Arthritis, also referred to as degenerative joint disease (DJD), can start at any age, but occurs most often in older cats. There are different kinds of arthritis.
Osteoarthritis is the most common form of arthritis in cats. Osteoarthritis (OA) happens when the cartilage layer in a joint becomes thin and allows the bones themselves to rub against each other. When the bones rub together it can be very painful. New bone growths, called bone spurs can grow along the rubbing bone edges. Bone spurs can cause additional pain if they rub or pinch nerve endings. With OA the onset of pain is often very gradual. Many cats experience worsening pain from OA long before they have obvious symptoms of that pain.
Septic Arthritis is a type of arthritis that happens when certain kinds of infections settle in one or more of a cat’s joints. Cats with septic arthritis will be in obvious pain, and the onset of that pain is usually very sudden. Septic arthritis is a rare kind of arthritis and is more likely to affect dogs than it is to affect cats. The main population of cats affected by septic arthritis are those who also have calicivirus. Wag! has a good article covering septic arthritis.
Immune Mediated Polyarthritis (IMPA) in cats is similar to Rheumatoid Arthritis (RA) in humans. IMPA is an autoimmune disorder in which the cat’s immune system attacks the joints, flooding them with white blood cells. The white blood cells cause the fluid in the joints to change, taking away its normal protective ability. IMPA, like SA, is more common in dogs than in cats.
Disease Information Sites
I haven’t found any websites devoted specifically to arthritis in cats.
Although the site is devoted to kidney disease, Tanya’s Comprehensive Guide to Chronic Kidney Disease has excellent coverage of arthritis in cats.
Harpsie’s Website, a sister site to Tanya’s, also has a page on arthritis. Harpsie’s arthritis page is not as up-to-date as the page on Tanya’s site, but has some information about Harpsie’s experience with arthritis, so it’s still worth reading.
Cats with Multiple Medical Conditions is a Facebook group that can help with managing arthritis in your cat.
Tanya’s Support Group is a groups.io (email) based group that can help with managing arthritis in your cat if they also have chronic kidney disease.
Cat’s Health and Behavior is another groups.io group, related to Tanya’s Support Group, and covers anything/everything related to cat health.