The most common arthritis in dogs and cats is osteoarthritis (OA), named because the problem is caused by the bones (osteo). Arthritis in pets can also be caused by infection (septic arthritis or bacterial arthritis in dogs) or by the body attacking itself in an immune system malfunction (rheumatoid arthritis).
With pet osteoarthritis, bones are damaged because cartilage has given way. Cartilage is there to act as a cushion preventing bone from hitting bone as the joint moves. If the cartilage dries, roughens, or chips, bare bones are exposed. The exposed bone flattens and loses resilience so the joint no longer moves smoothly; it jerks and creaks. In response, the body sends white blood cells to the joint, but instead of repairing damage, the white blood cells release enzymes and free radicals that make things worse. The synovial fluid bathing the joint loses viscosity because it is damaged by the white blood cells' enzymes. As the synovial fluid thins, it's no longer able to resist joint compression or provide lubrication. Eventually, even the capsule that surrounds the joint inflames. The pet then has painful arthritis.
Most dogs in their senior years have arthritis. In fact, osteoarthritis (OA) is the most common skeletal disease of dogs. Working, athletic, obese dogs-and those with diabetes or Cushing's disease-are especially prone. Trauma and injury, hip dysplasia or elbow dysplasia also predispose pets to arthritis. Cats get arthritis too, and estimates are that 12 million cats in the US (20% of cats) have feline arthritis.