Arthritis
Understanding this common disease
by Tom Lenz, D.V.M.. http://www.manepoints.com

You notice a subtle lameness in your horse, although others don't see it. You call your veterinarian, and after a careful check you hear a disturbing word: arthritis.

In humans and in horses, it means much the same thing. But what it means as far as enjoying your horse takes a bit of explaining.

Osteoarthritis, also known as degenerative joint disease, is a major cause of lameness in horses. The condition is a group of joint disorders. Most frequently it involves the hocks, front fetlocks, and coffin joints and navicular bones in the front feet.

To understand the disease, it is necessary to first understand how a normal joint functions.

The bones of a healthy joint fit closely together, with the ends of the opposing bone surfaces coated with a thick, cushion-like material called cartilage. Cartilage not only protects the ends of the bones from damaging each other, but it also absorbs the weight of the horse, somewhat like a shock absorber, each time the horse's hoof hits the ground.

The cartilage also provides a friction-free surface for the opposing ends of the leg bones to rub against as the horse walks or runs. The joint itself is composed of two parts: the fibrous outer joint capsule and the inner synovial membrane.

Strong collateral ligaments surround and connect the bone ends, providing stability and protection for the joint. The entire joint cavity is » filled with a thick, pale-yellow fluid that moves whenever the horse moves, filling the joint and bathing it. This vital liquid is called synovial fluid.

Healthy synovial fluid is critical in that it lubricates the soft tissue of the joint, and nourishes and carries wastes away from the cartilage. Cartilage doesn't have a blood supply of its own, which is why damaged cartilage doesn't heal well.

The viscous joint fluid is made of large, tangled molecules of hyaluronic acid (HA) that form a thick, impervious barrier to inflammatory cells that are trying to gain entrance to the joint. Good-quality HA is basic to the joint's health.

Repeated trauma or stress to a joint, which can occur during everyday use, training or racing, often starts damage to the tissues that enclose the joint, the delicate synovial membrane and the fibrous joint capsule. Lameness is not always present.

As the damage progresses, the horse will eventually become lame, and the joint will become hot and puffy. The tissue damage allows white blood cells from nearby blood vessels to invade the joint space.

The invading white blood cells release destructive enzymes and "free radicals" that begin to eat away at the joint cartilage. The enzymes also break the HA down to a thin, watery liquid that has poor

Left untreated or allowed to recur, this damage progresses to full-blown arthritis and, eventually, crippling degenerative joint disease (DJD).

Once damaged, articular cartilage does not repair itself. It does not have a good blood supply and must rely on the joint fluid which is already thin and watery. Once DJD has developed, it cannot be corrected, only managed, and often leads to premature retirement-sometimes, even, to putting the horse down.

Does my horse have arthritis?

THE BEST DEFENSE against arthritis and degenerative joint disease (DJD) is early detection.

Yet, because the early signs of joint disease are subtle, owners must be trained to look for mild joint swelling and heat, rather than lameness.

Early warning signs may include changes in performance, such as a reluctance to change leads, turn barrels, set a steer or take jumps. Although these problems may be related to behavior or training, the possibility of early joint disease should not be overlooked.

While all horses are at risk of developing DJD, several predisposing factors put some horses at greater risk.

If a horse has crooked legs or toes in or out, its joints will have uneven pressure placed on them. (For example, a horse that toes out will have greater pressure placed on the inside of the coffin, pastern, fetlock and knee joints.) This predisposes these joints to soft tissue inflammation and uneven wear of the cartilage, and may eventually lead to the development of DJD.

Older horses are more prone to arthritis. From birth to two years old, a horse's joints manufacture more new cartilage than they wear away. From two to about 15, cartilage replacement roughly equals normal joint cartilage wear. However, from about age 15 and above, cartilage wear begins to outstrip replacement.

As a result, the cartilage wears thin, increasing bone-to-bone concussion and injury to the joint. Tendons and ligaments in older horses also become less elastic, making them susceptible to tears, leading to joint instability and inflammation.

All joints are unique and respond to injury in slightly different ways, making early signs of joint disease difficult to see. Early signs may include heat or swelling of the joint; pain on joint flexion; and various degrees of lameness.

The vet's examination includes observing the horse trot in a straight line and in circles on a hard surface, as well as using nerve and joint blocks (local anesthesia) and X-rays. Other diagnostic tests such as joint fluid analysis, ultrasound and arthroscopy (insertion of a tiny flexible scope to show the inside of the joint) may also be warranted.

Once the affected joint or joints are isolated, additional X-rays will be used to determine the severity of the condition and to rule out other joint problems such as fractures, bone chips or foreign bodies.