, with a higher incidence in medium to large breeds such as German Shepherds, Labrador Retrievers, and Golden Retrievers. Certain breeds, like the Akita and Shar-Pei, may have a genetic predisposition to immune-mediated polyarthritis. The condition can occur at any age but is more frequent in young to middle-aged adults (2-6 years). No strong sex predilection is noted, though some studies suggest a slight female predominance. In cats, polyarthritis is less common but can be seen in association with viral infections, particularly in multi-cat households. Geographic distribution varies with infectious etiologies; for example, Borrelia burgdorferi (Lyme disease) is more prevalent in endemic areas of North America and Europe, while Ehrlichia and Anaplasma are common in tropical and subtropical regions. Seasonal patterns may be observed for vector-borne diseases.

The pathophysiology of polyarthritis involves a complex interplay of immune-mediated and inflammatory cascades. In immune-mediated polyarthritis, circulating immune complexes deposit in the synovial membrane, activating the complement system and attracting neutrophils. Neutrophils release proteolytic enzymes and reactive oxygen species, causing synovitis and cartilage degradation. In erosive forms, T-cell-mediated autoimmunity against type II collagen and other joint antigens leads to pannus formation, which invades and destroys cartilage and bone. In infectious polyarthritis, pathogens directly invade the synovium or trigger immune responses that result in inflammation. Bacterial components, such as lipopolysaccharides, stimulate Toll-like receptors on synoviocytes and macrophages, producing pro-inflammatory cytokines (TNF-α, IL-1, IL-6). These cytokines promote synovial proliferation, angiogenesis, and osteoclast activation, leading to joint destruction. Chronic inflammation can result in fibrosis, ankylosis, and permanent joint damage. Systemic signs arise from cytokine release, causing fever, lethargy, and acute-phase protein production.

Predisposing factors for polyarthritis include genetic susceptibility, as seen in certain dog breeds with major histocompatibility complex (MHC) haplotypes associated with autoimmune diseases. Concurrent infections, such as chronic bacterial infections (endocarditis, pyelonephritis) or viral infections (FeLV, FIV), can trigger immune-mediated polyarthritis. Drug administration, particularly sulfonamides, penicillins, and cephalosporins, may induce drug-induced lupus or polyarthritis. Environmental factors, such as exposure to ticks and other vectors, increase the risk of infectious polyarthritis. Age and sex may influence susceptibility, with young adults and females being overrepresented in some studies. Stress, vaccination, and hormonal factors have been proposed as triggers but lack strong evidence. In cats, retroviral infections are significant predisposing factors for chronic progressive polyarthritis.

Clinical signs of polyarthritis vary with the underlying cause and severity. Peracute cases may present with severe lameness, joint swelling, and fever. Acute cases show shifting-leg lameness, stiffness after rest, and reluctance to move. Subacute and chronic cases may exhibit muscle atrophy, joint deformities, and decreased range of motion. Systemic signs include fever (often >103°F), lethargy, anorexia, and weight loss. On physical examination, affected joints are warm, swollen, and painful on palpation. The carpi, tarsi, and stifles are commonly affected. In erosive forms, crepitus and joint instability may be noted. Cats may present with more subtle signs, such as hiding, decreased grooming, and reluctance to jump. In infectious polyarthritis, additional signs related to the primary infection (e.g., tick-borne disease) may be present, such as lymphadenopathy, splenomegaly, and petechiae.

Differential diagnoses for polyarthritis include: (1) Degenerative joint disease (osteoarthritis) – typically affects older animals, with radiographic evidence of osteophytes and subchondral bone sclerosis, and lacks systemic signs. (2) Trauma – history of injury, single joint involvement, and radiographic evidence of fractures or luxations. (3) Septic arthritis – usually monoarticular, with purulent joint fluid, positive culture, and rapid progression. (4) Systemic lupus erythematosus (SLE) – polyarthritis is a common feature, but positive ANA titer and multiple organ involvement (skin, kidney, hematologic) help differentiate. (5) Lyme disease – history of tick exposure, positive Borrelia serology, and response to doxycycline. (6) Rickettsial diseases (ehrlichiosis, anaplasmosis) – thrombocytopenia, leukopenia, and positive PCR or serology. (7) Mycoplasma arthritis – often in immunocompromised or young animals, with positive culture or PCR. (8) Fungal arthritis – endemic areas, positive fungal serology or culture, and granulomatous inflammation on cytology. (9) Neoplastic arthritis – paraneoplastic polyarthritis, often with underlying malignancy (e.g., carcinoma, lymphoma). (10) Drug-induced polyarthritis – history of recent drug administration, resolution after drug withdrawal.

The diagnostic algorithm for polyarthritis 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) for synovial fluid analysis, including cytology, culture, and possibly PCR. Synovial fluid should be evaluated for cell count, protein concentration, and presence of inflammatory cells. A neutrophilic inflammation with >90% neutrophils suggests immune-mediated or septic arthritis; culture and sensitivity are essential to rule out infection. Baseline bloodwork (CBC, biochemistry, urinalysis) is performed to assess systemic involvement and identify underlying causes. Serology and PCR for infectious agents (e.g., Borrelia, Ehrlichia, Anaplasma, FeLV/FIV) are recommended based on geographic exposure. Antinuclear antibody (ANA) testing is indicated if SLE is suspected. Radiographs of affected joints are taken to evaluate for erosive changes, which are characteristic of rheumatoid arthritis. If erosive disease is present, rheumatoid factor (RF) testing may be performed. In cases of suspected paraneoplastic polyarthritis, thoracic and abdominal imaging (radiographs, ultrasound) and possibly bone marrow aspiration are warranted. A therapeutic trial with immunosuppressive doses of glucocorticoids may be considered if immune-mediated disease is strongly suspected and infectious causes have been ruled out.

Hematology may reveal neutrophilia with a left shift, lymphopenia, and mild anemia of chronic disease. Thrombocytopenia may be present in rickettsial infections. Serum biochemistry may show hypoalbuminemia, hyperglobulinemia (especially in chronic inflammation), and elevated acute-phase proteins (C-reactive protein, serum amyloid A). Liver enzymes may be mildly elevated due to systemic inflammation. Urinalysis may reveal proteinuria if glomerulonephritis is concurrent (e.g., SLE). Blood gas analysis may show metabolic acidosis in severe systemic illness. Specific biomarkers: C-reactive protein (CRP) is often markedly elevated in immune-mediated polyarthritis and can be used to monitor response to therapy. Serology for ANA is positive in SLE but may be negative in idiopathic IMPA. Rheumatoid factor (RF) is positive in up to 70% of dogs with rheumatoid arthritis. PCR and serology for infectious agents (Borrelia, Ehrlichia, Anaplasma, Bartonella, Mycoplasma) are crucial to identify infectious etiologies. In cats, FeLV/FIV testing is recommended. Synovial fluid analysis is the cornerstone: normal fluid has <1,000 nucleated cells/μL; in inflammatory polyarthritis, cell counts exceed 5,000/μL, often >50,000/μL, with >90% neutrophils in immune-mediated cases. Protein concentration is elevated (>2.5 g/dL). Culture of synovial fluid is negative in immune-mediated cases but positive in septic arthritis.

Radiography of affected joints is essential to assess for erosive changes, which are characteristic of rheumatoid arthritis and chronic progressive polyarthritis in cats. Erosions appear as subchondral bone lysis, joint space narrowing, and periarticular osteophyte formation. In non-erosive immune-mediated polyarthritis, radiographs may be normal or show only soft tissue swelling and joint effusion. Ultrasonography can be used to evaluate joint effusion and synovial thickening, but is less commonly performed than radiography. Computed tomography (CT) provides detailed bone assessment and can detect early erosions not visible on radiographs. Magnetic resonance imaging (MRI) is superior for evaluating soft tissue structures, including synovial membrane and cartilage, and may be useful in chronic cases. In cases of suspected infectious arthritis, imaging of the primary infection site (e.g., thoracic radiographs for fungal disease, echocardiography for endocarditis) is indicated.

Synovial fluid analysis is the most important diagnostic test. In immune-mediated polyarthritis, the fluid is typically turbid, with low viscosity, and contains >5,000 nucleated cells/μL, predominantly neutrophils (often >90%). Macrophages and lymphocytes may be present in chronic cases. In erosive arthritis, the fluid may be less cellular but still inflammatory. Histopathology of synovial membrane biopsy is rarely needed but can be performed if the diagnosis is uncertain. Findings include synovial villous hypertrophy, hyperplasia of synoviocytes, infiltration of inflammatory cells (neutrophils, lymphocytes, plasma cells), and fibrin deposition. In rheumatoid arthritis, pannus formation with erosion of cartilage and bone is seen. Special stains (e.g., Gram stain, fungal stains) may be applied if infection is suspected. In infectious polyarthritis, organisms may be visualized in the synovial fluid or synovial membrane.

Treatment of polyarthritis depends on the underlying cause. For immune-mediated polyarthritis, immunosuppressive doses of glucocorticoids are the mainstay. Prednisolone or prednisone is administered at 1-2 mg/kg PO q12h for 2-4 weeks, then tapered gradually over 3-6 months. If response is inadequate or glucocorticoid-sparing is needed, additional immunosuppressive agents are used: azathioprine (2 mg/kg PO q24h for dogs, then q48h), cyclosporine (5-10 mg/kg PO q24h), or leflunomide (2-4 mg/kg PO q24h). For severe cases, cytarabine (5-10 mg/m² SC q24h for 2 days) or mycophenolate mofetil (10-20 mg/kg PO q12h) may be considered. In cats, chlorambucil (0.1-0.2 mg/kg PO q48h) is often used. For infectious polyarthritis, appropriate antimicrobial therapy is essential: doxycycline (5-10 mg/kg PO q12h for 14-28 days) for rickettsial and Borrelia infections; amoxicillin-clavulanate (12.5-25 mg/kg PO q12h) for bacterial arthritis; antifungal agents (itraconazole 5-10 mg/kg PO q24h) for fungal infections. Analgesics such as gabapentin (10-20 mg/kg PO q8-12h) or tramadol (2-5 mg/kg PO q8-12h) may be used for pain management. Non-steroidal anti-inflammatory drugs (NSAIDs) are generally avoided in immune-mediated polyarthritis due to potential renal and gastrointestinal side effects, but may be used cautiously in non-immune cases. Supportive care includes fluid therapy, nutritional support, and physical rehabilitation. In cases of septic arthritis, joint lavage and drainage may be necessary.

The prognosis for immune-mediated polyarthritis is generally good with appropriate immunosuppressive therapy, with remission rates of 70-80%. However, relapse is common, and long-term therapy may be required. Erosive forms, such as rheumatoid arthritis, have a guarded prognosis due to progressive joint destruction. Infectious polyarthritis carries a good prognosis if the underlying infection is treated early and effectively, but chronic cases may result in permanent joint damage. Negative prognostic indicators include severe erosive changes, concurrent systemic disease (e.g., SLE), and poor response to initial therapy. Mortality is low but can occur due to complications of immunosuppression (e.g., sepsis) or underlying disease.

Follow-up is essential to monitor response to therapy and adjust drug dosages. Re-evaluation should occur every 2-4 weeks initially, with physical examination, joint palpation, and assessment of lameness. Serial synovial fluid analysis may be performed to document resolution of inflammation. Bloodwork, including CBC and biochemistry, should be monitored regularly, especially when using immunosuppressive drugs, to detect bone marrow suppression or hepatotoxicity. C-reactive protein (CRP) can be used as a biomarker to guide therapy. Radiographs may be repeated every 3-6 months to assess progression of erosive changes. Once remission is achieved, immunosuppressive drugs are tapered slowly over several months. Long-term follow-up every 3-6 months is recommended to detect relapses.

Pearls: (1) Always perform arthrocentesis on multiple joints, even if only one is clinically affected, as polyarthritis may be subclinical in some joints. (2) Synovial fluid should be collected into EDTA tubes for cell count and cytology, and into sterile tubes for culture. (3) In immune-mediated polyarthritis, the synovial fluid is typically non-septic, with negative culture. (4) A positive ANA titer supports SLE, but a negative titer does not rule out immune-mediated polyarthritis. (5) Glucocorticoids should be started only after infectious causes have been ruled out, as they can exacerbate infections. Pitfalls: (1) Failing to rule out infectious causes before starting immunosuppressive therapy can be fatal. (2) Using NSAIDs in immune-mediated polyarthritis can increase the risk of gastrointestinal ulceration and renal injury, especially when combined with glucocorticoids. (3) Tapering immunosuppressive drugs too quickly can lead to relapse. (4) Overlooking concurrent diseases, such as neoplasia or chronic infections, that may be the underlying trigger.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: Prednisolone/prednisone: 1-2 mg/kg PO q12h for 2-4 weeks, then taper by 25% every 2-4 weeks. Azathioprine: 2 mg/kg PO q24h for dogs, then q48h after 2-4 weeks; monitor CBC and liver enzymes. Cyclosporine: 5-10 mg/kg PO q24h; adjust based on trough levels (target 400-600 ng/mL). Leflunomide: 2-4 mg/kg PO q24h; monitor for gastrointestinal signs. Mycophenolate mofetil: 10-20 mg/kg PO q12h; may cause diarrhea. Chlorambucil: 0.1-0.2 mg/kg PO q48h for cats. Doxycycline: 5-10 mg/kg PO q12h for 14-28 days; give with food to reduce vomiting. Amoxicillin-clavulanate: 12.5-25 mg/kg PO q12h for 14-21 days. Itraconazole: 5-10 mg/kg PO q24h for 3-6 months. Gabapentin: 10-20 mg/kg PO q8-12h for pain. Tramadol: 2-5 mg/kg PO q8-12h. All immunosuppressive drugs require monitoring for bone marrow suppression, hepatotoxicity, and renal function. Dosages should be adjusted in patients with renal or hepatic impairment. Contraindications: Glucocorticoids should be avoided in patients with systemic infections, diabetes mellitus, or gastrointestinal ulcers. Azathioprine is contraindicated in cats due to severe bone marrow suppression. Drug interactions: Glucocorticoids may interact with NSAIDs, increasing ulcer risk; azathioprine may interact with allopurinol, increasing toxicity.

Key studies and consensus guidelines: The ACVIM consensus statement on immune-mediated polyarthritis (2019) recommends a stepwise approach to diagnosis and treatment, emphasizing the importance of ruling out infectious causes. A study by Rondeau et al. (2013) reported that 70% of dogs with IMPA achieved remission with prednisone alone, while 30% required additional immunosuppressive agents. Another study by Colopy et al. (2010) found that combination therapy with prednisone and azathioprine was more effective than prednisone alone in reducing relapse rates. For infectious polyarthritis, the ISCAID guidelines (2017) recommend doxycycline as first-line therapy for rickettsial and Borrelia infections. A meta-analysis by Kohn et al. (2016) showed that early diagnosis and treatment of septic arthritis significantly improved outcomes. In cats, a study by Gerding et al. (2014) highlighted the association between feline chronic progressive polyarthritis and retroviral infections, recommending FeLV/FIV testing. Overall, the evidence supports a thorough diagnostic workup and tailored immunosuppressive or antimicrobial therapy based on the underlying etiology.