A disorder resembling human arthritis rarely results in erosive polyarthritis and progressive joint destruction in dogs. Small and toy breeds are most commonly affected by dog arthritis. The age of onset is variable (9 months to 13 years), but most affected dogs are young or middle-aged. Initially, dog arthritis is indistinguishable from idiopathic nonerosive polyarthritis, but the joints are destroyed over time, with distal joints most severely affected.

Arthritis in dogs may result when a triggering event or inciting antigen initiates an immune reaction against endogenous antigens, causing immune complexes to form. Immune complexes are deposited in the synovium, resulting in complement activation, the chemotactic attraction of inflammatory cells, the intraarticular release of cytokines, synovial cell proliferation, and progressive, severe, erosive inflammatory joint disease.

Granulation tissue arises from the inflamed synovium and extends across the joint underneath the articular cartilage. This vascular granulation tissue (e.g., pannus) begins to erode cartilage, and joint swelling and periarticular inflammation cause the joint capsule to stretch and collateral ligaments to rupture. Early treatment of arthritis in dogs is important to prevent irreversible changes and progressive disease.

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It is very important to start treatment for dog with arthritis as soon as possible. Medical treatment usually includes immunosuppressive drugs, gold salts, and chondroprotective agents. Despite their antiinflammatory and immunosuppressive effects, systemic corticosteroids do not seem to have any effect on the long-term progression of arthritis in people, and the response in dogs is variable. Initially, dog arthritis treatment starts with prednisone and azathioprine. Oral chondroprotective agents (cosequin or glucosamine and chondroitin sulfate) are routinely administered. Subjective improvement has also been observed in dogs receiving injectable chondroprotective agents (Adequan).

After 1 month of therapy, the dog is reexamined and synovial fluid is evaluated. If the fluid is noninflammatory, the corticosteroid dose is decreased and treatment with azathioprine is continued. If the fluid is still inflammatory, then daily administration of prednisone and azathioprine continues and methotrexate may be added to treatment. Monthly evaluation of synovial fluid is recommended. If inflammation of the synovial fluid presists after 2 months, additional therapy such as gold salts should be attempted. Toxicity is uncommon in dog arthritis treatment but may include fever, thrombocytopenia, leukopenia, dermatitis and stomatitis.

Some therapeutic success may be expected if the dog arthritis treatment is initiated before joint damage is severe. In most cases, however, damage to the articular cartilage is severe before the diagnosis is made. Many dogs require additional therapy to control joint discomfort. Palliative treatment with aspirin and glucosamine has been recommended but the additive gastrointestinal toxicities of corticosteroids must be considered. The concurrent administration of misoprostol can help decrease gastrointestinal adverse effects. Arthritis in dogs is a relentlessly progressive disorder, and even with the appropriate therapy most dogs show deterioration with time. Surgical procedures can occasionally be used to improve joint stability and pain. Synovectomy, arthroplasty, joint replacement, and arthrodesis may decrease painfulness and improve function.

Arthropathies are diseases that affect joints. Arthritis is the term used to denote inflammatory changes in the joint. Arthrosis denotes noninflammatory joint disease. Polyarthritis is inflammation that simultaneously affects several joints. Osteoarthritis or osteoarthrosis is the primarily noninflammatory degenerative joint disease characterized by degeneration of articular cartilage, hypertrophy of marginal bone, and synovial membrane changes. Ankylosis is the end result of degenerative joint or inflammatory diseases, in which the joint is fused after new bone production.

Clinical presentation
The history and clinical presentations vary depending on the arthropathy. Joint disease affects dogs of all ages and sizes; cats are affected less often. Osteoarthritis, the most frequently diagnosed arthropathy, is estimated to affect as much as 20% of the canine population over 1 year of age.

Physical examination findings
Dogs show varying degrees of lameness associated with arthropathies. Muscular assymetry (between limbs) and joint enlargement may be palpable. Abnormalities in range of motion, instability, and painful response and crepitation during joint manipulations may be noted.

Radiography
Radiography is an effective and essential means of screening affected joints. The radiographic findings in affected joints range from normal to proliferative or erosive bone lesions, or they can be limited to adjacent soft tissue changes. Computed tomography often is helpful for identifying joint incongruities and fragments in osteoarthritic joints. Ultrasonography may be helpful for evaluating intra-articular and extra-articular soft tissue structures.

Laboratory findings
The synovial fluid often is evaluated to help differentiate between arthropathies. Cytologic findings range from normal to the presence of phagocytic mononuclear cells, nondegenerative neutrophils, or degenerative neutrophils and organisms. Cytologic findings may help guide the clinician to a definitive diagnosis.

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