Feline Degenerative Joint Disease Study Guide

Overview and Clinical Importance

Degenerative joint disease (DJD), also known as osteoarthritis (OA), is a chronic, progressive disorder characterized by degradation of articular cartilage, subchondral bone remodeling, synovitis, and pain. DJD is now recognized as the primary source of chronic pain in cats, affecting well over 25% of the feline population. Despite its high prevalence, feline DJD remains significantly underdiagnosed due to cats' unique presentation of pain and mobility issues.

Unlike dogs, cats rarely exhibit overt lameness. Their small size and natural agility allow them to compensate for orthopedic disease, and bilateral involvement is common, making gait abnormalities difficult to detect. Behavioral changes are often the first and most reliable indicators of feline OA.

Prevalence and Epidemiology

Feline DJD is far more common than historically appreciated. Multiple radiographic studies have demonstrated remarkably high prevalence rates:

Most commonly affected joints: The elbow is most frequently affected, followed by the hip, stifle, tarsus, and shoulder. The lumbosacral spine is also commonly involved (40% of affected cats). Bilateral and symmetric involvement is typical, occurring in over 90% of affected cats.

Etiology and Pathophysiology

Primary vs Secondary OA

Primary (idiopathic) OA: Most cases of feline OA appear to be primary, meaning there is no obvious underlying cause. Age-related cartilage degeneration is the most important risk factor. Unlike dogs, where secondary OA from developmental conditions is common, cats more frequently develop primary OA.

Secondary OA: Develops as a consequence of: trauma or joint injury, hip dysplasia (more common in Maine Coons), developmental abnormalities, prior infection, or immune-mediated arthropathies. Only 11-25% of feline OA cases have an identifiable underlying cause.

Pathophysiologic Mechanisms

OA is a multifactorial condition arising from an imbalance in anabolic and catabolic processes within the joint. The sequence of pathologic changes includes:

• Cartilage degradation: Articular cartilage becomes brittle and fibrillated, eventually breaking away from the bone
• Subchondral bone changes: Exposure of subchondral bone leads to sclerosis and potential cyst formation
• Osteophyte formation: New bone develops at joint margins (periarticular osteophytes)
• Synovial inflammation: Joint capsule becomes inflamed and fibrotic; synovial fluid becomes thin and watery
• Pain signaling: Nerve Growth Factor (NGF) is upregulated and perpetuates chronic pain

Risk Factors

Clinical Presentation

Behavioral Changes - The Key to Diagnosis

Unlike dogs, cats with OA rarely exhibit overt lameness. Behavioral and lifestyle changes are the first indicators of disease and are typically what owners report. Many changes previously attributed to "normal aging" are now recognized as signs of OA pain.

Physical Examination Findings

Physical examination in cats is challenging due to their stoic nature and tendency to mask pain. Cats may resist joint manipulation due to pain OR anxiety, making interpretation difficult. Key findings include:

• Joint thickening and swelling: Palpable periarticular changes
• Crepitus: Grinding sensation during range of motion
• Decreased range of motion: Limited joint flexibility
• Pain on manipulation: Though cats often do not vocalize or show obvious pain response
• Muscle atrophy: Especially over hindquarters in chronic cases
• Altered posture: Difficulty getting up; hunched appearance

Diagnosis

Clinical Assessment

Diagnosis of feline OA relies on a combination of history, physical examination, and diagnostic imaging. Owner observation is crucial - validated questionnaires (such as the Feline Musculoskeletal Pain Index and MI-CAT) help identify mobility impairment. Video documentation of the cat's behavior at home is invaluable, as cats often freeze or behave abnormally in the veterinary clinic.

Radiographic Findings

Radiography remains the cornerstone of diagnosis. However, there is poor correlation between radiographic findings and clinical signs - radiographic changes may be present without clinical pain, and painful joints may have minimal radiographic abnormalities.

Treatment and Management

A multimodal approach is essential for managing feline OA. The goals are to alleviate pain, improve mobility, and enhance quality of life. Treatment options include pharmacological therapy, weight management, environmental modifications, nutraceuticals, and physical rehabilitation.

Pharmacological Treatment

Frunevetmab (Solensia) - Detailed Information

Frunevetmab is a felinized monoclonal antibody that targets Nerve Growth Factor (NGF), a key mediator of OA pain. By binding to NGF, frunevetmab prevents NGF from activating pain receptors, thereby reducing chronic pain signals. Unlike NSAIDs, it does not rely on renal or hepatic metabolism for elimination, making it safer for cats with concurrent kidney disease.

Key points: Administered monthly as a subcutaneous injection; improvement may be seen within 7-14 days; maximum effect by second or third dose; safe for cats with IRIS Stage 1-2 CKD; most common side effects are vomiting (occasional) and injection site pain; some cats develop dermatitis/pruritus on head and neck.

Non-Pharmacological Management

Surgical Options

Surgery is rarely indicated for feline OA and is reserved for cases where conservative management fails. Options include joint fusion (arthrodesis), total joint replacement (less common than in dogs), and femoral head ostectomy for hip disease.

Prognosis

While OA is progressive and incurable, appropriate multimodal management allows most cats to maintain good quality of life with normal life expectancy. Owner education is crucial for compliance and early recognition of disease progression. Regular monitoring and adjustment of treatment protocols are essential.