Feline Immune-Mediated Joint Disease – NAVLE Study Guide

Overview and Clinical Importance

Immune-mediated joint disease (IMJD) encompasses a spectrum of inflammatory arthropathies in cats resulting from aberrant immune responses targeting synovial tissues. While less common than in dogs, feline IMJD is increasingly recognized and represents an important differential for cats presenting with lameness, stiffness, and joint effusion. Unlike dogs where immune-mediated polyarthritis (IMPA) predominates, cats are more likely to have infectious causes of polyarthritis - making thorough diagnostic workup essential before initiating immunosuppressive therapy.

Feline IMJD can be classified as erosive or non-erosive based on radiographic findings, with distinct conditions in each category requiring different management approaches and carrying different prognoses. Understanding these classifications is critical for NAVLE success.

Classification of Feline Immune-Mediated Joint Disease

Feline IMJD is classified based on radiographic findings into erosive and non-erosive forms. This classification is essential for determining prognosis and treatment approach.

Non-Erosive Immune-Mediated Polyarthritis

Idiopathic (Type I) IMPA

Idiopathic IMPA is the most common form of non-erosive polyarthritis in cats when no underlying cause can be identified. It results from immune complex deposition in the synovial membrane, triggering neutrophilic inflammation.

Signalment and Clinical Signs

• Age: Young to middle-aged cats most commonly affected
• Sex: No sex predilection for non-erosive forms
• Breed: No breed predisposition
• Clinical signs: Lameness (100% of cases), pyrexia (approximately 55%), lethargy, decreased activity, stiff gait, reluctance to jump
• Commonly affected joints: Tarsi and carpi most frequently; stifles and elbows may also be involved

IMPA Classification by Association

Non-erosive IMPA can be further classified as associative (with identifiable comorbidity) or non-associative (idiopathic). A recent multicenter study found 65% of feline IMPA cases were non-associative and 35% were associative.

Systemic Lupus Erythematosus (SLE)

Systemic lupus erythematosus is a rare multisystemic autoimmune disease characterized by formation of antibodies against self-antigens with subsequent immune complex deposition in multiple tissues. Polyarthritis is a component of SLE but less common in cats (36%) than dogs (78%).

Clinical Features of Feline SLE

• Breed predisposition: Siamese, Himalayan, and Persian cats
• Cutaneous signs (60%): Erythema, scaling, erosions, ulceration, alopecia, depigmentation (face, ears, paws commonly affected)
• Hematologic abnormalities: Immune-mediated hemolytic anemia, thrombocytopenia, leukopenia
• Renal disease: Glomerulonephritis, proteinuria
• Other: Fever, oral ulceration, neurological signs

Diagnosis of SLE

• Antinuclear antibody (ANA) test: Positive in many cases but NOT pathognomonic; false positives occur
• Skin biopsy: Interface dermatitis; lupus band test may show IgG/C3 deposition at basement membrane
• CBC/Chemistry: Cytopenias, proteinuria
• Diagnosis: Requires multisystemic involvement; adapted from human American Rheumatism Association criteria

Erosive Immune-Mediated Polyarthritis

Feline Chronic Progressive Polyarthritis (FCPP)

Feline chronic progressive polyarthritis (FCPP) is the most important erosive joint disease in cats for NAVLE. First described in 1975, it primarily affects young to middle-aged male cats and has strong associations with retroviral infections, particularly feline leukemia virus (FeLV) and feline syncytia-forming virus (FeSFV/foamy virus).

Key Features of FCPP

Two Forms of FCPP

Memory Aid - FCPP = "F.C.P.P." F - Feline (cats only) C - Carpi and tarsi (distal joints) P - Progressive (worsens over time) P - Periosteal proliferation OR erosive Pattern

Feline Rheumatoid-Like Arthritis

Feline rheumatoid-like arthritis is rare and may represent the deforming variant of FCPP. It is characterized by rheumatoid factor production (autoantibodies against IgG) and progressive joint destruction. Siamese cats may be overrepresented. Unlike FCPP, systemic illness and fever are often absent, so cats may not be evaluated until severe joint deformities are evident.

Feline Calicivirus-Associated Polyarthritis

Feline calicivirus (FCV) can cause acute transient polyarthritis, often called "limping syndrome." This is an important differential diagnosis for young cats with acute lameness, particularly following vaccination or exposure to other cats.

Key Features of FCV Polyarthritis

• Age: Kittens and young cats most commonly affected
• Timing: May occur during acute FCV infection OR within days to weeks after modified-live virus vaccination
• Clinical signs: Acute shifting lameness, fever, joint pain; approximately 25% have concurrent oral ulceration
• Pathogenesis: Immune complex deposition; FCV antigens can be detected in synovial membranes
• Duration: Usually self-limiting; resolves within 48-96 hours (up to 2 weeks)
• Treatment: Supportive care; NSAIDs for pain if needed (meloxicam); no immunosuppression required
• Prognosis: Excellent; spontaneous recovery expected

Exam Focus: A kitten presenting with acute lameness 1-2 weeks after vaccination with ORAL ULCERS is classic for calicivirus-associated polyarthritis. This is SELF-LIMITING and does NOT require immunosuppression!

Diagnostic Approach

The diagnosis of feline IMPA requires systematic exclusion of infectious causes before initiating immunosuppressive therapy. Remember: cats are MORE likely to have infectious polyarthritis than dogs!

Arthrocentesis and Synovial Fluid Analysis

Arthrocentesis is the GOLD STANDARD for diagnosing inflammatory joint disease. Sample at least 2-3 joints, including the tarsal joints which most frequently yield diagnostic samples in cats.

Synovial Fluid Interpretation

IMPA Diagnostic Criteria

IMPA is diagnosed when: 1. Increased neutrophils (greater than 10% of nucleated cells) in synovial fluid from 2 or more joints 2. Non-degenerate neutrophils without intracellular bacteria 3. Negative bacterial culture 4. Response to immunosuppressive therapy

Additional Diagnostic Tests

Treatment

CRITICAL: Before initiating immunosuppression, rule out infectious causes (especially Mycoplasma) with culture, PCR, and/or a 2-week doxycycline trial. Immunosuppression in the presence of occult infection can be fatal.

First-Line Therapy: Prednisolone

Prednisolone is the mainstay of IMPA treatment in cats. Use PREDNISOLONE, not prednisone - cats poorly convert prednisone to the active form prednisolone.

Immunosuppressive Drug Protocols

Treatment Outcomes and Monitoring

• Response rate: 70% of cats with non-erosive IMPA achieve good outcome
• Multimodal therapy: 62% require second-line immunosuppressant in addition to prednisolone
• Meloxicam alone: 10% of cats respond to NSAID monotherapy (recurrence when tapered)
• Monitoring: Repeat arthrocentesis at 2-4 weeks to assess response; CBC monitoring for cytotoxic drugs
• Ligament laxity: Can develop in 15-25% of cases (even non-erosive); may be steroid-related

Prognosis