Chapter 124 Immune-Mediated Arthritis
Immune-mediated arthropathies are inflammatory, non-infectious, and considered either non-erosive or erosive, depending on the effect on articular cartilage. The arthropathies are characterized by synovitis and articular cartilage damage. Erosive arthritis is characterized by articular cartilage loss, and nonerosive arthritis is not. Nonerosive forms are more common in small animal practice. These conditions are not curable but can be treated to improve quality of life.
ETIOLOGY
• Little investigation of cytokines has been done in immune arthropathies in dogs and cats. It is well established that interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) activate metalloproteinases and cause cartilage degeneration. These cytokines diffuse from synovial membrane to articular cartilage (and vice versa). Studies in humans have shown higher levels of IL-1 and TNF-α in inflammatory arthropathies compared to osteoarthritis. Future investigation in small animals will probably show that these cytokines are involved in the pathology of both nonerosive and erosive inflammatory arthritis.
Nonerosive Forms
• Antigen-antibody complexes (type III hypersensitivity reaction) are potent stimuli for inflammatory mediator release, through complement fixation and subsequent infiltration of synovium by inflammatory cells. This inflammatory process releases cartilage-degrading enzymes (metalloproteinases) and leads to a self-perpetuating process of synovial inflammation and cartilage degradation. When such complexes involve antinuclear antibodies, this disease is termed systemic lupus erythematosus (SLE).
CLINICAL SIGNS
Canine Nonerosive Arthritis
• The signs may be acute onset of lameness, fever, lethargy, and swelling, especially of the distal joints.
• Initially, the range of motion in the joints may be decreased due to swelling, and later the joints may become unstable due to ligamentous damage.
• Type II—These are associated with remote infection from sites other than joints, for example, bacterial endocarditis or any other focus of infection that could act as a source of antigens. The joints are usually not infected; rather, immune complexes between host antibody and bacterial components cause the problem. Type II accounts for 25% of small animal cases.
• Type III—These are associated with chronic gastrointestinal disease and are also called enteropathic arthritis. They account for 15% of small animal cases.
Nonerosive Arthritis Syndromes
Several disease syndromes that do not fit in the above classification include:
Idiopathic Drug-Induced Polyarthritis
• Most commonly seen secondary to administration of antibiotics such as sulfonamide drugs and less commonly erythromycin, lincomycin, cephalosporins, and penicillins. Doberman pinschers have been identified as being particularly susceptible to the effects of sulfonamide drugs. The mechanisms of drug-induced polyarthritis could include a hapten reaction or direct immune complexes of host-produced antibody with the drug.
Canine Erosive Arthritis
Greyhound Polyarthritis
• Affects young greyhounds and has been described in Australia and the United States. The cause is unknown, but Mycoplasma spumans has been suspected.
