Arthritis is a painful, degenerative joint disease that affects many senior horses. It is the result of chronic inflammation. To understand how arthritis is related to inflammation, it is important to first understand the process of inflammation.

The Process of Inflammation in Horse Osteoarthritis

When an area is injured or infected, chemicals from injured tissues stimulate pain receptors which then start an impulse to the central nervous system (CNS). The brain processes the impulse and recognizes that there is a problem in that area, so it sends impulses to begin the immune response. The brain also creates the sensation of pain to prevent usage of the injured area so further damage will not happen.



Blood vessels dilate to increase blood flow to the injured area. The increased blood flow causes redness, swelling, and heat. The heat is caused not only by increased blood flow, but because blood from deeper, warmer tissues is delivered to the injured site.

Capillaries are the smallest blood vessels connecting arterioles (small arteries) with venules (small veins). There are gaps in the capillary walls that allow for the exchange of nutrients and wastes, and oxygen and carbon dioxide. During inflammation, the capillaries dilate, increasing the size of the gaps in their walls. This allows for white blood cells to leave the capillaries and move to the injured area where they are attracted by a chemical released by damaged tissues.

When the white blood cells (phagocytes) arrive, they begin eating everything in the area (phagocytosis). Think of it like dusting a table. Before you can dust, you first have to remove everything off of the table, like remote controls, books, vases, etc. When the table is cleared off, then you will begin to put everything where it belongs. The same thing happens in the body.

For example, when you cut yourself, your body goes through all of the above steps. It then begins to either regenerate by creating more cells and making the area like it was before, or it repairs the area by replacing the injured skin with scar tissue. This is where the process of inflammation is different in the joints.

As stated before, the phagocytes arrive in the injured area and eat everything, including cartilage. Cartilage is the cushioning at the ends of bones in joints that keeps the bones from grinding together. Since there is no blood flow in the joint capsule (the joint capsule consists of the ends of the bones, the cartilage, and serous fluid in and around the area), and chondrocytes (cartilage cells) reproduce slowly, cartilage is not easily replaced. Therefore, after an inflammatory response in a joint, and the phagocytes have cleaned up the area (including some of the cartilage), there is less cartilage in the joint than there was before the injury.

Sometimes, the lessened amount of cartilage may cause some pain. The destruction of cartilage from the immune response will also signal the brain that there is more damage in the joint. This will trigger the immune response again, causing inflammation and its consequences. With osteoarthritis, this can become a continuous cycle.

The continuous cycle of inflammation of the joint causes the breakdown of cartilage that is known as osteoarthritis.

Causes of Horse Osteoarthritis

While arthritis really cannot be prevented, there are some steps that can be taken to decrease your horse’s chance of getting arthritis. The biggest risk factor of arthritis is age, or time. Aging may lead to the wear-and-tear of joints and can become a bigger problem for some, leading to arthritis.

Increased weight is a significant and common risk factor for people, and can have the same affect on horses. The joints are only made for so much weight, and that is a healthy weight. Obese horses may be more likely to develop arthritis than horses of a healthy weight. The excess weight adds more stress to joints and can lead to wear-and-tear and inflammation.

Knowing that wear-and-tear can lead to arthritis may lead you to think that it is best to keep the horse still when it comes to joint health; however, this couldn’t be further from the truth. Inactivity is also a large risk factor for arthritis. Movement promotes the production of serous fluid in the joints, thus causing more lubrication. This extra lubrication leads to lesser resistance within the joint, decreasing the chances of inflammation due to wear-and-tear.

This article was first published between 1997 and 2002.

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