Poor Performance in The Sport Horse
Dr. Melissa Mazan
Degenerative Joint Disease
One of the most common causes of lameness in sport horses is degenerative joint disease (DJD), otherwise known as osteoarthritis. Areas that are often affected include the hocks (where it is known as bone spavin), the fetlocks, the pastern joints (where it is known as ringbone), coffin joints, and, less frequently, the carpal joints, usually called the knees by horsemen.
Degenerative joint disease, as the name implies, is a disease of wear and tear. The joints are lined by specialized tissue called hyaline cartilage that is vital to smooth joint function. This cartilage can become frayed and damaged due to the mechanical wear associated with exercise. Nasty inflammation results, which furthers the damage in a vicious cycle: damage, inflammation, damage from inflammation, more damage, and so on. The normally smooth glistening cartilage becomes eroded, exposing bone and causing pain. Horses show lameness that may respond to rest initially, but usually worsens with time. Most anyone with a knee problem can attest to the pain and recurrence of DJD.
Disease associated with the navicular bone, which resides within the hoof capsule, has shortened the athletic career of many horses. Researchers are still working hard at determining the ultimate cause of navicular disease. The accepted causes include increased pressure within the navicular bone itself and arthritis involving the navicular bone and surrounding structures such as tendons and the coffin joint surfaces. Although navicular disease has long been associated with quarter horses that possess large bodies and small feet, it is seen in most large sport horses, including racehorses and warmbloods.
Because many horses have navicular disease in both front feet, many owners do not realize that their horse is lame. Rather, they may report that the horse has developed a very short, choppy gait, a "shoulder lameness," or seems reluctant to go forward. These horses are really sore when turned on hard surfaces and actually start to limp.
Tying up Syndrome
Veterinarians know this disease as recurrent exertional rhabdomyolysis (RER), which literally means disintegration of muscle. It causes a painful breakdown of muscle, which is often accompanied by a high heart rate and respiratory rate, sweating, and anxiety.
RER was traditionally called Monday Morning Disease, because it was seen in draft horses who were given Sunday as a day of rest but fed the full grain ration anyways. These horses accumulated starch products over the weekend in their muscles, which rapidly broke down on Monday into lactic acid, causing them to tie up. We don't see this condition very often anymore.
We now know that RER affects all types of horses, but is most common in young fillies in training. It is seen in horses that are intermittently rested as well as in horses that are in continual work. Theories as to the cause of rhabdomyolysis include genetic defects in muscle function, carbohydrate (starch) overloading, thyroid imbalances, vitamin E and selenium deficiencies, and hormonal imbalances. The truth is that most cases are not associated with any of these causes, and the reason for the tying up is unclear.
Recently, an inherited disease, polysaccharide storage myopathy (PSSM), has been shown to be a cause of tying up in some quarter horses, draft horses, and warmbloods. Many horses are diagnosed with PSSM, but it is not certain that all of those accused of having PSSM have a genetic problem or have acquired the condition during life.
Horses with RER may show vague symptoms such as stiffness after work, a shortened stride, an odd lameness that comes and goes, stiffness and weight loss. In rare cases, this condition can cause the horses to collapse and be unable to get up on their own. Of course, this condition should not be confused with HYPP (hyperkalemic periodic paralysis), whereby horses collapse and show weakness rather than stiffness after work.